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Linux perf 1.1、perf_event内核框架

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为什么有了ftrace又出来一个perf？因为ftrace只管抓trace数据并没有分析，perf在trace数据分析方面做出了很多成果。

在trace数据采集方面，perf复用了ftrace的所有插桩点，并且加入了采样法(硬件PMU)。PMU是一种非常重要的数据采集方法，因为它大部分是硬件的，所以可以做到一些软件做不到的事情，获取到一些底层硬件的信息。

perf的基本包装模型是这样的，对每一个event分配一个对应的perf_event结构。所有对event的操作都是围绕perf_event来展开的：

• 通过perf_event_open系统调用分配到perf_event以后，会返回一个文件句柄fd，这样这个perf_event结构可以通过read/write/ioctl/mmap通用文件接口来操作。
• perf_event提供两种类型的trace数据：count和sample。count只是记录了event的发生次数，sample记录了大量信息(比如：IP、ADDR、TID、TIME、CPU、BT)。如果需要使用sample功能，需要给perf_event分配ringbuffer空间，并且把这部分空间通过mmap映射到用户空间。这和定位问题时从粗到细的思路是相符的，首先从counter的比例上找出问题热点在哪个模块，再使用详细记录抓取更多信息来进一步定位。具体分别对应“perf stat”和“perf record/report”命令。
• perf的开销应该是比ftrace要大的，因为它给每个event都独立一套数据结构perf_event，对应独立的attr和pmu。在数据记录时的开销肯定大于ftrace，但是每个event的ringbuffer是独立的所以也不需要ftrace复杂的ringbuffer操作。perf也有比ftrace开销小的地方，它的sample数据存储的ringbuffer空间会通过mmap映射到到用户态，这样在读取数据的时候就会少一次拷贝。不过perf的设计初衷也不是让成百上千的event同时使用，只会挑出一些event重点debug。

0、perf_event的组织

从上面的描述看per就是一个个perf_event并不复杂，那么复杂在哪里呢？真正的难点在于对event的组织，怎么把全局的event的资源，按照用户的需要分割成cpu维度/task维度。

我们在分析问题的时候，并不是一下子深入到底层event直接来看数据(如果不加区别event记录的是整系统的数据)，我们会遵从系统->cpu->进程来分析问题。针对实际的需求，perf使用cpu维度/task维度来组织perf_event。

我们具体来看看perf_event的组织方法：

• 1、cpu维度：

使用perf_event_context类型的链表来连接本cpu的相关perf_event。这样的链表共有两条(perf_hw_context = 0, perf_sw_context = 1)，链表指针存放在per_cpu变量pmu->pmu_cpu_context.ctx中由所有同类型的pmu共享。

• 2、task维度：
• 使用perf_event_context类型的链表来连接本task的相关perf_event。这样的链表共有两条(perf_hw_context = 0, perf_sw_context = 1)，链表指针存放在task->perf_event_ctxp[ctxn]变量中。

  

• 3、perf_event_open()系统调用使用cpu、pid两个参数来指定perf_event的cpu、task维度。

• pid == 0: event绑定到当前进程；

  pid > 0: event绑定到指定进程；

  pid == -1: event绑定到当前cpu的所有进程。

  cpu >= 0: event绑定到指定cpu；

  cpu == -1: event绑定到所有cpu；

  在同时指定的情况下task维度优先于cpu维度，所以pid、cpu组合起来有以下几种情况：

  组合1：pid >= 0, cpu >= 0。perf_event绑定到task维度的context。task在得到cpu调度运行的时候，context上挂载的本task相关的perf_event也开始运行。但是如果event指定的cpu不等于当前运行的cpu，event不会得到执行，这样就符合了这个组合的含义；

  组合2：pid >= 0, cpu == -1。perf_event绑定到task维度的context。只要task得到调度，该perf_event就会得到执行；

  组合3：pid == -1, cpu >= 0。perf_event绑定到cpu维度的context。只要该cpu运行，该perf_event就会得到执行。目前只有在cpu online的情况下才能绑定perf_event，cpu hotplug支持可能会有问题；

  组合4：pid == -1, cpu == -1。这种组合目前是非法的，相当于整系统所有cpu、所有进程。

• 4、group leader：

• cpu/task context使用->event_list链表来连接所有的perf_event。这些perf_event还允许以group的形式来组织，context使用->pinned_groups/flexible_groups链表来连接group leader的perf_event；group leader使用->sibling_list链表来连接所有的group member perf_event。组织形式参考上图。

  group的作用是在read count的时候可以一次性读出group中所有perf_event的count。

  perf_event_open()系统调用使用group_fd参数来指定perf_event的group_leader：>=0指定对于的perf_event为当前group_leader，== -1创建新的group_leader。

  pinned：可以看到group leader被放到两个链表中(->pinned_groups/flexible_groups)，attr.pinned=1指定放到高优先级链表->pinned_groups中。

  (具体参见后面perf_install_in_context()的代码解析)

• 5、perf_task_sched。
• 对于cpu维度的perf_event来说只要cpu online会一直运行，而对于task维度的perf_event来说只有在task得到调度运行的时候event才能运行。所以在每个cpu上同时只能有一个task维度的perf_evnt得到执行，cpu维度的context使用了pmu->pmu_cpu_context->task_ctx指针来保存当前运行的task context。

  

  perf驱动层的精髓就在于此：在合适的时间合适的开关对应的perf_event。(具体参见后面perf_event_task_sched_in()、perf_event_task_sched_out()的代码解析)

  在单个cpu上，多个任务调度时context/perf_event的开关情况：

  

  单个任务，在多个cpu上调度时context/perf_event的开关情况：

  

• 6、inherit：

• inherit属性指定如果perf_event绑定的task创建子进程，event自动的对子进程也进行追踪。这在实际使用时是非常有用的，我们追踪一个app，随后它创建出的子进程/线程都能自动的被追踪。

  父进程中所有attr.inherit=1的event被子进程所继承和复制，在使用PERF_FORMAT_GROUP读event的count值时，会把inherit所有子进程的值累加进来。(具体参见后面perf_event_init_task()、perf_read_group()的代码解析)

  

• 7、exclusive：
• 如果pmu有PERF_PMU_CAP_EXCLUSIVE属性，表明它要么只能被绑定为cpu维度、要么只能被绑定为task维度，不能混合绑定。(具体参见后面exclusive_event_init()的代码解析)
• 8、pmu的数据供给：

• 每个pmu拥有一个per_cpu的链表，perf_event需要在哪个cpu上获取数据就加入到哪个cpu的链表上。如果event被触发，它会根据当前的运行cpu给对应链表上的所有perf_event推送数据。

  cpu维度的context：this_cpu_ptr(pmu->pmu_cpu_context->ctx)上链接的所有perf_event会根据绑定的pmu，链接到pmu对应的per_cpu的->perf_events链表上。

  task维度的context：this_cpu_ptr(pmu->pmu_cpu_context->task_ctx)上链接的所有perf_event会根据绑定的pmu，链接到pmu对应的per_cpu的->perf_events链表上。perf_event还需要做cpu匹配，符合(event->cpu == -1 || event->cpu == smp_processor_id())条件的event才能链接到pmu上。

  

• 9、enable_on_exec：

• perf_event的状态(event->state)典型值有以下3种：

  disable：PERF_EVENT_STATE_OFF = -1, // 如果attr.disabled = 1，event->state的初始值

  enable/inactive：PERF_EVENT_STATE_INACTIVE = 0, // 如果attr.disabled = 0，event->state的初始值

  active：PERF_EVENT_STATE_ACTIVE = 1,

  attr.disabled属性决定了perf_event的初始化状态(disable/enable)。只有perf_event为enable以后才能参与schedule，在schedule过程中perf_event被使能时为active，关闭后恢复成enbale/inactive状态。

  perf_event变成enable状态有3种方法：

  1、attr.disabled = 0；

  2、attr.disabled = 1，创建后使用ioctl的PERF_EVENT_IOC_ENABLE命令来使能；

  3、attr.disabled = 1，attr.enable_on_exec = 1。这样使用execl执行新程序时使能event，这是一种非常巧妙的同步手段；

• 10、ringbuffer:

• 如果需要读取perf_event的sample类型的数据，需要先给perf_event分配一个对应的ringbuffer，为了减少开销这个ringbuffer会被mmap映射成用户态地址。

  如上图所示整个ringbuffer空间分成3部分：

  head：size = 1 page。主要用来存储控制数据，指针等等

  data：size = 2^n pages。主要用来存储perf_event的sample数据

  aux data：size = 2^n pages。作用暂时没有看明白

  如果perf_event支持inherit属性，那么它所有的子进程上继承的perf_event的sample数据，都会保存到父perf_event的ringbuffer中。perf_event可以inherit，但是ringbuffer不会重新分配，会共用父event的ringbuffer。

  

• 11、sample_period/sample_freq:

• 1、perf_event初始化

  perf_event初始化的时候将各种pmu注册到pmus链表。

  start_kernel() -> perf_event_init()：

  void __init perf_event_init(void)
  {
      int ret;
  
      /* (1) 初始化idr，给动态type的pmu来分配 */
      idr_init(&pmu_idr);
  
      /* (2) 初始化per_cpu变量swevent_htable */
      perf_event_init_all_cpus();
      init_srcu_struct(&pmus_srcu);
  
      /* (3) 注册"software" pmu */
      perf_pmu_register(&perf_swevent, "software", PERF_TYPE_SOFTWARE);
      perf_pmu_register(&perf_cpu_clock, NULL, -);
      perf_pmu_register(&perf_task_clock, NULL, -);
  
      /* (4) 注册"tracepoint" pmu */
      perf_tp_register();
      perf_cpu_notifier(perf_cpu_notify);
      idle_notifier_register(&perf_event_idle_nb);
      register_reboot_notifier(&perf_reboot_notifier);
  
      /* (5) 注册"breakpoint" pmu */
      ret = init_hw_breakpoint();
      WARN(ret, "hw_breakpoint initialization failed with: %d", ret);
  
      /* do not patch jump label more than once per second */
      jump_label_rate_limit(&perf_sched_events, HZ);
  
      /*
       * Build time assertion that we keep the data_head at the intended
       * location.  IOW, validation we got the __reserved[] size right.
       */
      BUILD_BUG_ON((offsetof(struct perf_event_mmap_page, data_head))
               != );
  }
  
  ↓
  
  int perf_pmu_register(struct pmu *pmu, const char *name, int type)
  {
      int cpu, ret;
  
      mutex_lock(&pmus_lock);
      ret = -ENOMEM;
      pmu->pmu_disable_count = alloc_percpu(int);
      if (!pmu->pmu_disable_count)
          goto unlock;
  
      /* (3.1) 如果name = null，则pmu->name=null、pmu->type=-1 */
      pmu->type = -;
      if (!name)
          goto skip_type;
  
      /* (3.1.1) pmu->name = name */
      pmu->name = name;
  
      /* (3.1.2) 如果type < 0，在idr中动态分配值给pmu->type */
      if (type < ) {
          type = idr_alloc(&pmu_idr, pmu, PERF_TYPE_MAX, , GFP_KERNEL);
          if (type < ) {
              ret = type;
              goto free_pdc;
          }
      }
      pmu->type = type;
  
      if (pmu_bus_running) {
          ret = pmu_dev_alloc(pmu);
          if (ret)
              goto free_idr;
      }
  
