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

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Message-ID: <CAM9d7cg-M_8V0O2rv_gx+1u=axpRmCp4XcBkkqsiGmDgeU2xZw@mail.gmail.com>
Date: Wed, 28 Feb 2024 13:19:12 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...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 12:16 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
flags.  But maybe we can update the flag when it sees exactly
the same callstack later.

Thanks,
Namhyung