Messages in this thread | | | Date | Mon, 3 Feb 2014 11:35:30 -0500 | From | Don Zickus <> | Subject | Re: [PATCH] x86, perf, p4: Counter corruption when using lots of perf groups |
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On Mon, Feb 03, 2014 at 10:19:29AM +0400, Cyrill Gorcunov wrote: > 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@openvz.org> > > thanks a lot!
thanks!
Cheers, Don
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