linux-kernel - Re: [PATCHSET 0/6] perf lock: Add contention subcommand (v1)

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Message-ID: <CAM9d7cjhVwRhVTiJM6hX8Y75SGJhpwYgMtSc_9PB0tzysJjVgQ@mail.gmail.com>
Date:   Wed, 3 Aug 2022 21:55:17 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCHSET 0/6] perf lock: Add contention subcommand (v1)

Hi Ingo,

On Wed, Aug 3, 2022 at 2:51 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Namhyung Kim <namhyung@...nel.org> wrote:
>
> > Hello,
> >
> > It's to add a new subcommand 'contention' (shortly 'con') to perf lock.
> >
> > The new subcommand is to handle the new lock:contention_{begin,end}
> > tracepoints and shows lock type and caller address like below:
> >
> >   $ perf lock contention
> >    contended   total wait     max wait     avg wait         type   caller
> >
> >          238      1.41 ms     29.20 us      5.94 us     spinlock   update_blocked_averages+0x4c
> >            1    902.08 us    902.08 us    902.08 us      rwsem:R   do_user_addr_fault+0x1dd
> >           81    330.30 us     17.24 us      4.08 us     spinlock   _nohz_idle_balance+0x172
> >            2     89.54 us     61.26 us     44.77 us     spinlock   do_anonymous_page+0x16d
> >           24     78.36 us     12.27 us      3.27 us        mutex   pipe_read+0x56
> >            2     71.58 us     59.56 us     35.79 us     spinlock   __handle_mm_fault+0x6aa
> >            6     25.68 us      6.89 us      4.28 us     spinlock   do_idle+0x28d
> >            1     18.46 us     18.46 us     18.46 us      rtmutex   exec_fw_cmd+0x21b
> >            3     15.25 us      6.26 us      5.08 us     spinlock   tick_do_update_jiffies64+0x2c
> >    ...
>
> Wouldn't it also be useful to display a lock contention percentage value,
> the ratio of fastpath vs. contended/wait events?
>
> That's usually the first-approximation metric to see how contended
> different locks are, and the average wait time quantifies it.

Yeah, that'd be nice to have.  But it requires some action in the fast path
which I don't want because I'd like to use this in production.  So these
new tracepoints were added only in the slow path.

Instead, I think we can display the ratio of (total) contended time vs.
wall clock time.  What do you think?

Thanks,
Namhyung