Re: Getting PMU stats on specific syscalls

[Leo Yan <leo.yan@linaro.org>]

On Tue, Jan 09, 2024 at 05:01:59PM -0800, Kees Cook wrote:

[...]

> >> I can't figure out how to limit collection to only time spent during the
> >> syscall I'm interested in. I tried building synthetic events, but that
> >> doesn't seem to work...
> >
> >Hmm.. ok.  it seems you want to use PMU only for a syscall.
> >I don't think perf supports that for now, but maybe it's possible
> >to attach custom BPF programs to read perf counters at syscall
> >enter and exit and to save the diff in a map.
> 
> Does BPF have access to the PMU counters? I don't know how to even approach writing that though. :P

An alternative is to use ftrace's function_graph tracer to measure a
function duration, e.g.

      # cd /sys/kernel/debug/tracing
      # echo syscall_function_name > set_graph_function
      # echo function_graph > current_tracer

      do some test

      # cat trace

I think you even can access the PMU counter for measurement duration
in user space - though you might cannot directly measure a specific
syscall, see:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c#n127

Thanks,
Leo