linux-kernel - Re: Use case for PERF_COUNT_HW_REF_CPU

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Message-ID: <CABPqkBR+okFaa2s1AbMHAgkBbW4QSwOceLuYvqE=6PQOvzksiw@mail.gmail.com>
Date:	Wed, 8 Feb 2012 10:51:51 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu
Subject: Re: Use case for PERF_COUNT_HW_REF_CPU_CYCLES generic PMU event

On Wed, Feb 8, 2012 at 10:43 AM, Anshuman Khandual
<khandual@...ux.vnet.ibm.com> wrote:
> Hello Stephane,
>
> I was going through the following discussion where we added the
> new HW generic event PERF_COUNT_HW_REF_CPU_CYCLES.
>
> https://lkml.org/lkml/2011/12/10/103
>
> (Sorry, for asking this question bit late)
>
> I am trying to understand the use case for this. Would this new event
> help us in generating (during a perf session) a CPU frequency invariant
> time metric against which we would plot our other perf event's measurements ?
> CPU frequency independent time measurement is it's primary purpose ? or we were
> finding a way to expose the fixed counter 2 which was not getting used before
> for not having an event encoding. I guess this would help us in finding equivalent
> PMU events or mechanisms in other architecture / platforms.
>
The goal was to expose a cycle event that is not subject to frequency scaling
nor turbo boost of any sort. An event that could be used to correlate with time.
An event that could also be used to compute idle time by comparing its value
with wall-clock time.

The fact that on Intel X86 this event is on fixed counter 2 is an
implementation
detail.

> --
> Anshuman Khandual
> Linux Technology Centre
> IBM Systems and Technology Group
>
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