      /* (3.2) 初始化cpu维度的perf_cpu_context，
         perf_cpu_context的作用是用来把某一维度的perf_event链接到一起 
       */
  skip_type:
      /* (3.2.1) 如果有相同task_ctx_nr类型的pmu已经创建perf_cpu_context结构， 
          直接引用
       */
      pmu->pmu_cpu_context = find_pmu_context(pmu->task_ctx_nr);
      if (pmu->pmu_cpu_context)
          goto got_cpu_context;
  
      ret = -ENOMEM;
      /* (3.2.2) 如果没有人创建本pmu task_ctx_nr类型的perf_cpu_context结构， 
          重新创建
       */
      pmu->pmu_cpu_context = alloc_percpu(struct perf_cpu_context);
      if (!pmu->pmu_cpu_context)
          goto free_dev;
  
      /* (3.2.3) 初始化per_cpu的perf_cpu_context结构 */
      for_each_possible_cpu(cpu) {
          struct perf_cpu_context *cpuctx;
  
          cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
          __perf_event_init_context(&cpuctx->ctx);
          lockdep_set_class(&cpuctx->ctx.mutex, &cpuctx_mutex);
          lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
          cpuctx->ctx.pmu = pmu;
  
          __perf_mux_hrtimer_init(cpuctx, cpu);
  
          cpuctx->unique_pmu = pmu;
      }
  
  got_cpu_context:
      /* (3.3) 给pmu赋值一些默认的操作函数 */
      if (!pmu->start_txn) {
          if (pmu->pmu_enable) {
              /*
               * If we have pmu_enable/pmu_disable calls, install
               * transaction stubs that use that to try and batch
               * hardware accesses.
               */
              pmu->start_txn  = perf_pmu_start_txn;
              pmu->commit_txn = perf_pmu_commit_txn;
              pmu->cancel_txn = perf_pmu_cancel_txn;
          } else {
              pmu->start_txn  = perf_pmu_nop_txn;
              pmu->commit_txn = perf_pmu_nop_int;
              pmu->cancel_txn = perf_pmu_nop_void;
          }
      }
  
      if (!pmu->pmu_enable) {
          pmu->pmu_enable  = perf_pmu_nop_void;
          pmu->pmu_disable = perf_pmu_nop_void;
      }
  
      if (!pmu->event_idx)
          pmu->event_idx = perf_event_idx_default;
  
      /* (3.4) 最重要的一步：将新的pmu加入到pmus链表中 */
      list_add_rcu(&pmu->entry, &pmus);
      atomic_set(&pmu->exclusive_cnt, );
      ret = ;
  unlock:
      mutex_unlock(&pmus_lock);
  
      return ret;
  
  free_dev:
      device_del(pmu->dev);
      put_device(pmu->dev);
  
  free_idr:
      if (pmu->type >= PERF_TYPE_MAX)
          idr_remove(&pmu_idr, pmu->type);
  
  free_pdc:
      free_percpu(pmu->pmu_disable_count);
      goto unlock;
  }
             

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  另外一个函数perf_event_sysfs_init()会在稍后的device_initcall中，为所有“pmu->name != null”的pmu创建对应的device：

  static int __init perf_event_sysfs_init(void)
  {
      struct pmu *pmu;
      int ret;
  
      mutex_lock(&pmus_lock);
  
      /* (1) 注册pmu_bus */
      ret = bus_register(&pmu_bus);
      if (ret)
          goto unlock;
  
      /* (2) 遍历pmus链表，创建pmu对应的device */
      list_for_each_entry(pmu, &pmus, entry) {
          /* 如果pmu->name = null或者pmu->type < 0，不创建 */
          if (!pmu->name || pmu->type < )
              continue;
  
          ret = pmu_dev_alloc(pmu);
          WARN(ret, "Failed to register pmu: %s, reason %d\n", pmu->name, ret);
      }
  
      /* (3) 设置pmu_bus_running */
      pmu_bus_running = ;
      ret = ;
  
  unlock:
      mutex_unlock(&pmus_lock);
  
      return ret;
  }
  device_initcall(perf_event_sysfs_init);           

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  可以在/sys路径下看到对应的device：

  # ls /sys/bus/event_source/devices/
  armv8_pmuv3 software tracepoint           

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  2、perf_event_open系统调用

  perf_event_open会创建event对应的perf_event结构，按照perf_event_attr参数把perf_event和对应的pmu以及perf_cpu_context(cpu维度/task维度)绑定，最后再把perf_event和perf_fops以及fd绑定，返回fd给系统进行文件操作。

  理解perf_event_open系统调用先理解它的5个参数：

  perf_event_open(struct perf_event_attr attr, pid_t pid, int cpu, int group_fd, unsigned long flags)           

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  • 参数1、struct perf_event_attr attr。该参数是最复杂也是最重要的参数：

  struct perf_event_attr {
  
      /*
       * Major type: hardware/software/tracepoint/etc.
       */
      /* () 指定pmu的type：
          enum perf_type_id {
              PERF_TYPE_HARDWARE          = ,
              PERF_TYPE_SOFTWARE          = ,
              PERF_TYPE_TRACEPOINT            = ,
              PERF_TYPE_HW_CACHE          = ,
              PERF_TYPE_RAW               = ,
              PERF_TYPE_BREAKPOINT            = ,
  
              PERF_TYPE_MAX,              
          };
       */
      __u32           type;
  
      /*
       * Size of the attr structure, for fwd/bwd compat.
       */
      /* (2) 整个perf_event_attr结构体的size */
      __u32           size;
  
      /*
       * Type specific configuration information.
       */
      /* () 不同type的pmu，config的含义也不同：
          、type = PERF_TYPE_HARDWARE：
              enum perf_hw_id {
                  PERF_COUNT_HW_CPU_CYCLES        = ,
                  PERF_COUNT_HW_INSTRUCTIONS      = ,
                  PERF_COUNT_HW_CACHE_REFERENCES      = ,
                  PERF_COUNT_HW_CACHE_MISSES      = ,
                  PERF_COUNT_HW_BRANCH_INSTRUCTIONS   = ,
                  PERF_COUNT_HW_BRANCH_MISSES     = ,
                  PERF_COUNT_HW_BUS_CYCLES        = ,
                  PERF_COUNT_HW_STALLED_CYCLES_FRONTEND   = ,
                  PERF_COUNT_HW_STALLED_CYCLES_BACKEND    = ,
                  PERF_COUNT_HW_REF_CPU_CYCLES        = ,
  
                  PERF_COUNT_HW_MAX,          
              };
          、type = PERF_TYPE_SOFTWARE：
              enum perf_sw_ids {
                  PERF_COUNT_SW_CPU_CLOCK         = ,
                  PERF_COUNT_SW_TASK_CLOCK        = ,
                  PERF_COUNT_SW_PAGE_FAULTS       = ,
                  PERF_COUNT_SW_CONTEXT_SWITCHES      = ,
                  PERF_COUNT_SW_CPU_MIGRATIONS        = ,
                  PERF_COUNT_SW_PAGE_FAULTS_MIN       = ,
                  PERF_COUNT_SW_PAGE_FAULTS_MAJ       = ,
                  PERF_COUNT_SW_ALIGNMENT_FAULTS      = ,
                  PERF_COUNT_SW_EMULATION_FAULTS      = ,
                  PERF_COUNT_SW_DUMMY         = ,
                  PERF_COUNT_SW_BPF_OUTPUT        = ,
  
                  PERF_COUNT_SW_MAX,          
              };
          、type = PERF_TYPE_TRACEPOINT：
              trace_point对应trace_event的id：“/sys/kernel/debug/tracing/events/x/x/id”
          、type = PERF_TYPE_HW_CACHE：
              enum perf_hw_cache_id {
                  PERF_COUNT_HW_CACHE_L1D         = ,
                  PERF_COUNT_HW_CACHE_L1I         = ,
                  PERF_COUNT_HW_CACHE_LL          = ,
                  PERF_COUNT_HW_CACHE_DTLB        = ,
                  PERF_COUNT_HW_CACHE_ITLB        = ,
                  PERF_COUNT_HW_CACHE_BPU         = ,
                  PERF_COUNT_HW_CACHE_NODE        = ,
  
                  PERF_COUNT_HW_CACHE_MAX,        
              };
       */
      __u64           config;
  
      /* (4) period/freq sample模式的具体数值 */
      union {
          __u64       sample_period;
          __u64       sample_freq;
      };
  
      /* () 在sample数据时，需要保存哪些数据：
          enum perf_event_sample_format {
              PERF_SAMPLE_IP              = U << ,
              PERF_SAMPLE_TID             = U << ,
              PERF_SAMPLE_TIME            = U << ,
              PERF_SAMPLE_ADDR            = U << ,
              PERF_SAMPLE_READ            = U << ,
              PERF_SAMPLE_CALLCHAIN           = U << ,
              PERF_SAMPLE_ID              = U << ,
              PERF_SAMPLE_CPU             = U << ,
              PERF_SAMPLE_PERIOD          = U << ,
              PERF_SAMPLE_STREAM_ID           = U << ,
              PERF_SAMPLE_RAW             = U << ,
              PERF_SAMPLE_BRANCH_STACK        = U << ,
              PERF_SAMPLE_REGS_USER           = U << ,
              PERF_SAMPLE_STACK_USER          = U << ,
              PERF_SAMPLE_WEIGHT          = U << ,
              PERF_SAMPLE_DATA_SRC            = U << ,
              PERF_SAMPLE_IDENTIFIER          = U << ,
              PERF_SAMPLE_TRANSACTION         = U << ,
              PERF_SAMPLE_REGS_INTR           = U << ,
  
              PERF_SAMPLE_MAX = U << , 
          };
       */
      __u64           sample_type;
  
      /* () 在read counter数据时，读取的格式:
           *
           * The format of the data returned by read() on a perf event fd,
           * as specified by attr.read_format:
           *
           * struct read_format {
           *  { u64       value;
           *    { u64     time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
           *    { u64     time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
           *    { u64     id;           } && PERF_FORMAT_ID
           *  } && !PERF_FORMAT_GROUP
           *
           *  { u64       nr;
           *    { u64     time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
           *    { u64     time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
           *    { u64     value;
           *      { u64   id;           } && PERF_FORMAT_ID
           *    }     cntr[nr];
           *  } && PERF_FORMAT_GROUP
           * };
           *
          enum perf_event_read_format {
              PERF_FORMAT_TOTAL_TIME_ENABLED      = U << ,
              PERF_FORMAT_TOTAL_TIME_RUNNING      = U << ,
              PERF_FORMAT_ID              = U << ,
              PERF_FORMAT_GROUP           = U << ,
  
              PERF_FORMAT_MAX = U << ,      
          };
       */
      __u64           read_format;
  
      /* (7) bit标志 */
                  /* () 定义event的初始状态为disable/enable。
                      如果初始被disable，后续可以通过ioctl/prctl来enable。 
                   */
      __u64           disabled       :  , /* off by default        */
  
                  /* (7.2) 如果该标志被设置，event进程对应的子孙后代的子进程都会计入counter */
                  inherit        :  , /* children inherit it   */
  
                  /* (7.3) 如果该标志被设置，event和cpu绑定。(只适用于硬件counter只适用于group leaders) */
                  pinned         :  , /* must always be on PMU */
  
                  /* (7.4) 如果该标志被设置，指定当这个group在CPU上时，它应该是唯一使用CPU计数器的group */
                  exclusive      :  , /* only group on PMU     */
  
