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

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Message-ID: <20110427132053.GD2936@mgebm.net>
Date:	Wed, 27 Apr 2011 09:20:54 -0400
From:	Eric B Munson <emunson@...bm.net>
To:	David Laight <David.Laight@...LAB.COM>
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

On Wed, 27 Apr 2011, David Laight wrote:

>  
> > 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.
> 

Which implies that it will only ever be 32 bits wide, just stored in 64.

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