Re: [PATCH v1 0/3] Use BPF filters for a "perf top -u" workaround

[Ian Rogers]

On Wed, May 15, 2024 at 10:04 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Wed, May 15, 2024 at 9:20 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > Allow uid and gid to be terms in BPF filters by first breaking the
> > connection between filter terms and PERF_SAMPLE_xx values. Calculate
> > the uid and gid using the bpf_get_current_uid_gid helper, rather than
> > from a value in the sample. Allow filters to be passed to perf top, this allows:
> >
> > $ perf top -e cycles:P --filter "uid == $(id -u)"
> >
> > to work as a "perf top -u" workaround, as "perf top -u" usually fails
> > due to processes/threads terminating between the /proc scan and the
> > perf_event_open.
>
> Fwiw, something I noticed playing around with this (my workload was
> `perf test -w noploop 100000` as different users) is that old samples
> appeared to linger around making terminated processes still appear in
> the top list. My guess is that there aren't other samples showing up
> and pushing the old sample events out of the ring buffers due to the
> filter. This can look quite odd and I don't know if we have a way to
> improve upon it, flush the ring buffers, histograms, etc. It appears
> to be a latent `perf top` issue that you could encounter on other low
> frequency events, but I thought I'd mention it anyway.

Oh, this is expected "perf top" behavior and "-z" fixes it:
```
$ man perf-top
..
       -z, --zero
          Zero history across display updates.
...
```
Why isn't "-z" the default? It would more naturally align with the
behavior of "top". I'll send a patch.

Thanks,
Ian

> Thanks,
> Ian
>
> > Ian Rogers (3):
> >   perf bpf filter: Give terms their own enum
> >   perf bpf filter: Add uid and gid terms
> >   perf top: Allow filters on events
> >
> >  tools/perf/Documentation/perf-record.txt     |  2 +-
> >  tools/perf/Documentation/perf-top.txt        |  4 ++
> >  tools/perf/builtin-top.c                     |  9 +++
> >  tools/perf/util/bpf-filter.c                 | 55 ++++++++++++----
> >  tools/perf/util/bpf-filter.h                 |  5 +-
> >  tools/perf/util/bpf-filter.l                 | 66 +++++++++----------
> >  tools/perf/util/bpf-filter.y                 |  7 +-
> >  tools/perf/util/bpf_skel/sample-filter.h     | 27 +++++++-
> >  tools/perf/util/bpf_skel/sample_filter.bpf.c | 67 +++++++++++++++-----
> >  9 files changed, 172 insertions(+), 70 deletions(-)
> >
> > --
> > 2.45.0.rc1.225.g2a3ae87e7f-goog
> >