系统宽perf_event_open

Question

使用perf，我们可以测量系统范围的计数器：

$ sudo perf stat -e cpu-cycles
^C
 Performance counter stats for 'system wide':

     4 247 009 923      cpu-cycles                                                  

       2,183469627 seconds time elapsed

在perf_event_open 手册上，我假设它相当于监视任何cpu (cpu == -1)上的任何pid (pid == -1)，但这似乎是不可能的：

   Arguments
   The pid and cpu arguments allow specifying which process and CPU
   to monitor:

   pid == 0 and cpu == -1
          This measures the calling process/thread on any CPU.

   pid == 0 and cpu >= 0
          This measures the calling process/thread only when running
          on the specified CPU.

   pid > 0 and cpu == -1
          This measures the specified process/thread on any CPU.

   pid > 0 and cpu >= 0
          This measures the specified process/thread only when
          running on the specified CPU.

   pid == -1 and cpu >= 0
          This measures all processes/threads on the specified CPU.
          This requires CAP_PERFMON (since Linux 5.8) or
          CAP_SYS_ADMIN capability or a
          /proc/sys/kernel/perf_event_paranoid value of less than 1.

   pid == -1 and cpu == -1
          This setting is invalid and will return an error.

这里的解决办法是什么？

Answer 1

看起来，这些选项中没有一个是聚合计数的，它们要么依赖于一个核心，要么跨上下文开关虚拟化计数器。

如果您查看系统范围内的perf stat -a所做的事情(例如使用strace -f perf stat)，您可以看到它在每个核心事件中调用perf_event_open一次。它必须将跨核事件的计数加起来；系统调用API不会为您做到这一点。