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

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Message-ID: <20120208175709.GK22598@amd.com>
Date:	Wed, 8 Feb 2012 18:57:09 +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 10:53:04AM -0700, David Ahern wrote:

> The segfault is in the event processing once the collection is done
> (perf-record), so the time length should not matter.

Yes, but I wanted to make sure there is a GUEST_KERNEL sample in there
to process. It usually takes some time (at least in my tests) to get a
spurious one.

> Either way, this fixes the segafult:
> 
> diff --git a/tools/perf/util/util.c b/tools/perf/util/util.c
> index 8131410..fb25d13 100644
> --- a/tools/perf/util/util.c
> +++ b/tools/perf/util/util.c
> @@ -6,7 +6,7 @@
>   * XXX We need to find a better place for these things...
>   */
>  bool perf_host  = true;
> -bool perf_guest = true;
> +bool perf_guest = false;
> 
>  void event_attr_init(struct perf_event_attr *attr)
>  {
> 
> which makes sense. It forces perf_session__find_machine_for_cpumode() to
> return the host machine always.

Great, thanks. I will send two patches tomorrow to fix Jason's problem
and change the default for perf_guest.


	Joerg

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