Re: [RFC] perf/core: what is exclude_idle supposed to do

[Arnaldo Carvalho de Melo <acme@redhat.com>]

Em Mon, Apr 16, 2018 at 10:04:53PM +0000, Stephane Eranian escreveu:
> Hi,
> 
> I am trying to understand what the exclude_idle event attribute is supposed
> to accomplish.
> As per the definition in the header file:
> 
>     exclude_idle   :  1, /* don't count when idle */
> 
> Naively, I thought it would simply stop the event when running in the
> context of the idle task (swapper, pid 0) on any CPU. That would seem to
> match the succinct description.
> 
> However, running a simple:
> 
> $ perf record -a -e cycles:I sleep 5
> perf_event_attr:
>     sample_type                      IP|TID|TIME|CPU|PERIOD
>    read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING|ID
>    exclude_idle                     1
> 
> on an idle machine, showed me that this is not what is actually happening:
> $ perf  script
>           swapper     0 [000] 1132634.287442:          1 cycles:I:
>   ffffffff8100b1fb __intel_pmu_enable_all.isra.17 ([kernel.kallsyms])
>           swapper     0 [001] 1132634.287498:          1 cycles:I:
>   ffffffff8100b1fb __intel_pmu_enable_all.isra.17 ([kernel.kallsyms])
> 
> 
> After looking at the code, it all made sense, it seems to current
> implementation is only relevant for sw events. I don't understand why.
> 
> I am left wondering what is the goal of exclude_idle?

To do something it is not doing, i.e. to do what you expected it to do.

There were messages exchanged recently where PeterZ said that this is
just that needs fixing.

- Arnaldo