linux-kernel - Re: [PATCH V2 1/1] perf/x86: Add Intel power cstate PMUs support

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Message-ID: <CABPqkBQafcye9jhtKr4wO-gR9j5roDdM9W4_UGGx74VgWGJRHQ@mail.gmail.com>
Date:	Thu, 6 Aug 2015 16:10:02 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/1] perf/x86: Add Intel power cstate PMUs support

On Thu, Aug 6, 2015 at 2:45 PM, Liang, Kan <kan.liang@...el.com> wrote:
>
>
>> -----Original Message-----
>> From: Stephane Eranian [mailto:eranian@...gle.com]
>> Sent: Thursday, August 06, 2015 4:38 PM
>> To: Liang, Kan
>> Cc: Peter Zijlstra; mingo@...hat.com; Arnaldo Carvalho de Melo;
>> ak@...ux.intel.com; LKML
>> Subject: Re: [PATCH V2 1/1] perf/x86: Add Intel power cstate PMUs
>> support
>>
>> On Thu, Aug 6, 2015 at 1:25 PM, Liang, Kan <kan.liang@...el.com> wrote:
>> >
>> >> >> >> >> +static cpumask_t power_cstate_core_cpu_mask;
>> >> >> >> >
>> >> >> >> > That one typically does not need a cpumask.
>> >> >> >> >
>> >> >> >> You need to pick one CPU out of the multi-core. But it is for
>> >> >> >> client parts thus there is only one socket. At least this is my
>> >> understanding.
>> >> >> >>
>> >> >> >
>> >> >> > CORE_C*_RESIDENCY are available for physical processor core.
>> >> >> > So logical processor in same physical processor core share the
>> >> >> > same counter.
>> >> >> > I think we need the cpumask to identify the default logical
>> >> >> > processor which do counting.
>> >> >> >
>> >> >> Did you restrict these events to system-wide mode only?
>> >> >>
>> >> Ok, so that means that your cpumask includes one HT per physical core.
>> >> But then, the result is not the simple aggregation of all the N/2 CPUs.
>> >
>> > The counter counts per physical core. The result is the aggregation of
>> > all HT cpus in same physical core.
>>
>> But then don't you need to divide by 2 to get a meaningful result?
>
> Rethink of it. I think I was unclear about the aggregation of all HT cpus
> in same physical core.
>
> physical core Cstate should equal to min(logical core C-state).
> So only all logical core enters C6-state, the physical core enters C6-state,
> then CORE_C6_RESIDENCY counts.
>
> So if we only count on one logical core/HT for CORE_C6_RESIDENCY.
> We don't need to divide by 2. The count result is the residency when all logical
> core in C6 (some may deeper).
>
Ok and here you are assuming you are only measuring one logical CPU per
physical core. If this is the case, then I think you are alright. But
I wonder what
you'd get when perf stat -a aggregates across all measured CPUs, i.e., one CPU
per core.
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