linux-kernel - Re: [PATCH 6/9] Include idle and iowait fields in cpuacct

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Date:	Tue, 20 Sep 2011 14:58:22 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, xemul@...allels.com,
	paul@...lmenage.org, lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	mingo@...e.hu, jbottomley@...allels.com
Subject: Re: [PATCH 6/9] Include idle and iowait fields in cpuacct

On Tue, 2011-09-20 at 09:36 -0300, Glauber Costa wrote:
> On 09/20/2011 06:21 AM, Peter Zijlstra wrote:
> > On Wed, 2011-09-14 at 17:04 -0300, Glauber Costa wrote:
> >> These are slightly different from the others though:
> >> (note to reviewers: might be better to put those in a separate
> >> array?)
> >>
> >> Since idle/iowait are a property of the system - by definition,
> >> no process from any cgroup is running when the system is idle,
> >> they are system wide. So what these fields really mean, are baselines
> >> for when the cgroup was created. It allows the cgroup to start
> >> counting idle/iowait from 0.
> >
> > Alternatively you can make iowait based on nr_uninterruptible per cgroup
> > and count all ticks _this_ cgroup was idle.
> You think?
> 
> Humm,humm... maybe...
> iowait can indeed be seen as a process group characteristic. I was 
> mainly concerned about overhead here, specially for the idle case:

The overhead of accounting per cgroup nr_uninterruptible is the worst I
think, that's in the sleep/wakeup paths.

> If we are idle, there is no task context we can draw from, since the 
> task in the cpu is the idle task. So we end up having to touch all 
> cgroups... Or am I missing something?
> 
> Sounds expensive.

Count the total number of ticks on the cpu (I think we already have
that) and subtract the number of ticks in this cgroup (I think we also
already have that), which should yield: number of ticks not in this
cgroup, aka number of ticks this cgroup was idle.
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