linux-kernel - Re: [PATCH] powerpc: perf: power_pmu_start restores incorrect values, breaking frequency events

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Date:	Thu, 16 Feb 2012 15:57:49 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	eranian@...gle.com, peterz@...radead.org, mingo@...e.hu,
	gleb@...hat.com, wcohen@...hat.com, vince@...ter.net,
	asharma@...com, andi@...stfloor.org, emunson@...bm.net,
	imunsie@....ibm.com, benh@...nel.crashing.org,
	sukadev@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: perf: power_pmu_start restores incorrect
 values, breaking frequency events

On Thu, Feb 16, 2012 at 03:48:22PM +1100, Anton Blanchard wrote:
> 
> perf on POWER stopped working after commit e050e3f0a71b (perf: Fix
> broken interrupt rate throttling). That patch exposed a bug in
> the POWER perf_events code.
> 
> Since the PMCs count upwards and take an exception when the top bit
> is set, we want to write 0x80000000 - left in power_pmu_start. We were
> instead programming in left which effectively disables the counter
> until we eventually hit 0x80000000. This could take seconds or longer.
> 
> With the patch applied I get the expected number of samples:
> 
> # taskset -c 0 yes > /dev/null &
> # perf record -C 0 -a sleep 10
> # perf report -D | grep SAMPLE | tail -1
>           SAMPLE events:       9948
> 
> Signed-off-by: Anton Blanchard <anton@...ba.org>

Acked-by: Paul Mackerras <paulus@...ba.org>
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