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

[Glauber Costa]

On 09/20/2011 09:58 AM, Peter Zijlstra wrote:
> 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.
No , no... remember steal time.

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