linux-kernel - Re: [PATCH 1/1] perf tools: Fix thread_map event synthesizing in top and record

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Message-ID: <20110210200000.GF20676@ghostprotocols.net>
Date:	Thu, 10 Feb 2011 18:00:00 -0200
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH 1/1] perf tools: Fix thread_map event synthesizing in
	top and record

Em Thu, Feb 10, 2011 at 02:40:29PM -0500, Jeff Moyer escreveu:
> Arnaldo Carvalho de Melo <acme@...radead.org> writes:
> > From: Arnaldo Carvalho de Melo <acme@...hat.com>
> > Jeff Moyer reported these messages:
> >
> >   Warning:  ... trying to fall back to cpu-clock-ticks
> >
> > couldn't open /proc/-1/status
> > couldn't open /proc/-1/maps
> > [ls output]
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.008 MB perf.data (~363 samples) ]
> >
> > That lead me and David Ahern to see that something was fishy on the thread
> > synthesizing routines, at least for the case where the workload is started
> > from 'perf record', as -1 is the default for target_tid in 'perf record --tid'
> > parameter, so somehow we were trying to synthesize the PERF_RECORD_MMAP and
> > PERF_RECORD_COMM events for the thread -1, a bug.
> >
> > So I investigated this and noticed that when we introduced support for
> > recording a process and its threads using --pid some bugs were introduced and
> > that the way to fix it was to instead of passing the target_tid to the event
> > synthesizing routines we should better pass the thread_map that has the list of
> > threads for a --pid or just the single thread for a --tid.
> 
> Thanks for looking into this, acme.  Honestly, I can't for the life of
> me figure out from your description how this problem would have affected
> systems.  The fix makes the warnings go away, but I am still left with
> the hung system I originally reported.

I wasn't looking at that part, as described, I looked just at the
/proc/-1/status part, that -1. I.e. the part I hack on, the tools.
 
> So, what does this patch actually fix?  And do you or others have any
> interest in trying to work out why perf is hanging my system?

Peter?

- Arnaldo
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