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

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Date: Wed, 28 Feb 2024 17:16:25 -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 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.

> than or equal to the monitoring period.  Maybe we can try with
> say, 90% of period but we can still miss something.
> 
> And collecting backtrace of other task would be racy as the it
> may not contend anymore.
> 
> > With that we wouldn't miss that info, however incomplete it is and the
> > user would try running again, perhaps for a longer time, or start
> > monitoring before the observed workload starts, etc.
> 
> Yeah, it can be useful.  Let me think about it more.
> 
> >
> > Anyway:
> >
> > Reviwed-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> 
> Thanks for your review!
> Namhyung
>