Subject: Re: [PATCH] Use pmc_overflow() to detect rolled back events

From: Sukadev Bhattiprolu <[email-masked]>
Date: Thu, 20 Sep 2012 17:38:05 -0700

Sukadev Bhattiprolu [[email-masked]] wrote:
| 
| >From 21e9d1775f0c6f37a39e5d682ff74693fa9a4004 Mon Sep 17 00:00:00 2001
| From: Sukadev Bhattiprolu <[email-masked]>
| Date: Tue, 7 Aug 2012 17:53:24 -0700
| Subject: [PATCH] Use pmc_overflow to detect rolled back events.
| 
| For certain speculative events on Power7, 'perf stat' reports far higher
| event count than 'perf record' for the same event.
| 
| As described in following commit, a performance monitor exception is raised
| even when the the performance events are rolled back.
| 
|         commit 0837e3242c73566fc1c0196b4ec61779c25ffc93
|         Author: Anton Blanchard <[email-masked]>
|         Date:   Wed Mar 9 14:38:42 2011 +1100
| 
| perf_event_interrupt() records an event only when an overflow occurs. But
| this check for overflow is a simple 'if (val < 0)'.
| 
| Because the events are rolled back, this check for overflow fails and the
| event is not recorded. perf_event_interrupt() later uses pmc_overflow() to
| detect the overflow and resets the counters and the events are lost completely.
| 
| To properly detect the overflow of rolled back events, use pmc_overflow()
| even when recording events.

Ben,

Sorry for the noise, but please revert this patch (following commit):

	commit 813312110bede27bffd082c25cd31730bd567beb
	Author: Sukadev Bhattiprolu <[email-masked]>
	Date:   Tue Aug 7 15:07:19 2012 +0000

While it does fix the problem described above and works for the limit-pmc
events, it seems to break on Power7 for other events and when the sample
period is low.

I am still investigating the problem and will follow up with a separate
mail.

Sukadev