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

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Date:	Thu, 9 Feb 2012 15:32:53 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	David Ahern <dsahern@...il.com>,
	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 Thu, Feb 09, 2012 at 02:34:47PM +0100, Ingo Molnar wrote:
> 
> * Joerg Roedel <joerg.roedel@....com> wrote:

> > Well, there are two options:
> > 
> > 	1) Make sure machine == NULL does not happen. Changing the
> > 	   default of perf_guest back to false does exactly this for
> > 	   David's problem.
> 
> So what if it's turned on by the user? Do we still crash 
> occasionally?

It is only turned on by perf-kvm, and this path should setup a machine
object for guest samples.

> 
> > 	2) Make sure that a machine == NULL pointer is never
> > 	   dereferenced
> > 
> > I was going to fix it with option 1. Do you suggest option 2 is better?
> 
> Looks like the better fix. You said:
> 
> > 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.
> 
> tooling should never be surprised by getting some unexpected 
> sample via the perf.data or the ring-buffer - regardless of 
> whether that functionality is default enabled or manually 
> enabled.

Yeah, right. Guest samples may also show up intentionally when the event
modifiers are used. So crashing on machine==NULL needs to be fixed.


	Joerg

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