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system wide perf_event_open

Time:07-21

Using the perf cli, we can measure system wide counters :

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

     4 247 009 923      cpu-cycles                                                  

       2,183469627 seconds time elapsed

On the perf_event_open manual, I assume it would be te equivalent of monitoring any pid (pid == -1) on any cpu (cpu == -1) but it doesn't seem to be possible :

   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.

What is the workaround here?

CodePudding user response:

It looks like none of those options aggregate counts for you, they either count on one core, or virtualize the counters across context switches.

If you look at what system-wide perf stat -a does (e.g. with strace -f perf stat), you can see it calls perf_event_open once per event per core. It has to add up the counts for an event across cores; the system-call API won't do that for you.

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