linux-kernel - Re: [PATCH v5 00/11] per-cgroup cpu-stat

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Date:	Tue, 22 Jan 2013 17:53:59 -0800
From:	Colin Cross <ccross@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Glauber Costa <glommer@...allels.com>, cgroups@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v5 00/11] per-cgroup cpu-stat

On Tue, Jan 22, 2013 at 5:02 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Mon, Jan 21, 2013 at 04:14:27PM +0400, Glauber Costa wrote:
>> > Android userspace is currently using both cpu and cpuacct, and not
>> > co-mounting them.  They are used for fundamentally different uses such
>> > that creating a single hierarchy for both of them while maintaining
>> > the existing behavior is not possible.
>> >
>> > We use the cpu cgroup primarily as a priority container.  A simple
>> > view is that each thread is assigned to a foreground cgroup when it is
>> > user-visible, and a background cgroup when it is not.  The foreground
>> > cgroup is assigned a significantly higher cpu.shares value such that
>> > when each group is fully loaded the background group will get 5% and
>> > the foreground group will get 95%.
>> >
>> > We use the cpuacct cgroup to measure cpu usage per uid, primarily to
>> > estimate one cause of battery usage.  Each uid gets a cgroup, and when
>> > spawning a task for a new uid we put it in the appropriate cgroup.
>>
>> As we are all in a way sons of Linus the Great, the fact that you have
>> this usecase should be by itself a reason for us not to deprecate it.
>>
>> I still view this, however, as a not common use case. And from the
>> scheduler PoV, we still have all the duplicate hierarchy walks. So
>> assuming we would carry on all the changes in this patchset, except the
>> deprecation, would it be okay for you?
>>
>> This way we could take steps to make sure the scheduler codepaths for
>> cpuacct are not taking during normal comounted operation, and you could
>> still have your setup unchanged.
>>
>> Tejun, any words here?
>
> I think the only thing we can do is keeping cpuacct around.  We can
> still optimize comounted cpu and cpuacct as the usual case.  That
> said, I'd really like to avoid growing new use cases for separate
> hierarchies for cpu and cpuacct (well, any controller actually).
> Having multiple hierarchies is fundamentally broken in that we can't
> say whether a given resource belongs to certain cgroup independently
> from the current task, and we're definitnely moving towards unified
> hierarchy.

I understand why it makes sense from a code perspective to combine cpu
and cpuacct, but by combining them you are enforcing a strange
requirement that to measure the cpu usage of a group of processes you
force them to be treated as a single scheduling entity by their parent
group, effectively splitting their time as if they were a single task.
 That doesn't make any sense to me.

> We are not gonna break multiple hierarchies but won't go extra miles
> to optimize or enable new features on it, so it would be best to move
> away from it.

I don't see how I can move away from it with the current design.

> Maybe we can generate a warning message on separate mounts?
>
> Thanks.
>
> --
> tejun
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