linux-kernel - Re: [PATCH] perf sched latency: Fix removed thread issue

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Message-ID: <20151103183310.GU21609@kernel.org>
Date:	Tue, 3 Nov 2015 15:33:10 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mohit Agrawal <moagrawa@...hat.com>
Subject: Re: [PATCH] perf sched latency: Fix removed thread issue

Em Tue, Nov 03, 2015 at 08:41:48AM +0100, Jiri Olsa escreveu:
> On Mon, Nov 02, 2015 at 07:53:53PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Mon, Nov 02, 2015 at 12:10:25PM +0100, Jiri Olsa escreveu:
> > > If machine's thread gets excited (EXIT event is received),
> > > we set thread->dead = true and it is later on removed from
> > > machine's tree if the pid is reused on new thread.
> > 
> > > The latency subcommand holds tree of working atoms sorted
> > > by thread's pid/tid. If there's new thread with same pid
> > 
> > Humm, wher is the latency subcommand handling the EXIT event?
> > 
> > I see:
> > 
> >    perf_sched__lat
> >      perf_sched__read_events
> >        session = perf_session__new(&file, false, &sched->tool);
> >        perf_session__process_events(session)
> > 
> > And sched->tool->exit() is not set, which will make
> > perf_session__process_events(), when calling perf_tool__fill_defaults()
> > set it to process_event_stub() which will do nothing for
> > PERF_RECORD_EXIT events, no?
> 
> yep, latency command does not handle EXIT event, but the
> thread is removed via FORK event.. the first changelog

So its not related to processing an EXIT event as described in the
changelog, ok. And I don't see where is that thread->dead is set, i.e.
the sequence is:

  machine__process_fork_event()
    machine__remove_thread()

The only place where thread->dead is set to true is in thread__exited()
and that is only called by machine__process_exit_event(), which is never
called by 'perf sched'.

It is not "later removed from machine's tree", it is removed straight
away, in __machine__remove_thread().

Anyway, I'm downloading that perf.data file to try to debug this here.

- Arnaldo

> paragraph might be a little misleading sorry ;-)
> 
> could you please change it to:
> 
> ---
> If machine's thread gets excited (EXIT event is received),
> we set thread->dead = true and it is later on removed from
> machine's tree if the pid is reused on new thread.
> 
> We dont handle EXIT command in 'perf sched latency',
> however the old thread is removed anyway when FORK
> event is received for new thrad with same pid/tid.
> ---
> 
> thanks,
> jirka
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