                  /* () exclude_user/exclude_kernel/exclude_hv/exclude_idle这几个标志用来标识，
                      不要记录对应场景的数据
                   */
                  exclude_user   :  , /* don't count user      */
                  exclude_kernel :  , /* ditto kernel          */
                  exclude_hv     :  , /* ditto hypervisor      */
                  exclude_idle   :  , /* don't count when idle */
  
                  /* (7.6) 允许记录PROT_EXEC mmap操作 */
                  mmap           :  , /* include mmap data     */
  
                  /* (7.7) 允许记录进程创建时的comm数据 */
                  comm           :  , /* include comm data     */
  
                  /* (7.8) 确定sample模式 = freq/period */
                  freq           :  , /* use freq, not period  */
  
  
                  inherit_stat   :  , /* per task counts       */
                  enable_on_exec :  , /* next exec enables     */
                  task           :  , /* trace fork/exit       */
                  watermark      :  , /* wakeup_watermark      */
                  /*
                   * precise_ip:
                   *
                   *   - SAMPLE_IP can have arbitrary skid
                   *   - SAMPLE_IP must have constant skid
                   *   - SAMPLE_IP requested to have  skid
                   *   - SAMPLE_IP must have  skid
                   *
                   *  See also PERF_RECORD_MISC_EXACT_IP
                   */
                  precise_ip     :  , /* skid constraint       */
                  mmap_data      :  , /* non-exec mmap data    */
                  sample_id_all  :  , /* sample_type all events */
  
                  exclude_host   :  , /* don't count in host   */
                  exclude_guest  :  , /* don't count in guest  */
  
                  exclude_callchain_kernel : , /* exclude kernel callchains */
                  exclude_callchain_user   : , /* exclude user callchains */
                  mmap2          :  , /* include mmap with inode data     */
                  comm_exec      :  , /* flag comm events that are due to an exec */
                  use_clockid    :  , /* use @clockid for time fields */
                  context_switch :  , /* context switch data */
                  constraint_duplicate : ,
  
                  __reserved_1   : ;
  
      union {
          __u32       wakeup_events;    /* wakeup every n events */
          __u32       wakeup_watermark; /* bytes before wakeup   */
      };
  
      __u32           bp_type;
      union {
          __u64       bp_addr;
          __u64       config1; /* extension of config */
      };
      union {
          __u64       bp_len;
          __u64       config2; /* extension of config1 */
      };
      __u64   branch_sample_type; /* enum perf_branch_sample_type */
  
      /*
       * Defines set of user regs to dump on samples.
       * See asm/perf_regs.h for details.
       */
      __u64   sample_regs_user;
  
      /*
       * Defines size of the user stack to dump on samples.
       */
      __u32   sample_stack_user;
  
      __s32   clockid;
      /*
       * Defines set of regs to dump for each sample
       * state captured on:
       *  - precise = : PMU interrupt
       *  - precise > : sampled instruction
       *
       * See asm/perf_regs.h for details.
       */
      __u64   sample_regs_intr;
  
      /*
       * Wakeup watermark for AUX area
       */
      __u32   aux_watermark;
      __u32   __reserved_2;   /* align to __u64 */
  }           

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  • 参数2、pid_t pid：

  pid == 0: event绑定到当前进程；

  pid > 0: event绑定到指定进程；

  pid < 0: event绑定到当前cpu的所有进程。

• 参数3、int cpu：

• cpu >= 0: event绑定到指定cpu；

  cpu == -1: event绑定到所有cpu；

  (注意’pid == -1’、’cpu == -1’同时存在是非法的)

• 参数4、int group_fd：

• group_fd = -1：创建一个新的group leader；

  group_fd > 0：加入到之前创建的group leader中。

• 参数5、unsigned long flags：

• #define PERF_FLAG_FD_NO_GROUP       (1UL << 0)
  #define PERF_FLAG_FD_OUTPUT     (1UL << 1)
  #define PERF_FLAG_PID_CGROUP        (1UL << 2) /* pid=cgroup id, per-cpu mode only */
  #define PERF_FLAG_FD_CLOEXEC        (1UL << 3) /* O_CLOEXEC */           

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  • 4
  perf_event_open()函数的完全解析如下：

  SYSCALL_DEFINE5(perf_event_open,
          struct perf_event_attr __user *, attr_uptr,
          pid_t, pid, int, cpu, int, group_fd, unsigned long, flags)
  {
      struct perf_event *group_leader = NULL, *output_event = NULL;
      struct perf_event *event, *sibling;
      struct perf_event_attr attr;
      struct perf_event_context *ctx, *uninitialized_var(gctx);
      struct file *event_file = NULL;
      struct fd group = {NULL, };
      struct task_struct *task = NULL;
      struct pmu *pmu;
      int event_fd;
      int move_group = ;
      int err;
      int f_flags = O_RDWR;
      int cgroup_fd = -;
  
      /* (1) 一系列的合法性判断和准备工作 */
      /* for future expandability... */
      if (flags & ~PERF_FLAG_ALL)
          return -EINVAL;
  
      /* (1.1) 权限判断 */
      if (perf_paranoid_any() && !capable(CAP_SYS_ADMIN))
          return -EACCES;
  
      /* (1.2) 拷贝用户态的perf_event_attr到内核态 */
      err = perf_copy_attr(attr_uptr, &attr);
      if (err)
          return err;
  
      if (attr.constraint_duplicate || attr.__reserved_1)
          return -EINVAL;
  
      if (!attr.exclude_kernel) {
          if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
              return -EACCES;
      }
  
      /* (1.3) 如果sample是freq mode，sample_freq的合法性判断 */
      if (attr.freq) {
          if (attr.sample_freq > sysctl_perf_event_sample_rate)
              return -EINVAL;
      /* (1.4) 如果sample是period mode，sample_period的合法性判断 */
      } else {
          if (attr.sample_period & (ULL << ))
              return -EINVAL;
      }
  
      /*
       * In cgroup mode, the pid argument is used to pass the fd
       * opened to the cgroup directory in cgroupfs. The cpu argument
       * designates the cpu on which to monitor threads from that
       * cgroup.
       */
      if ((flags & PERF_FLAG_PID_CGROUP) && (pid == - || cpu == -))
          return -EINVAL;
  
      if (flags & PERF_FLAG_FD_CLOEXEC)
          f_flags |= O_CLOEXEC;
  
      /* (1.5) 当前进程获取一个新的fd编号  */
      event_fd = get_unused_fd_flags(f_flags);
      if (event_fd < )
          return event_fd;
  
      /* (1.6) 如果当前event需要加入到指定的group leader中，获取到： 
          group_fd对应的fd结构 和 perf_event结构
       */
      if (group_fd != -) {
          err = perf_fget_light(group_fd, &group);
          if (err)
              goto err_fd;
          group_leader = group.file->private_data;
          if (flags & PERF_FLAG_FD_OUTPUT)
              output_event = group_leader;
          if (flags & PERF_FLAG_FD_NO_GROUP)
              group_leader = NULL;
      }
  
      /*
       * Take the group_leader's group_leader_mutex before observing
       * anything in the group leader that leads to changes in ctx,
       * many of which may be changing on another thread.
       * In particular, we want to take this lock before deciding
       * whether we need to move_group.
       */
      if (group_leader)
          mutex_lock(&group_leader->group_leader_mutex);
  
      /* (1.7) 找到pid对应的task_struct结构 */
      if (pid != - && !(flags & PERF_FLAG_PID_CGROUP)) {
          task = find_lively_task_by_vpid(pid);
          if (IS_ERR(task)) {
              err = PTR_ERR(task);
              goto err_group_fd;
          }
      }
  
      /* (1.8) 如果是加入到group leader，需要两者的attr.inherit属性一致 */
      if (task && group_leader &&
          group_leader->attr.inherit != attr.inherit) {
          err = -EINVAL;
          goto err_task;
      }
  
      get_online_cpus();
  
      if (task) {
          err = mutex_lock_interruptible(&task->signal->cred_guard_mutex);
          if (err)
              goto err_cpus;
  
          /*
           * Reuse ptrace permission checks for now.
           *
           * We must hold cred_guard_mutex across this and any potential
           * perf_install_in_context() call for this new event to
           * serialize against exec() altering our credentials (and the
           * perf_event_exit_task() that could imply).
           */
          err = -EACCES;
          if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
              goto err_cred;
      }
  
      /* (1.9) 创建cgroup fd，这和之前的group leader不一样 */
      if (flags & PERF_FLAG_PID_CGROUP)
          cgroup_fd = pid;
  
      /* (2) 重头戏：根据传入的参数，分配perf_event结构并初始化 */
      event = perf_event_alloc(&attr, cpu, task, group_leader, NULL,
                   NULL, NULL, cgroup_fd);
      if (IS_ERR(event)) {
          err = PTR_ERR(event);
          goto err_cred;
      }
  
      /* (3.1) 如果attr指定需要sample数据，但是pmu没有中断能力，返回出错(主要是针对硬件pmu) */
      if (is_sampling_event(event)) {
          if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
              err = -ENOTSUPP;
              goto err_alloc;
          }
      }
  
      /*
       * Special case software events and allow them to be part of
       * any hardware group.
       */
      pmu = event->pmu;
  
      /* (3.2) 如果用户指定时钟源，把event->clock设置为用户指定值 */
      if (attr.use_clockid) {
          err = perf_event_set_clock(event, attr.clockid);
          if (err)
              goto err_alloc;
      }
  
      /* (3.3) 如果event和group_leader的pmu type不一致的处理 */
      if (group_leader &&
          (is_software_event(event) != is_software_event(group_leader))) {
          /* (3.3.1) pmu type: event == software, group_leader != software 
              把event加入到group_leader中
           */
          if (is_software_event(event)) {
              /*
               * If event and group_leader are not both a software
               * event, and event is, then group leader is not.
               *
               * Allow the addition of software events to !software
               * groups, this is safe because software events never
               * fail to schedule.
               */
              pmu = group_leader->pmu;
  
          /* (3.3.2) pmu type: event != software, group_leader == software 
              尝试把整个group移入到hardware context中
           */
          } else if (is_software_event(group_leader) &&
                 (group_leader->group_flags & PERF_GROUP_SOFTWARE)) {
              /*
               * In case the group is a pure software group, and we
               * try to add a hardware event, move the whole group to
               * the hardware context.
               */
              move_group = ;
          }
      }
  
      /*
       * Get the target context (task or percpu):
       */
      /* (4) get到perf_event_context，根据perf_event类型得到cpu维度/task维度的context：
          如果pid=-1即task=NULL，获得cpu维度的context，即pmu注册时根据pmu->task_ctx_nr分配的pmu->pmu_cpu_context->ctx
          如果pid>=0即task!=NULL，获得task维度的context，即task->perf_event_ctxp[ctxn]，如果为空则重新创建
       */
      ctx = find_get_context(pmu, task, event);
      if (IS_ERR(ctx)) {
          err = PTR_ERR(ctx);
          goto err_alloc;
      }
  
      /* (5.1) event需要加入到group_leader，如果(pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE)，出错返回  */
      if ((pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE) && group_leader) {
          err = -EBUSY;
          goto err_context;
      }
  
      /*
       * Look up the group leader (we will attach this event to it):
       */
      /* (5.2) event需要加入到group_leader，对一些条件进行合法性判断  */
      if (group_leader) {
          err = -EINVAL;
  
