Messages in this thread | | | Date | Wed, 29 Jan 2014 15:17:17 -0500 | From | Don Zickus <> | Subject | Re: [PATCH] x86, perf, p4: Counter corruption when using lots of perf groups |
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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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