linux-kernel - Re: [PATCH 3/5] scheduler: cpuacct: Enable platform hooks to track cpuusage for CPU frequencies

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Message-ID: <20101122065129.072bba06@schatten.dmk.lab>
Date:	Mon, 22 Nov 2010 06:51:29 +0100
From:	Florian Mickler <florian@...kler.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Mike Chan <mike@...roid.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 3/5] scheduler: cpuacct: Enable platform hooks to track
 cpuusage for CPU frequencies

On Sat, 20 Nov 2010 11:48:24 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2010-11-19 at 18:08 -0800, John Stultz wrote:
> > From: Mike Chan <mike@...roid.com>
> > 
> > Introduce new platform callback hooks for cpuacct for tracking CPU frequencies
> > 
> > Not all platforms / architectures have a set CPU_FREQ_TABLE defined
> > for CPU transition speeds. In order to track time spent in at various
> > CPU frequencies, we enable platform callbacks from cpuacct for this accounting.
> > 
> > Architectures that support overclock boosting, or don't have pre-defined
> > frequency tables can implement their own bucketing system that makes sense
> > given their cpufreq scaling abilities.
> > 
> > New file:
> > cpuacct.cpufreq reports the CPU time (in nanoseconds) spent at each CPU
> > frequency.
> 
> I utterly detest all such accounting crap.. it adds ABI constraints it
> add runtime overhead. etc.. 
> 
> Can't you get the same information by using the various perf bits? If
> you trace the cpufreq changes you can compute the time spend in each
> power state, if you additionally trace the sched_switch you can compute
> it for each task.
> 
> 
This is probably used for "on-site" debugging of production systems.

I.e. when someone sends them a problem report using an
bugreport-tool, they gather all useful information they can get on the
system because they only have one-way communication with their bug
reporters.

Do the perf bits work for such a usecase? If I guess correctly, the
perf bits need a userspace part that computes what would be in the
cpuacct.cpufreq file? 

Regards,
Flo
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