          /*
           * Do not allow a recursive hierarchy (this new sibling
           * becoming part of another group-sibling):
           */
          /* (5.2.1) 不允许递归的->group_leader */
          if (group_leader->group_leader != group_leader)
              goto err_context;
  
          /* All events in a group should have the same clock */
          /* (5.2.2) event加入gruop，需要时钟源一致 */
          if (group_leader->clock != event->clock)
              goto err_context;
  
          /*
           * Do not allow to attach to a group in a different
           * task or CPU context:
           */
          /* (5.2.3) event加入gruop，需要task/cpu的context一致 */
          if (move_group) {
              /*
               * Make sure we're both on the same task, or both
               * per-cpu events.
               */
              if (group_leader->ctx->task != ctx->task)
                  goto err_context;
  
              /*
               * Make sure we're both events for the same CPU;
               * grouping events for different CPUs is broken; since
               * you can never concurrently schedule them anyhow.
               */
              if (group_leader->cpu != event->cpu)
                  goto err_context;
          } else {
              if (group_leader->ctx != ctx)
                  goto err_context;
          }
  
          /*
           * Only a group leader can be exclusive or pinned
           */
          /* (5.2.4) 只有group才能设置exclusive/pinned属性 */
          if (attr.exclusive || attr.pinned)
              goto err_context;
      }
  
      /* (5.3) 设置output_event */
      if (output_event) {
          err = perf_event_set_output(event, output_event);
          if (err)
              goto err_context;
      }
  
      /* (6) 分配perf_event对应的file结构： 
          file->private_data = event; // file和event结构链接在一起
          file->f_op = perf_fops；    // file的文件操作函数集
          后续会把fd和file链接到一起：fd_install(event_fd, event_file);
       */
      event_file = anon_inode_getfile("[perf_event]", &perf_fops, event,
                      f_flags);
      if (IS_ERR(event_file)) {
          err = PTR_ERR(event_file);
          event_file = NULL;
          goto err_context;
      }
  
      if (move_group) {
          gctx = group_leader->ctx;
          mutex_lock_double(&gctx->mutex, &ctx->mutex);
      } else {
          mutex_lock(&ctx->mutex);
      }
  
      /* (7.1) 根据attr计算：event->read_size、event->header_size、event->id_header_size 
          并判断是否有超长
       */
      if (!perf_event_validate_size(event)) {
          err = -E2BIG;
          goto err_locked;
      }
  
      /*
       * Must be under the same ctx::mutex as perf_install_in_context(),
       * because we need to serialize with concurrent event creation.
       */
      /* (7.2) 如果是排他性event：(pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE) 
          检查context链表中现有的event是否允许新的event插入
       */
      if (!exclusive_event_installable(event, ctx)) {
          /* exclusive and group stuff are assumed mutually exclusive */
          WARN_ON_ONCE(move_group);
  
          err = -EBUSY;
          goto err_locked;
      }
  
      WARN_ON_ONCE(ctx->parent_ctx);
  
      /*
       * This is the point on no return; we cannot fail hereafter. This is
       * where we start modifying current state.
       */
  
      /* (8) 如果group和当前event的pmu type不一致，
          尝试更改context到当前event
       */
      if (move_group) {
          /*
           * See perf_event_ctx_lock() for comments on the details
           * of swizzling perf_event::ctx.
           */
          /* (8.1) 把group_leader从原有的context中remove */
          perf_remove_from_context(group_leader, false);
  
          /* (8.2) 把所有group_leader的子event从原有的context中remove */
          list_for_each_entry(sibling, &group_leader->sibling_list,
                      group_entry) {
              perf_remove_from_context(sibling, false);
              put_ctx(gctx);
          }
  
          /*
           * Wait for everybody to stop referencing the events through
           * the old lists, before installing it on new lists.
           */
          synchronize_rcu();
  
          /*
           * Install the group siblings before the group leader.
           *
           * Because a group leader will try and install the entire group
           * (through the sibling list, which is still in-tact), we can
           * end up with siblings installed in the wrong context.
           *
           * By installing siblings first we NO-OP because they're not
           * reachable through the group lists.
           */
          /* (8.3) 把所有group_leader的子event安装到新的context中 */
          list_for_each_entry(sibling, &group_leader->sibling_list,
                      group_entry) {
              perf_event__state_init(sibling);
              perf_install_in_context(ctx, sibling, sibling->cpu);
              get_ctx(ctx);
          }
  
          /*
           * Removing from the context ends up with disabled
           * event. What we want here is event in the initial
           * startup state, ready to be add into new context.
           */
          /* (8.4) 把group_leader安装到新的context中 */
          perf_event__state_init(group_leader);
          perf_install_in_context(ctx, group_leader, group_leader->cpu);
          get_ctx(ctx);
  
          /*
           * Now that all events are installed in @ctx, nothing
           * references @gctx anymore, so drop the last reference we have
           * on it.
           */
          put_ctx(gctx);
      }
  
      /*
       * Precalculate sample_data sizes; do while holding ctx::mutex such
       * that we're serialized against further additions and before
       * perf_install_in_context() which is the point the event is active and
       * can use these values.
       */
      /* (9.1) 重新计算：event->read_size、event->header_size、event->id_header_size */
      perf_event__header_size(event);
      perf_event__id_header_size(event);
  
      /* (10) 把event安装到context当中 */
      perf_install_in_context(ctx, event, event->cpu);
      perf_unpin_context(ctx);
  
      if (move_group)
          mutex_unlock(&gctx->mutex);
      mutex_unlock(&ctx->mutex);
      if (group_leader)
          mutex_unlock(&group_leader->group_leader_mutex);
  
      if (task) {
          mutex_unlock(&task->signal->cred_guard_mutex);
          put_task_struct(task);
      }
  
      put_online_cpus();
  
      event->owner = current;
  
      /* (9.2) 把当前进程创建的所有event，加入到current->perf_event_list链表中 */
      mutex_lock(&current->perf_event_mutex);
      list_add_tail(&event->owner_entry, &current->perf_event_list);
      mutex_unlock(&current->perf_event_mutex);
  
      /*
       * Drop the reference on the group_event after placing the
       * new event on the sibling_list. This ensures destruction
       * of the group leader will find the pointer to itself in
       * perf_group_detach().
       */
      fdput(group);
      /* (9.3) 把fd和file进行链接 */
      fd_install(event_fd, event_file);
      return event_fd;
  
  err_locked:
      if (move_group)
          mutex_unlock(&gctx->mutex);
      mutex_unlock(&ctx->mutex);
  /* err_file: */
      fput(event_file);
  err_context:
      perf_unpin_context(ctx);
      put_ctx(ctx);
  err_alloc:
      /*
       * If event_file is set, the fput() above will have called ->release()
       * and that will take care of freeing the event.
       */
      if (!event_file)
          free_event(event);
  err_cred:
      if (task)
          mutex_unlock(&task->signal->cred_guard_mutex);
  err_cpus:
      put_online_cpus();
  err_task:
      if (task)
          put_task_struct(task);
  err_group_fd:
      if (group_leader)
          mutex_unlock(&group_leader->group_leader_mutex);
      fdput(group);
  err_fd:
      put_unused_fd(event_fd);
      return err;
  }
  
  |→
  
  static struct perf_event *
  perf_event_alloc(struct perf_event_attr *attr, int cpu,
           struct task_struct *task,
           struct perf_event *group_leader,
           struct perf_event *parent_event,
           perf_overflow_handler_t overflow_handler,
           void *context, int cgroup_fd)
  {
      struct pmu *pmu;
      struct perf_event *event;
      struct hw_perf_event *hwc;
      long err = -EINVAL;
  
      if ((unsigned)cpu >= nr_cpu_ids) {
          if (!task || cpu != -)
              return ERR_PTR(-EINVAL);
      }
  
      /* (2.1) 分配perf_event空间 */
      event = kzalloc(sizeof(*event), GFP_KERNEL);
      if (!event)
          return ERR_PTR(-ENOMEM);
  
      /*
       * Single events are their own group leaders, with an
       * empty sibling list:
       *
      /* (2.2) 如果group_fd == -1，那么group_leader = self */
      if (!group_leader)
          group_leader = event;
  
      mutex_init(&event->group_leader_mutex);
      mutex_init(&event->child_mutex);
      INIT_LIST_HEAD(&event->child_list);
  
      INIT_LIST_HEAD(&event->group_entry);
      INIT_LIST_HEAD(&event->event_entry);
      INIT_LIST_HEAD(&event->sibling_list);
      INIT_LIST_HEAD(&event->rb_entry);
      INIT_LIST_HEAD(&event->active_entry);
      INIT_LIST_HEAD(&event->drv_configs);
      INIT_HLIST_NODE(&event->hlist_entry);
  
  
      init_waitqueue_head(&event->waitq);
      init_irq_work(&event->pending, perf_pending_event);
  
      mutex_init(&event->mmap_mutex);
  
      atomic_long_set(&event->refcount, );
      event->cpu      = cpu;
      event->attr     = *attr;
      event->group_leader = group_leader;
      event->pmu      = NULL;
      event->oncpu        = -;
  
      event->parent       = parent_event;
  
      event->ns       = get_pid_ns(task_active_pid_ns(current));
      event->id       = atomic64_inc_return(&perf_event_id);
  
      event->state        = PERF_EVENT_STATE_INACTIVE;
  
      /* (2.3) 如果task != null */
      if (task) {
          event->attach_state = PERF_ATTACH_TASK;
          /*
           * XXX pmu::event_init needs to know what task to account to
           * and we cannot use the ctx information because we need the
           * pmu before we get a ctx.
           */
          event->hw.target = task;
      }
  
      event->clock = &local_clock;
      if (parent_event)
          event->clock = parent_event->clock;
  
      if (!overflow_handler && parent_event) {
          overflow_handler = parent_event->overflow_handler;
          context = parent_event->overflow_handler_context;
      }
  
      event->overflow_handler = overflow_handler;
      event->overflow_handler_context = context;
  
      /* (2.4) 根据attr.disabled的值来设置event的初始化值：
          event->state =  PERF_EVENT_STATE_OFF/PERF_EVENT_STATE_INACTIVE
       */
      perf_event__state_init(event);
  
      pmu = NULL;
  
      /* (2.5) 根据event的attr->freq和attr->sample_period/sample_freq来初始化event->hw:
          hwc->sample_period
          hwc->last_period
          hwc->period_left
       */
      hwc = &event->hw;
      hwc->sample_period = attr->sample_period;
      if (attr->freq && attr->sample_freq)
          hwc->sample_period = ;
      hwc->last_period = hwc->sample_period;
  
      local64_set(&hwc->period_left, hwc->sample_period);
  
      /*
       * we currently do not support PERF_FORMAT_GROUP on inherited events
       */
      /* (2.6) inherit时不支持PERF_FORMAT_GROUP */
      if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP))
          goto err_ns;
  
      /* (2.7) 如果!(attr.sample_type & PERF_SAMPLE_BRANCH_STACK)：
          则attr.branch_sample_type = 0
       */
      if (!has_branch_stack(event))
          event->attr.branch_sample_type = ;
  
      /* (2.8) 如果(flags & PERF_FLAG_PID_CGROUP)：
          将当前event加入cgroup 
       */
      if (cgroup_fd != -) {
          err = perf_cgroup_connect(cgroup_fd, event, attr, group_leader);
          if (err)
              goto err_ns;
      }
  
