Re: [PATCH 07/12] perf, x86: Avoid checkpointed counters causing excessive TSX aborts v3

[Andi Kleen]

On Tue, Jan 29, 2013 at 01:30:19AM +0100, Stephane Eranian wrote:
> >> The counter is reinstated to its state before the critical section but
> >> the PMI cannot be
> >> cancelled and there is no state left behind to tell what to do with it.
> >
> > The PMI is effectively spurious, but we use it to set back. Don't know
> > what you mean with "cancel". It already happened of course.
> >
> But when you do this, it seems you making INT_CP events unusable
> for sampling, because you're resetting their value under the cover.
> So what happens when you sample, especially with a fixed period?

Sampling is forbidden for checkpointed events, the setup code
enforces that. It's unlikely to be useful anyways.

The main use case for checkpointing is perf stat -T and related
counting usages.

> >>
> >> > +               if (event->attr.sample_period > 0 &&
> >> > +                   event->attr.sample_period < 0x7fffffff)
> >> > +                       return -EIO;
> >> > +       }
> Explain the 0x7fffffff to me? Is that the max period set by default when you
> just count?

Originally I had just > 0, but then I found that perf stat from the
guest doesn't work anymore because it sets an very high overflow
to accumulate counters.

The 0x7fffffff is a somewhat arbitary threshold to detect this case.


-Andi
-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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