Re: [PATCH] POWER: perf_event: Skip updating kernel counters ifregister value shrinks
From: Eric B Munson
Date: Wed Apr 06 2011 - 17:27:41 EST
On Thu, 31 Mar 2011, Benjamin Herrenschmidt wrote:
> On Wed, 2011-03-30 at 14:36 -0400, Eric B Munson wrote:
> > On Wed, 30 Mar 2011, Benjamin Herrenschmidt wrote:
> > > On Tue, 2011-03-29 at 10:25 -0400, Eric B Munson wrote:
> > > > Here I made the assumption that the hardware would never remove more events in
> > > > a speculative roll back than it had added. This is not a situation I
> > > > encoutered in my limited testing, so I didn't think underflow was possible. I
> > > > will send out a V2 using the signed 32 bit delta and remeber to CC stable
> > > > this time.
> > >
> > > I'm not thinking about underflow but rollover... or that isn't possible
> > > with those counters ? IE. They don't wrap back to 0 after hitting
> > > ffffffff ?
> > >
> > They do roll over to 0 after ffffffff, but I thought that case was already
> > covered by the perf_event_interrupt. Are you concerned that we will reset a
> > counter and speculative roll back will underflow that counter?
> No, but take this part of the patch:
> > --- a/arch/powerpc/kernel/perf_event.c
> > +++ b/arch/powerpc/kernel/perf_event.c
> > @@ -416,6 +416,15 @@ static void power_pmu_read(struct perf_event *event)
> > prev = local64_read(&event->hw.prev_count);
> > barrier();
> > val = read_pmc(event->hw.idx);
> > + /*
> > + * POWER7 can roll back counter values, if the new value is
> > + * smaller than the previous value it will cause the delta
> > + * and the counter to have bogus values. If this is the
> > + * case skip updating anything until the counter grows again.
> > + * This can lead to a small lack of precision in the counters.
> > + */
> > + if (val < prev)
> > + return;
> > } while (local64_cmpxchg(&event->hw.prev_count, prev, val) != prev);
> Doesn't that mean that power_pmu_read() can only ever increase the value of
> the perf_event and so will essentially -stop- once the counter rolls over ?
> Similar comments every where you do this type of comparison.
Sorry for the nag, but am I missing something about the way the register and
the previous values are reset in the overflow interrupt handler?
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