linux-kernel - Re: [PATCH 2/9] perf: Deny optimized switch for events read by PERF_SAMPLE

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Message-ID: <20140908163933.GI17728@krava.brq.redhat.com>
Date:	Mon, 8 Sep 2014 18:39:33 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Jen-Cheng(Tommy) Huang" <tommy24@...ech.edu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/9] perf: Deny optimized switch for events read by
 PERF_SAMPLE_READ

On Mon, Sep 08, 2014 at 12:11:19PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 08, 2014 at 12:00:19PM +0200, Jiri Olsa wrote:
> 
> > > no.. so the value of the counter is the sum of all the inherited events.
> > > It doesn't matter if you flip it or not the sum is not affected.
> > > 
> > > PERF_SAMPLE_READ should return the value.
> > 
> > so I want to be able to do the leader sampling over child processes
> > that means:
> >   - have event group with sampling leader, and the rest
> >     of the group events' periods being read on leader's sample
> >     via PERF_SAMPLE_READ sample_type
> >   - for each child process created I want it to do the
> >     same thing as the parent - sample on leader, read
> >     values of other events in group via PERF_SAMPLE_READ
> 
> The thing is, PERF_SAMPLE_READ should return the exact same thing as an
> actual read(), and for read() the value reported is the sum of all
> events.

maybe we could make this configurable? like via new per_event_attr::option bit,
keeping by default the behaviour you said

> 
> I _think_ I know what you want, and I think I see why you'd want that,
> but I'm not sure how to best do that.. Hmm.. maybe look at
> 'perf_event_attr::inherit_stat' that appears to be similar.
> 
> At which point read() vs inherit_stat might be 'broken'. Hard to know if
> anybody uses that :-(

I'll check, thanks

jirka
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