Re: [PATCH 1/2] powerpc/perf: Fix finding overflowed PMC in interrupt

[Michael Neuling <mikey@neuling.org>]

Anshuman Khandual <khandual@linux.vnet.ibm.com> wrote:

> On 11/06/2012 07:23 AM, Michael Neuling wrote:
> 
> > +	if (!found && pvr_version_is(PVR_POWER7)) {
> > +		/* check active counters for special buggy p7 overflow */
> > +		for (i = 0; i < cpuhw->n_events; ++i) {
> > +			event = cpuhw->event[i];
> > +			if (!event->hw.idx || is_limited_pmc(event->hw.idx))
> >  				continue;
> > -			val = read_pmc(i + 1);
> > -			if (pmc_overflow(val))
> > -				write_pmc(i + 1, 0);
> > +			if (pmc_overflow_power7(val[event->hw.idx - 1])) {
> 
> 
> I have couple of questions. 
> 
> Can the buggy overflow happen on any of the available counters PMC1-PMC4 ?

No.  It's limited to certain events and I believe it can only happen on
PMC2 and 4.  This code doesn't bother trying to make this distinction
though.

> Will this approach never reset an actual user defined event (with
> sample period < 256) ? Is this related to the counter or the event
> which it is counting ? Just wondering if we have to do something more
> than checking for the count < 256. Just a thought.


I don't understand what you mean by these questions.  Can you explain a
bit more?

Mikey