Re: PEBS bug on HSW: "Unexpected number of pebs records 10" (was:Re: [GIT PULL] perf changes for v3.12)

[Peter Zijlstra]

On Mon, Sep 23, 2013 at 05:25:19PM +0200, Stephane Eranian wrote:
> > Its not just a broken threshold. When a PEBS event happens it can re-arm
> > itself but only if you program a RESET value !0. We don't do that, so
> > each counter should only ever fire once.
> >
> > We must do this because PEBS is broken on NHM+ in that the
> > pebs_record::status is a direct copy of the overflow status field at
> > time of the assist and if you use the RESET thing nothing will clear the
> > status bits and you cannot demux the PEBS events back to the event that
> > generated them.
> >
> Trying to understand this problem better. You are saying that in case you
> are sampling multiple PEBS events there is a problem if you allow more
> than one record per PEBS buffer because the overflow status is not reset
> properly.

That is what I wrote; but I'm not entire sure that's correct. I think it
will reset the overflow bits once it does an actual reset after the PEBS
assist triggers, but see below.

> For instance, if first record is caused by counter 0, ovfl_status=0x1,
> then counter
> is reset. Then, if counter 1 is the cause of the next record, then
> that record has the
> ovfl_status=0x3 instead of ovfl_status=0x2? Is that what you are saying?
> 
> If so then yes, I agree this is a serious bug and we need to have Intel fix it.

But there's still the case where with 2 counters you can get:

cnt0 overflows; sets status |= 1 << 0, arms PEBS0 assist
cnt1 overflows; sets status |= 1 << 1, arms PEBS1 assist

PEBS0 ready to trigger
PEBS1 ready to trigger

Cnt1 event -> PEBS1 trigger, writes entry with status := 0x03
Cnt0 event -> PEBS0 trigger, writes entry with status := 0x03

At which point you'll have 2 events with the same status overflow bits
in 'reverse' order.

If we'd set RESET, the second entry would have status : 0x01, which
would be unambiguous again. But we'd still not know where to place the
0x03 entry.

With more PEBSn counters enabled and a threshold > 1 the chance of
having such scenarios is greatly increased.

The threshold := 1 case tries to avoid these cases by getting them out
as fast as possible and hopefully avoiding the second trigger.
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