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. Cheers, Ben. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
