Re: [PATCH v7 0/6] perf record: Track sideband events for all CPUs when tracing selected CPUs

[Ravi Bangoria]

On 26-Aug-23 8:56 AM, Yang Jihong wrote:
> User space tasks can migrate between CPUs, track sideband events for all
> CPUs.

I've tested this series with simple test program and it mostly works fine.

Tested-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>

Since we are recording sideband data on all cpus, perf will endup recording
lots of unnecessary data, esp. on large systems. E.g. on a 128 cpu system:

Without patch:
  $ sudo ./perf record -C 10 -o perf.data.without -- sleep 10
  $ du -d1 -ha ./perf.data.without
  3.0M    ./perf.data.without

  $ sudo ./perf report --stats -i perf.data.without
  Aggregated stats:
             TOTAL events:      47011
              MMAP events:         51  ( 0.1%)
              COMM events:       1549  ( 3.3%)
              EXIT events:        105  ( 0.2%)
              FORK events:       1544  ( 3.3%)
            SAMPLE events:      38226  (81.3%)
             MMAP2 events:       5485  (11.7%)
             ...
  cycles:P stats:
            SAMPLE events:      38226

With patch:
  $ sudo ./perf record -C 10 -o perf.data.with -- sleep 10
  $ du -d1 -ha ./perf.data.with
  15M     ./perf.data.with

  $ sudo ./perf report --stats -i perf.data.with
  Aggregated stats:
             TOTAL events:     160164
              MMAP events:         51  ( 0.0%)
              COMM events:      12879  ( 8.0%)
              EXIT events:      11192  ( 7.0%)
              FORK events:      12669  ( 7.9%)
            SAMPLE events:      38464  (24.0%)
             MMAP2 events:      84844  (53.0%)
             ...
  cycles:P stats:
            SAMPLE events:      38464

Number of actual samples are same ~38K. However, the perf.data file is 5x
bigger because of additional sideband data.

I'm pretty sure we don't need most of those additional data. So, thinking
loud, should we post-process perf.data file and filter out unnecessary data?

Thanks,
Ravi