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

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Date: Thu, 16 May 2024 10:34:06 -0700
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, Changbin Du <changbin.du@...wei.com>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: [PATCH v1 0/3] Use BPF filters for a "perf top -u" workaround

On Wed, May 15, 2024 at 10:04 PM Ian Rogers <irogers@...gle.com> wrote:
>
> On Wed, May 15, 2024 at 9:20 PM Ian Rogers <irogers@...gle.com> 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.

Some other thoughts:

 - It is kind of annoying with the --filter option (either on top or
record) that there first needs to be an event to filter on. It'd be
nice if we could just filter the default event.

 - Should "perf top --uid=1234" be removed or turned into  an alias
for '--filter "uid == $(id -u)"' given the --uid option generally
doesn't work?

 - What should happen to the perf top --pid and --tid options, should
they be filters? Should they fallback on /proc scanning if there
aren't sufficient BPF permissions? The plumbing for that is going to
be messy.

 - There should probably be a way to filter on cgroups.

 - Does the user care that there are 3 kinds of filter that will work
differently? Could we break them apart to make it more explicit, I may
want tracepoint events with a BPF filter. How can we ensure 1 syntax
for the 3 kinds of filter.

 - Filtering on register values could be potentially interesting, for
example, sampling on memcpy-s where the length is over a threshold. We
have a register capture test:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/shell/record.sh#n81
Perhaps the filter could look something like 'perf record -g -e
mem:$ADDRESS_OF_MEMCPY:x --filter "reg:rdx > 1024"' -  this makes me
think we need to make a more convenient way to specify memory
addresses as symbols.

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
> >