linux-kernel - Re: [PATCH v2] perf lock contention: Account contending locks too

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Message-ID: <ZeC9ki-4SGa-iU0C@x1>
Date: Thu, 29 Feb 2024 14:23:30 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org, Song Liu <song@...nel.org>,
	bpf@...r.kernel.org
Subject: Re: [PATCH v2] perf lock contention: Account contending locks too

On Wed, Feb 28, 2024 at 01:19:12PM -0800, Namhyung Kim wrote:
> On Wed, Feb 28, 2024 at 12:16 PM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > On Wed, Feb 28, 2024 at 12:01:55PM -0800, Namhyung Kim wrote:
> > > On Wed, Feb 28, 2024 at 4:22 AM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > > > On Tue, Feb 27, 2024 at 09:33:35PM -0800, Namhyung Kim wrote:
> > > > > Currently it accounts the contention using delta between timestamps in
> > > > > lock:contention_begin and lock:contention_end tracepoints.  But it means
> > > > > the lock should see the both events during the monitoring period.

> > > > > Actually there are 4 cases that happen with the monitoring:

> > > > >                 monitoring period
> > > > >             /                       \
> > > > >             |                       |
> > > > >  1:  B------+-----------------------+--------E
> > > > >  2:    B----+-------------E         |
> > > > >  3:         |           B-----------+----E
> > > > >  4:         |     B-------------E   |
> > > > >             |                       |
> > > > >             t0                      t1

> > > > > where B and E mean contention BEGIN and END, respectively.  So it only
> > > > > accounts the case 4 for now.  It seems there's no way to handle the case
> > > > > 1.  The case 2 might be handled if it saved the timestamp (t0), but it
> > > > > lacks the information from the B notably the flags which shows the lock
> > > > > types.  Also it could be a nested lock which it currently ignores.  So
> > > > > I think we should ignore the case 2.

> > > > Perhaps have a separate output listing locks that were found to be with
> > > > at least tE - t0 time, with perhaps a backtrace at that END time?

> > > Do you mean long contentions in case 3?  I'm not sure what do
> > > you mean by tE, but they started after t0 so cannot be greater

> > case 2

> >                 monitoring period
> >             /                       \
> >             |                       |
> >  2:    B----+-------------E         |
> >             |             |         |
> >             t0            tE        t1
> >
> > We get a notification for event E, right? We don´t have one for B,
> > because it happened before we were monitoring.
> 
> Ah, ok.  But there should be too many events in case 2 and
> I don't think users want to see them all.  And they don't have

So maybe a summary, something like:

  N locks that were locked before this session started have been
  released, no further info besides this histogram of in-session
  durations:

    0-N units of time: ++
  N+1-M units of time: ++++
    ...

> flags.  But maybe we can update the flag when it sees exactly
> the same callstack later.

  The callstack, if going all the way to userspace may have the workload
targeted in the command line ( some pid, tid, CPU, etc) and thus would
point for things the user probably is interested than some other lock
that may affect it but indirectly.

- Arnaldo