linux-kernel - Re: [PATCH] perf_events: Enable idle state tracing for pseries (ppc64)

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Date:	Tue, 21 Jun 2011 07:42:13 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	deepthi <deepthi@...ux.vnet.ibm.com>
Cc:	linuxppc-dev@...abs.org, linux-pm@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_events: Enable idle state tracing for pseries
 (ppc64)

On Mon, 2011-06-20 at 22:48 +0530, deepthi wrote:
> On Friday 17 June 2011 09:54 AM, Benjamin Herrenschmidt wrote:
> > On Wed, 2011-06-01 at 18:05 +0530, Deepthi Dharwar wrote:
> >> Hi,
> >>
> >> Please find below a patch, which has perf_events added for pseries (ppc64)
> >> platform in order to emit the trace required for perf timechart. 
> >> It essentially enables perf timechart for pseries platfrom to analyse
> >> power savings events like cpuidle states.
> > 
> > Unless I'm mistaken, you added traces to dedicated CPU idle sleep but
> > not shared processor. Any reason ?
> > 
> Yes, the traces were added only to dedicated CPU idle sleep and not for 
> shared processor. This was added only for RFC purpose, and looking for 
> comments from trace implementation point of view. This can be
> easily extended to the latter too.

Please do both.

> > Also I don't really know that tracing stuff but what's the point of
> > having start/end _and trace_cpu_idle if you're going to always start &
> > end around a single occurence of trace_cpu_idle ?
> > 
> power_start/end are the APIs that were used initially
> and they are going to be deprecated in the upcoming kernel releases.
> trace_cpu_idle call is going to replace power start/end routines. 
> To maintain backward compatibility and uniformity, both the routines 
> have been used.
> (ref:https://lkml.org/lkml/2010/11/14/60ref:https://lkml.org/lkml/2010/11/14/60)

Backward compatible with what ? Userspace ? Do we care in that specific
case since it's a new feature ?

> > Wouldn't there be a way to start/end and then trace the snooze and
> > subsequent cede within the same start/end section or that makes no
> > sense ?
> > 
> We wanted to find the residency time of both Snooze as well as cede 
> separately. Knowing this will help us tweak our cpuidle code. So, both 
> have been captured separately.
> 
> > Also would there be any interest in doing the tracing more generically
> > in idle.c ?
> > 
> Yes, this tracing is already implemented for Intel platform. This would
> be a part of cpuidle framework. Going further, once the power cpuidle 
> framework is ported and ready, we will extend this trace there as well.
> (ref:https://lkml.org/lkml/2011/6/7/375)

So do we need to apply this patch at all since the cpuidle stuff is
happening too ?

Cheers,
Ben.


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