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Date:	Wed, 29 Jan 2014 15:17:17 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, perf, p4: Counter corruption when using lots of
 perf groups

On Thu, Jan 30, 2014 at 12:06:57AM +0400, Cyrill Gorcunov wrote:
> On Wed, Jan 29, 2014 at 02:37:50PM -0500, Don Zickus wrote:
> > On a P4 box stressing perf with
> > 
> > ./perf record -o perf.data ./perf stat -v ./perf bench all
> > 
> > it was noticed that a slew of unknown NMIs would pop out rather quickly.
> > 
> > Painfully debugging this ancient platform, led me to notice cross cpu counter
> > corruption.
> > 
> > The P4 machine is special in that it has 18 counters, half are used for cpu0
> > and the other half is for cpu1 (or all 18 if hyperthreading is disabled).  But
> > the splitting of the counters has to be actively managed by the software.
> > 
> > In this particular bug, one of the cpu0 specific counters was being used by
> > cpu1 and caused all sorts of random unknown nmis.
> > 
> > I am not entirely sure on the corruption path, but what happens is:
> > 
> > o perf schedules a group with p4_pmu_schedule_events()
> > o inside p4_pmu_schedule_events(), it notices an hwc pointer is being reused
> >   but for a different cpu, so it 'swaps' the config bits and returns the
> >   updated 'assign' array with a _new_ index.
> > o perf schedules another group with p4_pmu_schedule_events()
> > o inside p4_pmu_schedule_events(), it notices an hwc pointer is being reused
> >   (the same one as above) but for the _same_ cpu [BUG!!], so it updates the
> >   'assign' array to use the _old_ (wrong cpu) index because the _new_ index is in
> >   an earlier part of the 'assign' array (and hasn't been committed yet).
> > o perf commits the transaction using the wrong index and corrupts the other cpu
> 
> Thanks for the fix Don! I fear I won't be able to look precisely tonight, so
> could it wait until tomorrow? (If it's critical sure such fix should do the
> trick).

There is no rush.  Early next week is fine too. :-)

Cheers,
Don

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