linux-kernel - Re: [PATCH] x86, perf, p4: Counter corruption when using lots of perf groups

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Date:	Mon, 3 Feb 2014 10:19:29 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Don Zickus <dzickus@...hat.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 Wed, Jan 29, 2014 at 03:17:17PM -0500, Don Zickus wrote:
> > > 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. :-)

Hi Don, sorry for delay. I thought maybe extending match_prev_assignment()
would be better (ie to figure out if previous event can run without
reprogramming the counter) but this makes code only harder (and what
is worse -- having no physical accees to p4 machine leaves no chance
to test changes). So eventually I think your patch does the same thing
as I had in mind but in different way. Thus

Acked-by: Cyrill Gorcunov <gorcunov@...nvz.org>

thanks a lot!
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