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From:   Ian Rogers <irogers@google.com>
Date:   Tue, 24 Oct 2023 21:13:00 -0700
Message-ID: <CAP-5=fWd440mN-tuL-h-9HkeDNcWK8V0jOqBZjXwFb7dhuaWjg@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] perf lock contention: Clear lock addr after use
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-perf-users@vger.kernel.org, Song Liu <song@kernel.org>,
        Hao Luo <haoluo@google.com>, bpf@vger.kernel.org
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On Fri, Oct 20, 2023 at 1:47=E2=80=AFPM Namhyung Kim <namhyung@kernel.org> =
wrote:
>
> It checks the current lock to calculated the delta of contention time.
> The address is saved in the tstamp map which is allocated at begining of
> contention and released at end of contention.
>
> But it's possible for bpf_map_delete_elem() to fail.  In that case, the
> element in the tstamp map kept for the current lock and it makes the
> next contention for the same lock tracked incorrectly.  Specificially
> the next contention begin will see the existing element for the task and
> it'd just return.  Then the next contention end will see the element and
> calculate the time using the timestamp for the previous begin.
>
> This can result in a large value for two small contentions happened from
> time to time.  Let's clear the lock address so that it can be updated
> next time even if the bpf_map_delete_elem() failed.
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

> ---
>  tools/perf/util/bpf_skel/lock_contention.bpf.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/=
util/bpf_skel/lock_contention.bpf.c
> index 4900a5dfb4a4..b11179452e19 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -389,6 +389,7 @@ int contention_end(u64 *ctx)
>
>         duration =3D bpf_ktime_get_ns() - pelem->timestamp;
>         if ((__s64)duration < 0) {
> +               pelem->lock =3D 0;
>                 bpf_map_delete_elem(&tstamp, &pid);
>                 __sync_fetch_and_add(&time_fail, 1);
>                 return 0;
> @@ -422,6 +423,7 @@ int contention_end(u64 *ctx)
>         data =3D bpf_map_lookup_elem(&lock_stat, &key);
>         if (!data) {
>                 if (data_map_full) {
> +                       pelem->lock =3D 0;
>                         bpf_map_delete_elem(&tstamp, &pid);
>                         __sync_fetch_and_add(&data_fail, 1);
>                         return 0;
> @@ -445,6 +447,7 @@ int contention_end(u64 *ctx)
>                                 data_map_full =3D 1;
>                         __sync_fetch_and_add(&data_fail, 1);
>                 }
> +               pelem->lock =3D 0;
>                 bpf_map_delete_elem(&tstamp, &pid);
>                 return 0;
>         }
> @@ -458,6 +461,7 @@ int contention_end(u64 *ctx)
>         if (data->min_time > duration)
>                 data->min_time =3D duration;
>
> +       pelem->lock =3D 0;
>         bpf_map_delete_elem(&tstamp, &pid);
>         return 0;
>  }
> --
> 2.42.0.655.g421f12c284-goog
>