Re: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second eventon intel perf counter

[Stephane Eranian]

On Fri, Sep 3, 2010 at 1:11 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, 2010-09-03 at 13:02 +0200, Stephane Eranian wrote:
>>
>> > One thing we still need to do is on init detect if the BIOS is using one
>> > of the PMCs and simply disable all of perf and print a nice big message
>> > to the user to request a new BIOS from their vendor.
>> >
>> Given then way perf_events operate, that is your only choice at this point.
>
> Well, it wouldn't be too hard to cure that, but the BIOS should simply
> keep its grubby paws of the PMU -- I'm really not interested in
> co-operating on that point.
>
>> But I am sure neither my system nor yours is subject to this particular issue
>
> Sure, worth checking though, not sure Don did on his machine.
>
>> yet there is some unexplained errors with OVF_STATUS.
>
> Right.
>
>> Here is an example of what I gathered on a Westmere:
>>
>> This is coming into the interrupt handler:
>> - status  = overflow status coming from GLOBAL_OVF_STATUS
>> - status2 = inspection of the counters
>> - act = cpuc->active_mask[0]
>>
>> In case both status don't match, I dump the state of the active events
>> incl. the counter values(val).
>>
>> [ Â822.813808] CPU2 irqin status=0x6 status2=0x4 act=0x7
>> [ Â822.813818] CPU2 cfg=0x13003c idx=0 sel=53003c val=ffffa833f298
>> [ Â822.813821] CPU2 cfg=0x12003c idx=1 sel=52003c val=fffffe130229
>> [ Â822.813823] CPU2 cfg=0x11003c idx=2 sel=51003c val=5e9
>>
>> Here only counter2 has overflowed, yet the handler will also process counter1
>> which is wrong.
>
> Right, we could easily revert to scanning all counters like we do for
> all other interrupt handlers.
>
Well, that's the question! Looks like this may be more reliable, yet more
costly. And also you'd have to deal with PEBS separately, though using
OVF_STATUS for that may be sufficient.

>> The other thing I noticed is that in intel_pmu_disable_event(), the event
>> stopped sometimes has overflowed. Looks like OVF_STAUS is stale.
>> Maybe OVF_STATUS is not cleared properly somewhere, possibly when
>> an event gets disabled.
>
> Right, the code pretty much assumes that if it overflows a PMI will be
> generated. So you're saying a pending PMI might get canceled when we
> clear the EN bit? Most icky.
>
No, I don't think that cancels it. But that may be a reason why there are
back-to-back NMIs, with nothing to process sometimes (event not in the
active_mask anymore).
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