linux-kernel - Re: [PATCH] perf record: Synthesize cgroup events only if needed

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Message-ID: <CAM9d7cgBbSEy9r+Kh5-06bTii5GED81gdtYiJ7AOkQKo9NbVhw@mail.gmail.com>
Date:   Mon, 30 Nov 2020 22:26:58 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH] perf record: Synthesize cgroup events only if needed

Hi Jiri,

On Sat, Nov 28, 2020 at 12:46 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Fri, Nov 27, 2020 at 02:43:56PM +0900, Namhyung Kim wrote:
> > It didn't check the tool->cgroup_events bit which is set when
> > the --all-cgroups option is given.  Without it, samples will not have
> > cgroup info so no reason to synthesize.
> >
> > We can check the PERF_RECORD_CGROUP records after running perf record
> > *WITHOUT* the --all-cgroups option:
> >
> > Before:
> >   $ perf report -D | grep CGROUP
> >   0 0 0x8430 [0x38]: PERF_RECORD_CGROUP cgroup: 1 /
> >           CGROUP events:          1
> >           CGROUP events:          0
> >           CGROUP events:          0
> >
> > After:
> >   $ perf report -D | grep CGROUP
> >           CGROUP events:          0
> >           CGROUP events:          0
> >           CGROUP events:          0
> >
> > Fixes: 8fb4b67939e16 ("perf record: Add --all-cgroups option")
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  tools/perf/util/synthetic-events.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> > index 8a23391558cf..d9c624377da7 100644
> > --- a/tools/perf/util/synthetic-events.c
> > +++ b/tools/perf/util/synthetic-events.c
> > @@ -563,6 +563,9 @@ int perf_event__synthesize_cgroups(struct perf_tool *tool,
> >       char cgrp_root[PATH_MAX];
> >       size_t mount_len;  /* length of mount point in the path */
> >
> > +     if (!tool || !tool->cgroup_events)
> > +             return 0;
>
> can !tool actually happen here? or it's just being extra cautious

Actually I just copied the namespace code. :).
It's only called from perf record and top, and neither of them pass NULL.

>
> Acked-by: Jiri Olsa <jolsa@...hat.com>

Thanks,
Namhyung