      /* (2.9) 至关重要的一步：
          根据attr.type在pmus链表中找到对应的pmu，
          并且调用pmu->event_init(event)来初始化event
       */
      pmu = perf_init_event(event);
      if (!pmu)
          goto err_ns;
      else if (IS_ERR(pmu)) {
          err = PTR_ERR(pmu);
          goto err_ns;
      }
  
      /* (2.10) exclusive类型pmu的event的处理 */
      err = exclusive_event_init(event);
      if (err)
          goto err_pmu;
  
      if (!event->parent) {
          if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) {
              err = get_callchain_buffers();
              if (err)
                  goto err_per_task;
          }
      }
  
      /* symmetric to unaccount_event() in _free_event() */
      /* (2.11) 关于event操作的一些统计 */
      account_event(event);
  
      return event;
  
  err_per_task:
      exclusive_event_destroy(event);
  
  err_pmu:
      if (event->destroy)
          event->destroy(event);
      module_put(pmu->module);
  err_ns:
      if (is_cgroup_event(event))
          perf_detach_cgroup(event);
      if (event->ns)
          put_pid_ns(event->ns);
      kfree(event);
  
      return ERR_PTR(err);
  }
  
  |→
  
  static struct perf_event_context *
  find_get_context(struct pmu *pmu, struct task_struct *task,
          struct perf_event *event)
  {
      struct perf_event_context *ctx, *clone_ctx = NULL;
      struct perf_cpu_context *cpuctx;
      void *task_ctx_data = NULL;
      unsigned long flags;
      int ctxn, err;
      int cpu = event->cpu;
  
      /* (4.1) 如果task=null即pid=-1，获取cpu维度的context */
      if (!task) {
          /* Must be root to operate on a CPU event: */
          /* (4.1.1) 权限判断 */
          if (event->owner != EVENT_OWNER_KERNEL && perf_paranoid_cpu() &&
              !capable(CAP_SYS_ADMIN))
              return ERR_PTR(-EACCES);
  
          /*
           * We could be clever and allow to attach a event to an
           * offline CPU and activate it when the CPU comes up, but
           * that's for later.
           */
          /* (4.1.2) attr指定的cpu是否online */
          if (!cpu_online(cpu))
              return ERR_PTR(-ENODEV);
  
          /* (4.1.3) 根据cpu获取到对应pmu的cpu维度context：per_cpu_ptr(pmu->pmu_cpu_context, cpu)->ctx */
          cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
          ctx = &cpuctx->ctx;
          get_ctx(ctx);
          ++ctx->pin_count;
  
          return ctx;
      }
  
      /* (4.2) 如果task!=null即pid>=0，获取task维度的context */
  
      err = -EINVAL;
      ctxn = pmu->task_ctx_nr;
      if (ctxn < )
          goto errout;
  
      /* (4.2.1) 部分架构context需要分配ctx->task_ctx_data */
      if (event->attach_state & PERF_ATTACH_TASK_DATA) {
          task_ctx_data = kzalloc(pmu->task_ctx_size, GFP_KERNEL);
          if (!task_ctx_data) {
              err = -ENOMEM;
              goto errout;
          }
      }
  
  retry:
      /* (4.2.2) 获取task维度的context：task->perf_event_ctxp[pmu->task_ctx_nr]
          如果此前无人创建过此context，则分配空间创建
       */
      ctx = perf_lock_task_context(task, ctxn, &flags);
      /* (4.2.2.1) task此context已经创建，则使用现成的 */
      if (ctx) {
          clone_ctx = unclone_ctx(ctx);
          ++ctx->pin_count;
  
          if (task_ctx_data && !ctx->task_ctx_data) {
              ctx->task_ctx_data = task_ctx_data;
              task_ctx_data = NULL;
          }
          raw_spin_unlock_irqrestore(&ctx->lock, flags);
  
          if (clone_ctx)
              put_ctx(clone_ctx);
      /* (4.2.2.2) 否则，重新创建task->perf_event_ctxp[pmu->task_ctx_nr] */
      } else {
          ctx = alloc_perf_context(pmu, task);
          err = -ENOMEM;
          if (!ctx)
              goto errout;
  
          if (task_ctx_data) {
              ctx->task_ctx_data = task_ctx_data;
              task_ctx_data = NULL;
          }
  
          err = ;
          mutex_lock(&task->perf_event_mutex);
          /*
           * If it has already passed perf_event_exit_task().
           * we must see PF_EXITING, it takes this mutex too.
           */
          if (task->flags & PF_EXITING)
              err = -ESRCH;
          else if (task->perf_event_ctxp[ctxn])
              err = -EAGAIN;
          else {
              get_ctx(ctx);
              ++ctx->pin_count;
              rcu_assign_pointer(task->perf_event_ctxp[ctxn], ctx);
          }
          mutex_unlock(&task->perf_event_mutex);
  
          if (unlikely(err)) {
              put_ctx(ctx);
  
              if (err == -EAGAIN)
                  goto retry;
              goto errout;
          }
      }
  
      kfree(task_ctx_data);
      return ctx;
  
  errout:
      kfree(task_ctx_data);
      return ERR_PTR(err);
  }
  
  |→
  
  static void
  perf_install_in_context(struct perf_event_context *ctx,
              struct perf_event *event,
              int cpu)
  {
      struct task_struct *task = ctx->task;
  
      lockdep_assert_held(&ctx->mutex);
  
      /* (10.1) context赋值给event->ctx */
      event->ctx = ctx;
      if (event->cpu != -)
          event->cpu = cpu;
  
      /* (10.2) 如果是cpu维度的context，
          使用cpu同步机制来调用指定的cpu上运行__perf_install_in_context() 
          绑定context、event
       */
      if (!task) {
          /*
           * Per cpu events are installed via an smp call and
           * the install is always successful.
           */
          cpu_function_call(cpu, __perf_install_in_context, event);
          return;
      }
  
  retry:
      /* (10.3) 如果是task维度的context，且task当前正在running
          使用cpu同步机制调用指定的task的运行cpu(即task_cpu(p))上运行__perf_install_in_context()
          绑定context、event
       */
      if (!task_function_call(task, __perf_install_in_context, event))
          return;
  
      raw_spin_lock_irq(&ctx->lock);
      /*
       * If we failed to find a running task, but find the context active now
       * that we've acquired the ctx->lock, retry.
       */
      if (ctx->is_active) {
          raw_spin_unlock_irq(&ctx->lock);
          /*
           * Reload the task pointer, it might have been changed by
           * a concurrent perf_event_context_sched_out().
           */
          task = ctx->task;
          goto retry;
      }
  
      /*
       * Since the task isn't running, its safe to add the event, us holding
       * the ctx->lock ensures the task won't get scheduled in.
       */
      /* (10.4) 如果是task维度的context，但是task当前不在runnning
          可以安全的绑定event和context
       */
      add_event_to_ctx(event, ctx);
      raw_spin_unlock_irq(&ctx->lock);
  }
  
  ||→
  
  static void add_event_to_ctx(struct perf_event *event,
                     struct perf_event_context *ctx)
  {
      u64 tstamp = perf_event_time(event);
  
      /* (10.4.1) 将event加入到context的相关链表 */
      list_add_event(event, ctx);
  
      /* (10.4.2) 将event加入到group_leader的链表 */
      perf_group_attach(event);
      event->tstamp_enabled = tstamp;
      event->tstamp_running = tstamp;
      event->tstamp_stopped = tstamp;
  }
  
  |||→
  
  static void
  list_add_event(struct perf_event *event, struct perf_event_context *ctx)
  {
      WARN_ON_ONCE(event->attach_state & PERF_ATTACH_CONTEXT);
  
      /* (10.4.1.1) 设置event->attach_state的PERF_ATTACH_CONTEXT */
      event->attach_state |= PERF_ATTACH_CONTEXT;
  
      /*
       * If we're a stand alone event or group leader, we go to the context
       * list, group events are kept attached to the group so that
       * perf_group_detach can, at all times, locate all siblings.
       */
      /* (10.4.1.2) 如果event是group_leader 
          则将其event->group_entry加入到顶级group链表：ctx->flexible_groups/pinned_groups
          ctx->flexible_groups/pinned_groups链表只链接group_leader的event
       */
      if (event->group_leader == event) {
          struct list_head *list;
  
          if (is_software_event(event))
              event->group_flags |= PERF_GROUP_SOFTWARE;
  
          list = ctx_group_list(event, ctx);
          list_add_tail(&event->group_entry, list);
      }
  
      if (is_cgroup_event(event))
          ctx->nr_cgroups++;
  
      /* (10.4.1.3) 将event->event_entry加入到链表：ctx->event_list 
          ctx->event_list链表链接context下所有的event
       */
      list_add_rcu(&event->event_entry, &ctx->event_list);
      ctx->nr_events++;
      if (event->attr.inherit_stat)
          ctx->nr_stat++;
  
      ctx->generation++;
  }
  
  |||→
  
  static void perf_group_attach(struct perf_event *event)
  {
      struct perf_event *group_leader = event->group_leader, *pos;
  
      /*
       * We can have double attach due to group movement in perf_event_open.
       */
      /* (10.4.2.1) 设置event->attach_state的PERF_ATTACH_GROUP */
      if (event->attach_state & PERF_ATTACH_GROUP)
          return;
  
      event->attach_state |= PERF_ATTACH_GROUP;
  
      /* (10.4.2.2) 如果event本身就是group_leader，不需要继续操作 */
      if (group_leader == event)
          return;
  
      WARN_ON_ONCE(group_leader->ctx != event->ctx);
  
      /* (10.4.2.3) move_group的处理？ */
      if (group_leader->group_flags & PERF_GROUP_SOFTWARE &&
              !is_software_event(event))
          group_leader->group_flags &= ~PERF_GROUP_SOFTWARE;
  
      /* (10.4.2.4) 把event->group_entry加入到group_leader->sibling_list链表 */
      list_add_tail(&event->group_entry, &group_leader->sibling_list);
      group_leader->nr_siblings++;
  
      /* (10.4.2.5) 重新计算group_leader的event->read_size、event->header_size */
      perf_event__header_size(group_leader);
  
      /* (10.4.2.6) 重新计算group_leader所有子event的event->read_size、event->header_size */
      list_for_each_entry(pos, &group_leader->sibling_list, group_entry)
          perf_event__header_size(pos);
  }
             

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  经过perf_event_open()调用以后返回perf_event对应的fd，后续的文件操作对应perf_fops：

  static const struct file_operations perf_fops = {
      .llseek         = no_llseek,
      .release        = perf_release,
      .read           = perf_read,
      .poll           = perf_poll,
      .unlocked_ioctl     = perf_ioctl,
      .compat_ioctl       = perf_compat_ioctl,
      .mmap           = perf_mmap,
      .fasync         = perf_fasync,
  };           

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  后续会对其重点的函数perf_read()、perf_ioctl()、perf_mmap()一一进行解析。

  2.1、inherit

  进程通过task contex(task->perf_event_ctxp[ctxn])挂载了很多面向task的perf_event，如果event支持inherit属性，当进程创建子进程时需要给子进程创建task context和继承的event。

  copy_process() -> perf_event_init_task() -> perf_event_init_context() -> inherit_task_group() -> inherit_group() -> inherit_event():

  int perf_event_init_task(struct task_struct *child)
  {
      int ctxn, ret;
  
