Messages in this thread | | | Date | Wed, 1 Sep 2010 15:04:45 +0200 | Subject | Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event on intel perf counter | From | Stephane Eranian <> |
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Don,
Found your patch on LKML (I am not on it).
In your changelog you said:
> During testing of a patch to stop having the perf subsytem swallow nmis, > it was uncovered that Nehalem boxes were randomly getting unknown nmis > when using the perf tool. > > Moving the ack'ing of the PMI closer to when we get the status allows > the hardware to properly re-set the PMU bit signaling another PMI was > triggered during the processing of the first PMI. This allows the new > logic for dealing with the shortcomings of multiple PMIs to handle the > extra NMI by 'eat'ing it later.
> Now one can wonder why are we getting a second PMI when we disable all > the PMUs in the beginning of the NMI handler to prevent such a case, for > that I do not know. But I know the fix below helps deal with this quirk. >
I am assuming you're talking about back-to-back NMIs here, not nested NMIs. I don't quite understand the scenario here. Is it the case that you handled 1 overflow, and then right as you return from the interrupt, you get a second PMI with a ovfl_status=0 ?
What events did you measure? Which counters did you use? Did you have HT turned on?
> Tested on multiple Nehalems where the problem was occuring. With the > patch, the code now loops a second time to handle the second PMI (whereas > before it was not).
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