On Tue, Aug 06, 2013 at 11:28:05PM +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.
>
> Signed-off-by: Michael Ellerman <[email protected]>
Tested-by/Acked-by: Jiri Olsa <[email protected]>
thanks,
jirka
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