      /* () 初始化child task的perf_event相关结构 */
      memset(child->perf_event_ctxp, , sizeof(child->perf_event_ctxp));
      mutex_init(&child->perf_event_mutex);
      INIT_LIST_HEAD(&child->perf_event_list);
  
      /* () 进程通过task contex(task->perf_event_ctxp[ctxn])挂载了很多面向task的perf_event，
          如果event支持inherit属性，当进程创建子进程时需要给子进程创建task context和继承的event
       */
      for_each_task_context_nr(ctxn) {
          ret = perf_event_init_context(child, ctxn);
          if (ret) {
              perf_event_free_task(child);
              return ret;
          }
      }
  
      return ;
  }
  
  |→
  
  static int perf_event_init_context(struct task_struct *child, int ctxn)
  {
      struct perf_event_context *child_ctx, *parent_ctx;
      struct perf_event_context *cloned_ctx;
      struct perf_event *event;
      struct task_struct *parent = current;
      int inherited_all = ;
      unsigned long flags;
      int ret = ;
  
      /* () 父进程即为当前进程(current) */
      if (likely(!parent->perf_event_ctxp[ctxn]))
          return ;
  
      /*
       * If the parent's context is a clone, pin it so it won't get
       * swapped under us.
       */
      parent_ctx = perf_pin_task_context(parent, ctxn);
      if (!parent_ctx)
          return ;
  
      /*
       * No need to check if parent_ctx != NULL here; since we saw
       * it non-NULL earlier, the only reason for it to become NULL
       * is if we exit, and since we're currently in the middle of
       * a fork we can't be exiting at the same time.
       */
  
      /*
       * Lock the parent list. No need to lock the child - not PID
       * hashed yet and not running, so nobody can access it.
       */
      mutex_lock(&parent_ctx->mutex);
  
      /*
       * We dont have to disable NMIs - we are only looking at
       * the list, not manipulating it:
       */
      /* () 遍历父进程context上的高优先级group leader event链表：parent_ctx->pinned_groups，
          在子进程上复制需要inherit的event
       */
      list_for_each_entry(event, &parent_ctx->pinned_groups, group_entry) {
          ret = inherit_task_group(event, parent, parent_ctx,
                       child, ctxn, &inherited_all);
          if (ret)
              break;
      }
  
      /*
       * We can't hold ctx->lock when iterating the ->flexible_group list due
       * to allocations, but we need to prevent rotation because
       * rotate_ctx() will change the list from interrupt context.
       */
      raw_spin_lock_irqsave(&parent_ctx->lock, flags);
      parent_ctx->rotate_disable = ;
      raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
  
      /* () 遍历父进程context上的低优先级group leader event链表：parent_ctx->flexible_groups，
          在子进程上复制需要inherit的event
       */
      list_for_each_entry(event, &parent_ctx->flexible_groups, group_entry) {
          ret = inherit_task_group(event, parent, parent_ctx,
                       child, ctxn, &inherited_all);
          if (ret)
              break;
      }
  
      raw_spin_lock_irqsave(&parent_ctx->lock, flags);
      parent_ctx->rotate_disable = ;
  
      child_ctx = child->perf_event_ctxp[ctxn];
  
      /* () 如果inherited_all>，表示父进程挂载的所有event都是inherit属性，都被子进程复制继承， 
          那么给子进程的task context->parent_ctx赋值为父进程的context
       */
      if (child_ctx && inherited_all) {
          /*
           * Mark the child context as a clone of the parent
           * context, or of whatever the parent is a clone of.
           *
           * Note that if the parent is a clone, the holding of
           * parent_ctx->lock avoids it from being uncloned.
           */
          cloned_ctx = parent_ctx->parent_ctx;
          if (cloned_ctx) {
              child_ctx->parent_ctx = cloned_ctx;
              child_ctx->parent_gen = parent_ctx->parent_gen;
          } else {
              child_ctx->parent_ctx = parent_ctx;
              child_ctx->parent_gen = parent_ctx->generation;
          }
          get_ctx(child_ctx->parent_ctx);
      }
  
      raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
      mutex_unlock(&parent_ctx->mutex);
  
      perf_unpin_context(parent_ctx);
      put_ctx(parent_ctx);
  
      return ret;
  }
  
  ||→
  
  static int
  inherit_task_group(struct perf_event *event, struct task_struct *parent,
             struct perf_event_context *parent_ctx,
             struct task_struct *child, int ctxn,
             int *inherited_all)
  {
      int ret;
      struct perf_event_context *child_ctx;
  
      /* () 如果event不支持inherit，直接返回 */
      if (!event->attr.inherit) {
          *inherited_all = ;
          return ;
      }
  
      /* () 如果子进程的context为空，分配创建 */
      child_ctx = child->perf_event_ctxp[ctxn];
      if (!child_ctx) {
          /*
           * This is executed from the parent task context, so
           * inherit events that have been marked for cloning.
           * First allocate and initialize a context for the
           * child.
           */
  
          child_ctx = alloc_perf_context(parent_ctx->pmu, child);
          if (!child_ctx)
              return -ENOMEM;
  
          child->perf_event_ctxp[ctxn] = child_ctx;
      }
  
      /* () 子进程对父进程的event进行复制继承 */
      ret = inherit_group(event, parent, parent_ctx,
                  child, child_ctx);
  
      if (ret)
          *inherited_all = ;
  
      return ret;
  }
  
  |||→
  
  static int inherit_group(struct perf_event *parent_event,
            struct task_struct *parent,
            struct perf_event_context *parent_ctx,
            struct task_struct *child,
            struct perf_event_context *child_ctx)
  {
      struct perf_event *leader;
      struct perf_event *sub;
      struct perf_event *child_ctr;
  
      /* () 对group_leader event进行复制继承， 
          新建group leader的继承者是新的group leader
       */
      leader = inherit_event(parent_event, parent, parent_ctx,
                   child, NULL, child_ctx);
      if (IS_ERR(leader))
          return PTR_ERR(leader);
  
      /* () 对group_leader下面的子event进行复制继承： 
          如果父进程group leader下面有树成员，他会把这棵树给子进程也复制一份，
          ---oops：这里有个疑问，如果父进程group成员不是group leader所在的父进程，但是inherit会给子进程复制一个event，这样合理吗？
       */
      list_for_each_entry(sub, &parent_event->sibling_list, group_entry) {
          child_ctr = inherit_event(sub, parent, parent_ctx,
                          child, leader, child_ctx);
          if (IS_ERR(child_ctr))
              return PTR_ERR(child_ctr);
      }
      return ;
  }
  
  ||||→
  
  static struct perf_event *
  inherit_event(struct perf_event *parent_event,
            struct task_struct *parent,
            struct perf_event_context *parent_ctx,
            struct task_struct *child,
            struct perf_event *group_leader,
            struct perf_event_context *child_ctx)
  {
      enum perf_event_active_state parent_state = parent_event->state;
      struct perf_event *child_event;
      unsigned long flags;
  
      /*
       * Instead of creating recursive hierarchies of events,
       * we link inherited events back to the original parent,
       * which has a filp for sure, which we use as the reference
       * count:
       */
      /* () 获得子进程新建event的parent event，
          如果是多级子进程，把所有子进程的event都挂在原始parent下面，而不是一级一级的挂载
       */
      if (parent_event->parent)
          parent_event = parent_event->parent;
  
      /* () 对group_leader event进行复制，重新分配初始化： 
          和父event创建不同，这指定了子event的parent：event->parent = parent_event;
          随后子event也会被加入到父event的parent_event->child_list链表中
       */
      child_event = perf_event_alloc(&parent_event->attr,
                         parent_event->cpu,
                         child,
                         group_leader, parent_event,
                         NULL, NULL, -);
      if (IS_ERR(child_event))
          return child_event;
  
      if (is_orphaned_event(parent_event) ||
          !atomic_long_inc_not_zero(&parent_event->refcount)) {
          free_event(child_event);
          return NULL;
      }
  
      get_ctx(child_ctx);
  
      /*
       * Make the child state follow the state of the parent event,
       * not its attr.disabled bit.  We hold the parent's mutex,
       * so we won't race with perf_event_{en, dis}able_family.
       */
      /* () 如果父event的状态已经enable，子event的状态也为enable */
      if (parent_state >= PERF_EVENT_STATE_INACTIVE)
          child_event->state = PERF_EVENT_STATE_INACTIVE;
      else
          child_event->state = PERF_EVENT_STATE_OFF;
  
      if (parent_event->attr.freq) {
          u64 sample_period = parent_event->hw.sample_period;
          struct hw_perf_event *hwc = &child_event->hw;
  
          hwc->sample_period = sample_period;
          hwc->last_period   = sample_period;
  
          local64_set(&hwc->period_left, sample_period);
      }
  
      child_event->ctx = child_ctx;
      child_event->overflow_handler = parent_event->overflow_handler;
      child_event->overflow_handler_context
          = parent_event->overflow_handler_context;
  
      /*
       * Precalculate sample_data sizes
       */
      perf_event__header_size(child_event);
      perf_event__id_header_size(child_event);
  
      /*
       * Link it up in the child's context:
       */
      /* () 将新的event加入到子进程的context中 */
      raw_spin_lock_irqsave(&child_ctx->lock, flags);
      add_event_to_ctx(child_event, child_ctx);
      raw_spin_unlock_irqrestore(&child_ctx->lock, flags);
  
      /*
       * Link this into the parent event's child list
       */
      /* () 将子event加入到父event的->child_list链表中 */
      WARN_ON_ONCE(parent_event->ctx->parent_ctx);
      mutex_lock(&parent_event->child_mutex);
      list_add_tail(&child_event->child_list, &parent_event->child_list);
      mutex_unlock(&parent_event->child_mutex);
  
      return child_event;
  }
             

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  2.2、perf task sched

  task context上链接的perf_event需要跟随task进程调度一起动态启动停止，在task得到调度时相关perf_event开始工作，在task被其他任务调度出去时相关perf_event停止工作。

  为了支持这种行为，在task switch时调用perf的回调函数：perf_event_task_sched_out()/perf_event_task_sched_in()

  context_switch() -> prepare_task_switch() -> perf_event_task_sched_out():

  static inline void perf_event_task_sched_out(struct task_struct *prev,
                           struct task_struct *next)
  {
      /* (1) "software" pmu 子类型为“context-switches”的数据采集点 */
      perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, , );
  
      /* (2) 随着taskswitch，和task关联的相关event需要停止工作：即sched_out 
          perf_sched_events.key：是进行perf_sched_in/out的开关，在event分配(perf_event_alloc() -> account_event())和释放(_free_event() -> unaccount_event())时操作
       */
      if (static_key_false(&perf_sched_events.key))
          __perf_event_task_sched_out(prev, next);
  }
  
  |→
  
  void __perf_event_task_sched_out(struct task_struct *task,
                   struct task_struct *next)
  {
      int ctxn;
  
      /* (2.1) 回调函数pmu->sched_task的调用点 */
      if (__this_cpu_read(perf_sched_cb_usages))
          perf_pmu_sched_task(task, next, false);
  
      /* (2.2) */
      if (atomic_read(&nr_switch_events))
          perf_event_switch(task, next, false);
  
      /* (2.3) 遍历task->[perf_event_ctxp]中的context， 
          逐个关闭context链接中的每个perf_event
       */
      for_each_task_context_nr(ctxn)
          perf_event_context_sched_out(task, ctxn, next);
  
