linux-kernel - Re: [PATCH v2 1/2] perf tools: Add support for pinned modifier

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Message-ID: <87bo5ayqg1.fsf@sejong.aot.lge.com>
Date:	Wed, 07 Aug 2013 15:18:06 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Michael Ellerman <michael@...erman.id.au>
Cc:	<linux-kernel@...r.kernel.org>, acme@...stprotocols.net,
	a.p.zijlstra@...llo.nl, jolsa@...hat.com
Subject: Re: [PATCH v2 1/2] perf tools: Add support for pinned modifier

Hi Michael,

On Tue,  6 Aug 2013 23:28:05 +1000, Michael Ellerman wrote:
> This commit adds support for a new modifier "D", which requests that the
> event, or group of events, be pinned to the PMU.
>
> The "p" modifier is already taken for precise, and "P" may be used in
> future to mean "fully precise".
>
> So we use "D", which stands for pinneD - and looks like a padlock, or if
> you're using the ":D" syntax perf smiles at you.
>
> This is an oft-requested feature from our HW folks, who want to be able
> to run a large number of events, but also want 100% accurate results for
> instructions per cycle.
>
> Comparison of results with and without pinning:
>
> $ perf stat -e '{cycles,instructions}:D' -e cycles,instructions,...
>
>   79,590,480,683 cycles         #  0.000 GHz
>  166,123,716,524 instructions   #  2.09  insns per cycle
>                                 #  0.11  stalled cycles per insn
>
>   79,352,134,463 cycles         #  0.000 GHz                     [11.11%]
>  165,178,301,818 instructions   #  2.08  insns per cycle
>                                 #  0.11  stalled cycles per insn [11.13%]
>
> As you can see although perf does a very good job of scaling the values
> in the non-pinned case, there is some small discrepancy.
>
> The patch is fairly straight forward, the one detail is that we need to
> make sure we only request pinning for the group leader when we have a
> group.

Acked-by: Namhyung Kim <namhyung@...nel.org>

Thanks,
Namhyung
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