linux-kernel - Re: [PATCH v3 1/3] perf lock contention: Clear lock addr after use

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Date:   Thu, 26 Oct 2023 10:11:12 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Song Liu <song@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>, bpf@...r.kernel.org,
        linux-perf-users@...r.kernel.org, Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/3] perf lock contention: Clear lock addr after use

On Fri, 20 Oct 2023 13:47:39 -0700, Namhyung Kim 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.
> 
> [...]

Applied to perf-tools-next, thanks!