linux-kernel - Re: [RFC/PATCH] perf report: Support latency profiling in system-wide mode

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Message-ID: <aBm1x2as1fraHXHz@google.com>
Date: Tue, 6 May 2025 00:09:59 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.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, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [RFC/PATCH] perf report: Support latency profiling in
 system-wide mode

On Tue, May 06, 2025 at 08:46:20AM +0200, Dmitry Vyukov wrote:
> On Tue, 6 May 2025 at 08:43, Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Tue, May 06, 2025 at 07:55:25AM +0200, Dmitry Vyukov wrote:
> > > Where does the patch check that this mode is used only for system-wide profiles?
> > > Is it that PERF_SAMPLE_CPU present only for system-wide profiles?
> >
> > Basically yes, but you can use --sample-cpu to add it.
> 
> Are you sure? --sample-cpu seems to work for non-system-wide profiles too.

Yep, that's why I said "Basically".  So it's not 100% guarantee.

We may disable latency column by default in this case and show warning
if it's requested.  Or we may add a new attribute to emit sched-switch
records only for idle tasks and enable the latency report only if the
data has sched-switch records.

What do you think?

Thanks,
Namhyung

> 
> > In util/evsel.c::evsel__config():
> >
> >         if (target__has_cpu(&opts->target) || opts->sample_cpu)
> >                 evsel__set_sample_bit(evsel, CPU);