perf_event_open on Android

[hosunnyc]

Hi, I was hoping to gain some help on here regarding perf_event_open even though this is for the Android NDK. I wanted to make my own executable (similar to simpleperf), using the examples on ARM | Use perf_event_open for counting. It has two examples:

1. Getting values of a single counter
2. Getting values of multiple counters at once

The single counter example worked fine on the Android device, but I couldn't get the multiple counter example working on an Android. The same code for multiple counters does seem to work on an Ubuntu 22.04. I was wondering why the same code might not work for Android and if you might have insights as to why that is. To be more precise, the code stops at this part:

  // Read the group of counters and print result
  read(fd[0], &counter_results, sizeof(struct read_format));
  printf("Num events captured: %" PRIu64 "\n", counter_results.nr);
  for(int i = 0; i < counter_results.nr; i++) {
    for(int j = 0; j < TOTAL_EVENTS ;j++){
      if(counter_results.values[i].id == id[j]){
        pe_val[i] = counter_results.values[i].value;
      }
    }
  }

I think the issue might lie in the defined read_format struct, but wasn't able to find an issue based on the perf_event_open manual. The device I am using is a Pixel 7 Pro with Android 13. And I compiled using 27.2.12479018/build/cmake/android.toolchain.cmake" -DANDROID_ABI="arm64-v8a" -DANDROID_PLATFORM=android-31.

Tagged @yabinc, since @yabinc typically works on simpleperf. Any help is appreciated!

[yabinc]

The multiple counter example is requesting 6 hardware counters at the same time. It will fail on Pixel 7. Because Pixel 7 only leaves 3 free counters to userspace. You may try it on a newer device, like Pixel 9, which has 18 free counters.
The free counters can be shown by running simpleperf stat --print-hw-counter.

[hosunnyc]

I see. I know that the Pixel 7 CPUs (Cortex X1, A78, and A55) allow monitoring of 6 simultaneous PMCs and I have also succeeded doing so with simpleperf when running under sudo/root access with --use-devfreq-counters. Could you tell me what additional steps I need to achieve this, given that my executable will run under root access/sudo? Thank you.

[yabinc]

In that case, you may copy simpleperf code implementing --use-devfreq-counters, maybe setting it directly in adb root. The code is in https://cs.android.com/android/platform/superproject/main/+/main:system/extras/simpleperf/cmd_stat.cpp;drc=fdd0073f4a5824894d653b26aad4cb008ec23867;l=314

[hosunnyc]

Thank you. I checked that out and it does increase it to 6PMCs. Unfortunately, the issue seems to be something else when it comes to reading the values with PERF_FORMAT_GROUP. I'm assuming it might be the read format:

struct read_format {
  uint64_t nr;
  struct {
    uint64_t value;
    uint64_t id;
  } values[TOTAL_EVENTS];
};

Could you maybe let me know how or the format simpleperf reads the counters when PERF_FORMAT_GROUP is enabled? Just so I can get an idea where the issue might be.

[yabinc]

simpleperf stat command doesn't use PERF_FORMAT_GROUP. Even when grouping events, it reads events separately for each perf_event, as shown in https://cs.android.com/android/platform/superproject/main/+/main:system/extras/simpleperf/event_selection_set.cpp;l=835;drc=8fae81cc40d98877629b28a16fcc72b736423bdb

I am not saying PERF_FORMAT_GROUP isn't working. I also don't think there is anything special on Android for this. Just the function isn't used by simpleperf.