linux-kernel - RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

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Date:	Wed, 27 Apr 2011 14:13:25 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric B Munson" <emunson@...bm.net>
Cc:	<benh@...nel.crashing.org>, <a.p.zijlstra@...llo.nl>,
	<paulus@...ba.org>, <mingo@...e.hu>, <acme@...stprotocols.net>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<anton@...ba.org>, <stable@...nel.org>
Subject: RE: [PATCH V4] POWER: perf_event: Skip updating kernel counters if register value shrinks

 
> prev and val are both 64 bit variables holding 32 bit numbers, we do
not
> accumulate in either, they are both replaced by values directly from
the
> registers.
> So prev > val will not always be true.

The code seems to be:
    prev = local64_read(&event->hw.prev_count);
    val = read_pmc(event->hw.idx);
    delta = check_and_compute_delta(prev, val);
    local64_add(delta, &event->count);
Which looks very much like 'prev' being a 64bit counter generated
from the 32bit pmc register.

	David


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