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

Anshuman Khandual [off-list ref] wrote:

On 11/06/2012 07:23 AM, Michael Neuling wrote:

quoted

+	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