      /*
       * if cgroup events exist on this CPU, then we need
       * to check if we have to switch out PMU state.
       * cgroup event are system-wide mode only
       */
      /* (2.4) task cgroup相关sched_out */
      if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
          perf_cgroup_sched_out(task, next);
  }
  
  ||→
  
  static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
                       struct task_struct *next)
  {
      struct perf_event_context *ctx = task->perf_event_ctxp[ctxn];
      struct perf_event_context *next_ctx;
      struct perf_event_context *parent, *next_parent;
      struct perf_cpu_context *cpuctx;
      int do_switch = ;
  
      if (likely(!ctx))
          return;
  
      cpuctx = __get_cpu_context(ctx);
      if (!cpuctx->task_ctx)
          return;
  
      rcu_read_lock();
      next_ctx = next->perf_event_ctxp[ctxn];
      if (!next_ctx)
          goto unlock;
  
      parent = rcu_dereference(ctx->parent_ctx);
      next_parent = rcu_dereference(next_ctx->parent_ctx);
  
      /* If neither context have a parent context; they cannot be clones. */
      if (!parent && !next_parent)
          goto unlock;
  
      /* (2.3.1) 如果curr task和next task的context刚好是parent关系，我们使用快捷路径来切换任务， 
          context的parent关系，只有在创建子进程，且所有的父进程event都有inherit属性被子进程全部复制继承，
       */
      if (next_parent == ctx || next_ctx == parent || next_parent == parent) {
          /*
           * Looks like the two contexts are clones, so we might be
           * able to optimize the context switch.  We lock both
           * contexts and check that they are clones under the
           * lock (including re-checking that neither has been
           * uncloned in the meantime).  It doesn't matter which
           * order we take the locks because no other cpu could
           * be trying to lock both of these tasks.
           */
          raw_spin_lock(&ctx->lock);
          raw_spin_lock_nested(&next_ctx->lock, SINGLE_DEPTH_NESTING);
          if (context_equiv(ctx, next_ctx)) {
              /*
               * XXX do we need a memory barrier of sorts
               * wrt to rcu_dereference() of perf_event_ctxp
               */
              task->perf_event_ctxp[ctxn] = next_ctx;
              next->perf_event_ctxp[ctxn] = ctx;
              ctx->task = next;
              next_ctx->task = task;
  
              swap(ctx->task_ctx_data, next_ctx->task_ctx_data);
  
              do_switch = ;
  
              perf_event_sync_stat(ctx, next_ctx);
          }
          raw_spin_unlock(&next_ctx->lock);
          raw_spin_unlock(&ctx->lock);
      }
  unlock:
      rcu_read_unlock();
  
      /* (2.3.2) 慢速路径的task context切换：
       */
      if (do_switch) {
          raw_spin_lock(&ctx->lock);
          ctx_sched_out(ctx, cpuctx, EVENT_ALL);
          cpuctx->task_ctx = NULL;
          raw_spin_unlock(&ctx->lock);
      }
  }
  
  |||→
  
  static void ctx_sched_out(struct perf_event_context *ctx,
                struct perf_cpu_context *cpuctx,
                enum event_type_t event_type)
  {
      struct perf_event *event;
      int is_active = ctx->is_active;
  
      ctx->is_active &= ~event_type;
      if (likely(!ctx->nr_events))
          return;
  
      update_context_time(ctx);
      update_cgrp_time_from_cpuctx(cpuctx);
      if (!ctx->nr_active)
          return;
  
      perf_pmu_disable(ctx->pmu);
      /* (2.3.2.1) 遍历context的高优先级event链表->pinned_groups：
          任务已经被切出，关联的所有event需要停工
       */
      if ((is_active & EVENT_PINNED) && (event_type & EVENT_PINNED)) {
          list_for_each_entry(event, &ctx->pinned_groups, group_entry)
              group_sched_out(event, cpuctx, ctx);
      }
  
      /* (2.3.2.2) 遍历context的低优先级event链表->flexible_groups：
          任务已经被切出，关联的所有event需要停工
       */
      if ((is_active & EVENT_FLEXIBLE) && (event_type & EVENT_FLEXIBLE)) {
          list_for_each_entry(event, &ctx->flexible_groups, group_entry)
              group_sched_out(event, cpuctx, ctx);
      }
      perf_pmu_enable(ctx->pmu);
  }
  
  ||||→
  
  static void
  group_sched_out(struct perf_event *group_event,
          struct perf_cpu_context *cpuctx,
          struct perf_event_context *ctx)
  {
      struct perf_event *event;
      int state = group_event->state;
  
      /* (2.3.2.2.1) 对group leader event进行停工 */
      event_sched_out(group_event, cpuctx, ctx);
  
      /*
       * Schedule out siblings (if any):
       */
      /* (2.3.2.2.2) 对group leader下的子event进行停工 */
      list_for_each_entry(event, &group_event->sibling_list, group_entry)
          event_sched_out(event, cpuctx, ctx);
  
      if (state == PERF_EVENT_STATE_ACTIVE && group_event->attr.exclusive)
          cpuctx->exclusive = ;
  }
  
  |||||→
  
  static void
  event_sched_out(struct perf_event *event,
            struct perf_cpu_context *cpuctx,
            struct perf_event_context *ctx)
  {
      u64 tstamp = perf_event_time(event);
      u64 delta;
  
      WARN_ON_ONCE(event->ctx != ctx);
      lockdep_assert_held(&ctx->lock);
  
      /*
       * An event which could not be activated because of
       * filter mismatch still needs to have its timings
       * maintained, otherwise bogus information is return
       * via read() for time_enabled, time_running:
       */
      if (event->state == PERF_EVENT_STATE_INACTIVE
          && !event_filter_match(event)) {
          delta = tstamp - event->tstamp_stopped;
          event->tstamp_running += delta;
          event->tstamp_stopped = tstamp;
      }
  
      /* (2.3.2.2.1.1) 如果当前event不是开工状态(PERF_EVENT_STATE_ACTIVE)直接返回 */
      if (event->state != PERF_EVENT_STATE_ACTIVE)
          return;
  
      perf_pmu_disable(event->pmu);
  
      event->tstamp_stopped = tstamp;
  
      /* (2.3.2.2.1.2) 调用event的停工函数：pmu->del() */
      event->pmu->del(event, );
      event->oncpu = -;
  
      /* (2.3.2.2.1.3) 状态设置为停工：PERF_EVENT_STATE_INACTIVE */
      event->state = PERF_EVENT_STATE_INACTIVE;
      if (event->pending_disable) {
          event->pending_disable = ;
          event->state = PERF_EVENT_STATE_OFF;
      }
  
      if (!is_software_event(event))
          cpuctx->active_oncpu--;
      if (!--ctx->nr_active)
          perf_event_ctx_deactivate(ctx);
      if (event->attr.freq && event->attr.sample_freq)
          ctx->nr_freq--;
      if (event->attr.exclusive || !cpuctx->active_oncpu)
          cpuctx->exclusive = ;
  
      if (is_orphaned_child(event))
          schedule_orphans_remove(ctx);
  
      perf_pmu_enable(event->pmu);
  }           

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  context_switch() -> finish_task_switch() -> perf_event_task_sched_in():

  static inline void perf_event_task_sched_in(struct task_struct *prev,
                          struct task_struct *task)
  {
      /* (1) 新进程上相关的event需要开工 */
      if (static_key_false(&perf_sched_events.key))
          __perf_event_task_sched_in(prev, task);
  
      if (perf_sw_migrate_enabled() && task->sched_migrated) {
          struct pt_regs *regs = this_cpu_ptr(&__perf_regs[]);
  
          perf_fetch_caller_regs(regs);
          ___perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, , regs, );
          task->sched_migrated = ;
      }
  }
  
  |→
  
  void __perf_event_task_sched_in(struct task_struct *prev,
                  struct task_struct *task)
  {
      struct perf_event_context *ctx;
      int ctxn;
  
      /* (1.1) 遍历task->[perf_event_ctxp]中的context， 
          逐个打开context链接中的每个perf_event
       */
      for_each_task_context_nr(ctxn) {
          ctx = task->perf_event_ctxp[ctxn];
          if (likely(!ctx))
              continue;
  
          perf_event_context_sched_in(ctx, task);
      }
      /*
       * if cgroup events exist on this CPU, then we need
       * to check if we have to switch in PMU state.
       * cgroup event are system-wide mode only
       */
      if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
          perf_cgroup_sched_in(prev, task);
  
      if (atomic_read(&nr_switch_events))
          perf_event_switch(task, prev, true);
  
      if (__this_cpu_read(perf_sched_cb_usages))
          perf_pmu_sched_task(prev, task, true);
  }
  
  ||→
  
  static void perf_event_context_sched_in(struct perf_event_context *ctx,
                      struct task_struct *task)
  {
      struct perf_cpu_context *cpuctx;
  
      cpuctx = __get_cpu_context(ctx);
      if (cpuctx->task_ctx == ctx)
          return;
  
      perf_ctx_lock(cpuctx, ctx);
      perf_pmu_disable(ctx->pmu);
      /*
       * We want to keep the following priority order:
       * cpu pinned (that don't need to move), task pinned,
       * cpu flexible, task flexible.
       */
      /* (1.1.1) 将新task context对应的本cpu维度的ctx->flexible_groups停工
          ctx->pinned_groups这时不会停工，就体现出优先级了？
       */
      cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
  
      /* (1.1.2) 切换本cpu的当前task context指针：cpuctx->task_ctx */
      if (ctx->nr_events)
          cpuctx->task_ctx = ctx;
  
      /* (1.1.3) 将新对应的本cpu维度的ctx->flexible_groups开工 
          将task context对应的ctx->flexible_groups、ctx->pinned_groups开工
       */
      perf_event_sched_in(cpuctx, cpuctx->task_ctx, task);
  
      perf_pmu_enable(ctx->pmu);
      perf_ctx_unlock(cpuctx, ctx);
  }
  
  |||→
  
  static void perf_event_sched_in(struct perf_cpu_context *cpuctx,
                  struct perf_event_context *ctx,
                  struct task_struct *task)
  {
      cpu_ctx_sched_in(cpuctx, EVENT_PINNED, task);
      if (ctx)
          ctx_sched_in(ctx, cpuctx, EVENT_PINNED, task);
      cpu_ctx_sched_in(cpuctx, EVENT_FLEXIBLE, task);
      if (ctx)
          ctx_sched_in(ctx, cpuctx, EVENT_FLEXIBLE, task);
  }
  
  ||||→
  
  static void cpu_ctx_sched_in(struct perf_cpu_context *cpuctx,
                   enum event_type_t event_type,
                   struct task_struct *task)
  {
      struct perf_event_context *ctx = &cpuctx->ctx;
  
      ctx_sched_in(ctx, cpuctx, event_type, task);
  }
  
  |||||→
  
  static void
  ctx_sched_in(struct perf_event_context *ctx,
           struct perf_cpu_context *cpuctx,
           enum event_type_t event_type,
           struct task_struct *task)
  {
      u64 now;
      int is_active = ctx->is_active;
  
      ctx->is_active |= event_type;
      if (likely(!ctx->nr_events))
          return;
  
      now = perf_clock();
      ctx->timestamp = now;
      perf_cgroup_set_timestamp(task, ctx);
      /*
       * First go through the list and put on any pinned groups
       * in order to give them the best chance of going on.
       */
      if (!(is_active & EVENT_PINNED) && (event_type & EVENT_PINNED))
          ctx_pinned_sched_in(ctx, cpuctx);
  
