linux-kernel - Re: perf: record segfaults for cycles event when collecting data on a VM

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Message-ID: <20120208174434.GI22598@amd.com>
Date:	Wed, 8 Feb 2012 18:44:34 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	David Ahern <dsahern@...il.com>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Wang <jasowang@...hat.com>
Subject: Re: perf: record segfaults for cycles event when collecting data on
 a VM

On Wed, Feb 08, 2012 at 09:55:35AM -0700, David Ahern wrote:
> perf record -e cycles -p 21483 -- sleep 1
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.012 MB perf.data (~503 samples) ]
> Segmentation fault

The problem is similar to the one Jason is seeing. I am working on a fix
right now.
Bottom line is that the perf-tool may receive samples tagged as
GUEST_KERNEL even when guest-sampling is disabled (probably a
race-condition). The perf-tool can not find a valid machine pointer for
such a sample and passes NULL down to the other functions. And some
functions don't seem to handle this.

David, can you try to change the default for perf_guest back to false
amd re-test? Not with 'sleep 1' probably, on my setup it takes a busy
guest and a few seconds to trigger.

	Joerg

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