linux-kernel - Re: [PATCH] perf record: enable multiplexing scaling via -R

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Date:   Fri, 1 Sep 2017 10:31:43 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "mingo@...e.hu" <mingo@...e.hu>, "Liang, Kan" <kan.liang@...el.com>
Subject: Re: [PATCH] perf record: enable multiplexing scaling via -R

On Fri, Sep 01, 2017 at 01:21:03AM -0700, Stephane Eranian wrote:

SNIP

> >> I am also surprised to see that perf record keep inherit=1 in
> >> system-wide mode. I don't think this
> >> is relavant in this mode. But the kernel this fails in this case,
> >> which I think is a bug. In system-wide
> >> mode, the attr-.no_inherit should be ignored. We can fix perf record
> >> to avoid this in system-wide.
> >>
> >> The cmdline above works for both per-thread and system-wide modes.
> >>
> >> So I think we do not need my patch or variations thereof, everything
> >> is there, though a bit difficult
> >> to combine.
> >
> > hum, how about we introduce new modifier to attach timing info, like:
> >   $ perf record -e cycles:T ....
> >
> > modifiers might be scares resource, but we don't add them every day,
> > and this requirement looks generic
> >
> It is not just a matter of modifier, you need to have the kernel
> record just what you want.
> AFAIK, the only way for the the kernel to record timings on sampling events
> is to force PERF_SAMPLE_READ. So the T modifier would have to also set
> that format,
> at which point, I wonder how useful it is compared to S.

well :S is meant to be used for leader sampling,
so in addition to what you described it also
disables sampling on all other group members,
which I don't think you want

> 
> Alternatively, we could improve the kernel to support recording timing with
> PERF_SAMPLE_TIMINGS as a pair of u64 to represent time_enabled, time_running.
> That would avoid the whole PERF_SAMPLE_READ and the extra u64 it records.
> Recording the value of the sampling event is not very useful because
> it keeps being
> reset for each period.

sounds good.. or if we mind taking another bit out of the sample_type
for this we could have new read_format bit PERF_FORMAT_NO_COUNT, which
would omit the count values

jirka