linux-kernel - Re: [PATCH 2/4] perf_event: add PERF_COUNT_HW_REF_CPU

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Date:	Sun, 11 Dec 2011 09:01:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	acme@...radead.org, robert.richter@....com, ming.m.lin@...el.com,
	andi@...stfloor.org, asharma@...com
Subject: Re: [PATCH 2/4] perf_event: add PERF_COUNT_HW_REF_CPU_CYCLES generic
 PMU event


* Stephane Eranian <eranian@...gle.com> wrote:

> This event counts the number of reference core cpu cycles. 
> Reference means that the event increments at a constant rate 
> which is not subject to core CPU frequency adjustments. The 
> event may not count when the processor is in halted (low 
> power) state. As such, it may not be equivalent to wall clock 
> time. However, when the processor is not halted state, the 
> event keeps a constant correlation with wall clock time.
> 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> ---
>  include/linux/perf_event.h |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 564769c..0885561 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -54,6 +54,7 @@ enum perf_hw_id {
>  	PERF_COUNT_HW_BUS_CYCLES		= 6,
>  	PERF_COUNT_HW_STALLED_CYCLES_FRONTEND	= 7,
>  	PERF_COUNT_HW_STALLED_CYCLES_BACKEND	= 8,
> +	PERF_COUNT_HW_REF_CPU_CYCLES		= 9,

Btw., that was what 'bus cycles' tried to do a long time ago: 
the constant, non-variable baseline heartbeat of the system.

We could simply rename that, fix the event tables where needed, 
and keep the ABI.

Just a small detail: how about naming it 'constant [frequency] 
CPU cycles'? That's a concept that might be present on other 
platforms as well.

CONST_FREQ_CYCLES or so. The 'CPU' can be dropped as well 
because it's not really the CPU's frequency anymore.

Thanks,

	Ingo
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