Subject: Re: [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks

From: Eric B Munson <[email-masked]>
Date: Thu, 31 Mar 2011 12:14:56 -0400

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 being so dense on this, but I think that when a counter overflows
both the register value and the previous value are reset so we should continue
seeing new event counts after the overflow interrupt handler puts the counter
back into a sane state.  What am I not seeing?

Eric