linux-kernel - Re: [GIT PULL] perf fixes

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Message-ID: <20110124203802.GC29975@ghostprotocols.net>
Date:	Mon, 24 Jan 2011 18:38:02 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] perf fixes

Em Tue, Jan 25, 2011 at 06:27:31AM +1000, Linus Torvalds escreveu:
> On Tue, Jan 25, 2011 at 6:17 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > It only happens for me with "g". With "perf record -af sleep 10" it worked.
> 
> Actually, it's something subtler than that. It must depend on the
> actual data, because now when I tried it again, it worked with 'g'
> too. I hadn't saved the old perf.data that caused the lockup (it got
> overwritten by the non-g test), and now when I try to re-create it it
> doesn't hang on the result.
> 
> So it's probably some very specific data pattern that causes it.
> 
> (And I don't know if it's a hard hang - it could just be something
> _very_ slow. But we're talking half a minute kind of slow).

Was this on a freshly installed machine? Or on a freshly updated one?

Probably its the build-id collecting at the end of a session, on the
first run you had a cold cache and it had to figure out which binaries
to cache on ~/.debug, second time it was already cached so it was fast.

So one way to try to reproduce would be to:

rm -rf ~/.debug

and then try it again.

To double check, you can try to disable the build-id cache with:

--no-buildid-cache or -N

i.e.:

perf record -afN sleep 10

- Arnaldo
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