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

[Joerg Roedel <joerg.roedel@amd.com>]

On Thu, Feb 09, 2012 at 02:34:47PM +0100, Ingo Molnar wrote:
> 
> * Joerg Roedel <joerg.roedel@amd.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

-- 
AMD Operating System Research Center

Advanced Micro Devices GmbH Einsteinring 24 85609 Dornach
General Managers: Alberto Bozzo
Registration: Dornach, Landkr. Muenchen; Registerger. Muenchen, HRB Nr. 43632