      /* Then walk through the lower prio flexible groups */
      if (!(is_active & EVENT_FLEXIBLE) && (event_type & EVENT_FLEXIBLE))
          ctx_flexible_sched_in(ctx, cpuctx);
  }
  
  ||||||→
  
  static void
  ctx_pinned_sched_in(struct perf_event_context *ctx,
              struct perf_cpu_context *cpuctx)
  {
      struct perf_event *event;
  
      list_for_each_entry(event, &ctx->pinned_groups, group_entry) {
          if (event->state <= PERF_EVENT_STATE_OFF)
              continue;
          if (!event_filter_match(event))
              continue;
  
          /* may need to reset tstamp_enabled */
          if (is_cgroup_event(event))
              perf_cgroup_mark_enabled(event, ctx);
  
          if (group_can_go_on(event, cpuctx, ))
              group_sched_in(event, cpuctx, ctx);
  
          /*
           * If this pinned group hasn't been scheduled,
           * put it in error state.
           */
          if (event->state == PERF_EVENT_STATE_INACTIVE) {
              update_group_times(event);
              event->state = PERF_EVENT_STATE_ERROR;
          }
      }
  }
  
  |||||||→
  
  static int
  group_sched_in(struct perf_event *group_event,
             struct perf_cpu_context *cpuctx,
             struct perf_event_context *ctx)
  {
      struct perf_event *event, *partial_group = NULL;
      struct pmu *pmu = ctx->pmu;
      u64 now = ctx->time;
      bool simulate = false;
  
      if (group_event->state == PERF_EVENT_STATE_OFF)
          return ;
  
      pmu->start_txn(pmu, PERF_PMU_TXN_ADD);
  
      if (event_sched_in(group_event, cpuctx, ctx)) {
          pmu->cancel_txn(pmu);
          perf_mux_hrtimer_restart(cpuctx);
          return -EAGAIN;
      }
  
      /*
       * Schedule in siblings as one group (if any):
       */
      list_for_each_entry(event, &group_event->sibling_list, group_entry) {
          if (event_sched_in(event, cpuctx, ctx)) {
              partial_group = event;
              goto group_error;
          }
      }
  
      if (!pmu->commit_txn(pmu))
          return ;
  
  group_error:
      /*
       * Groups can be scheduled in as one unit only, so undo any
       * partial group before returning:
       * The events up to the failed event are scheduled out normally,
       * tstamp_stopped will be updated.
       *
       * The failed events and the remaining siblings need to have
       * their timings updated as if they had gone thru event_sched_in()
       * and event_sched_out(). This is required to get consistent timings
       * across the group. This also takes care of the case where the group
       * could never be scheduled by ensuring tstamp_stopped is set to mark
       * the time the event was actually stopped, such that time delta
       * calculation in update_event_times() is correct.
       */
      list_for_each_entry(event, &group_event->sibling_list, group_entry) {
          if (event == partial_group)
              simulate = true;
  
          if (simulate) {
              event->tstamp_running += now - event->tstamp_stopped;
              event->tstamp_stopped = now;
          } else {
              event_sched_out(event, cpuctx, ctx);
          }
      }
      event_sched_out(group_event, cpuctx, ctx);
  
      pmu->cancel_txn(pmu);
  
      perf_mux_hrtimer_restart(cpuctx);
  
      return -EAGAIN;
  }
  
  ||||||||→
  
  static int
  event_sched_in(struct perf_event *event,
           struct perf_cpu_context *cpuctx,
           struct perf_event_context *ctx)
  {
      u64 tstamp = perf_event_time(event);
      int ret = ;
  
      lockdep_assert_held(&ctx->lock);
  
      if (event->state <= PERF_EVENT_STATE_OFF)
          return ;
  
      WRITE_ONCE(event->oncpu, smp_processor_id());
      /*
       * Order event::oncpu write to happen before the ACTIVE state
       * is visible.
       */
      /* 设置event为开工状态：PERF_EVENT_STATE_ACTIVE */
      smp_wmb();
      WRITE_ONCE(event->state, PERF_EVENT_STATE_ACTIVE);
  
      /*
       * Unthrottle events, since we scheduled we might have missed several
       * ticks already, also for a heavily scheduling task there is little
       * guarantee it'll get a tick in a timely manner.
       */
      if (unlikely(event->hw.interrupts == MAX_INTERRUPTS)) {
          perf_log_throttle(event, );
          event->hw.interrupts = ;
      }
  
      /*
       * The new state must be visible before we turn it on in the hardware:
       */
      smp_wmb();
  
      perf_pmu_disable(event->pmu);
  
      perf_set_shadow_time(event, ctx, tstamp);
  
      perf_log_itrace_start(event);
  
      /* 调用event的开工函数：pmu->add() */
      if (event->pmu->add(event, PERF_EF_START)) {
          event->state = PERF_EVENT_STATE_INACTIVE;
          event->oncpu = -;
          ret = -EAGAIN;
          goto out;
      }
  
      event->tstamp_running += tstamp - event->tstamp_stopped;
  
      if (!is_software_event(event))
          cpuctx->active_oncpu++;
      if (!ctx->nr_active++)
          perf_event_ctx_activate(ctx);
      if (event->attr.freq && event->attr.sample_freq)
          ctx->nr_freq++;
  
      if (event->attr.exclusive)
          cpuctx->exclusive = ;
  
      if (is_orphaned_child(event))
          schedule_orphans_remove(ctx);
  
  out:
      perf_pmu_enable(event->pmu);
  
      return ret;
  }           

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  如果是pid>0，cpu!=-1的event，在sched_in的时候会调用event_filter_match()判断当前cpu和event绑定的cpu(event->cpu)是否一致，只有符合条件event才能被使能：

  ctx_pinned_sched_in()/ctx_flexible_sched_in() -> event_filter_match()：

  static inline int
  event_filter_match(struct perf_event *event)
  {
      /* 如果没有指定cpu：event->cpu == -1 
          或者event绑定cpu和当前cpu一致：event->cpu == smp_processor_id()
          cgroup的filter条件：perf_cgroup_match(event)
          pmu的filter条件：pmu_filter_match(event)
          上述条件符合的情况下，才能event才能被使能
       */
      return (event->cpu == - || event->cpu == smp_processor_id())
          && perf_cgroup_match(event) && pmu_filter_match(event);
  }           

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  2.3、cgroup

  暂不分析

  3、perf_ioctl

  通过perf_event_open()系统调用获得perf_event对应的fd后，可以通过操作fd的ioctl命令来配置perf_event。

  static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
  {
      struct perf_event *event = file->private_data;
      struct perf_event_context *ctx;
      long ret;
  
      ctx = perf_event_ctx_lock(event);
      ret = _perf_ioctl(event, cmd, arg);
      perf_event_ctx_unlock(event, ctx);
  
      return ret;
  }
  
  ↓
  
  static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned long arg)
  {
      void (*func)(struct perf_event *);
      u32 flags = arg;
  
      switch (cmd) {
      case PERF_EVENT_IOC_ENABLE:
          func = _perf_event_enable;
          break;
      case PERF_EVENT_IOC_DISABLE:
          func = _perf_event_disable;
          break;
      case PERF_EVENT_IOC_RESET:
          func = _perf_event_reset;
          break;
  
      case PERF_EVENT_IOC_REFRESH:
          return _perf_event_refresh(event, arg);
  
      case PERF_EVENT_IOC_PERIOD:
          return perf_event_period(event, (u64 __user *)arg);
  
      case PERF_EVENT_IOC_ID:
      {
          u64 id = primary_event_id(event);
  
          if (copy_to_user((void __user *)arg, &id, sizeof(id)))
              return -EFAULT;
          return ;
      }
  
      case PERF_EVENT_IOC_SET_OUTPUT:
      {
          int ret;
          if (arg != -) {
              struct perf_event *output_event;
              struct fd output;
              ret = perf_fget_light(arg, &output);
              if (ret)
                  return ret;
              output_event = output.file->private_data;
              ret = perf_event_set_output(event, output_event);
              fdput(output);
          } else {
              ret = perf_event_set_output(event, NULL);
          }
          return ret;
      }
  
      case PERF_EVENT_IOC_SET_FILTER:
          return perf_event_set_filter(event, (void __user *)arg);
  
      case PERF_EVENT_IOC_SET_BPF:
          return perf_event_set_bpf_prog(event, arg);
  
      case PERF_EVENT_IOC_SET_DRV_CONFIGS:
          return perf_event_drv_configs(event, (void __user *)arg);
  
      default:
          return -ENOTTY;
      }
  
      if (flags & PERF_IOC_FLAG_GROUP)
          perf_event_for_each(event, func);
      else
          perf_event_for_each_child(event, func);
  
      return ;
  }           

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  3.1、_perf_event_enable

  简单分析一下enable命令。该命令的主要作用就是把处于PERF_EVENT_STATE_OFF状态的event设置成PERF_EVENT_STATE_INACTIVE，以便该event能参与到perf sched当中去。

  static void _perf_event_enable(struct perf_event *event)
  {
      struct perf_event_context *ctx = event->ctx;
      struct task_struct *task = ctx->task;
  
      if (!task) {
          /*
           * Enable the event on the cpu that it's on
           */
          cpu_function_call(event->cpu, __perf_event_enable, event);
          return;
      }
  
      raw_spin_lock_irq(&ctx->lock);
      if (event->state >= PERF_EVENT_STATE_INACTIVE)
          goto out;
  
      /*
       * If the event is in error state, clear that first.
       * That way, if we see the event in error state below, we
       * know that it has gone back into error state, as distinct
       * from the task having been scheduled away before the
       * cross-call arrived.
       */
      if (event->state == PERF_EVENT_STATE_ERROR)
          event->state = PERF_EVENT_STATE_OFF;
  
  retry:
      if (!ctx->is_active) {
          __perf_event_mark_enabled(event);
          goto out;
      }
  
      raw_spin_unlock_irq(&ctx->lock);
  
      if (!task_function_call(task, __perf_event_enable, event))
          return;
  
      raw_spin_lock_irq(&ctx->lock);
  
      /*
       * If the context is active and the event is still off,
       * we need to retry the cross-call.
       */
      if (ctx->is_active && event->state == PERF_EVENT_STATE_OFF) {
          /*
           * task could have been flipped by a concurrent
           * perf_event_context_sched_out()
           */
          task = ctx->task;
          goto retry;
      }
  
  out:
      raw_spin_unlock_irq(&ctx->lock);
  }
  
  ↓
  
  static void __perf_event_mark_enabled(struct perf_event *event)
  {
      struct perf_event *sub;
      u64 tstamp = perf_event_time(event);
  
      event->state = PERF_EVENT_STATE_INACTIVE;
      event->tstamp_enabled = tstamp - event->total_time_enabled;
      list_for_each_entry(sub, &event->sibling_list, group_entry) {
          if (sub->state >= PERF_EVENT_STATE_INACTIVE)
              sub->tstamp_enabled = tstamp - sub->total_time_enabled;
      }
  }           

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为什么有了ftrace又出来一个perf？因为ftrace只管抓trace数据并没有分析，perf在trace数据分析方面做出了很多成果。