linux-kernel - Re: [PATCH 3/3] cgroup: Implement cgroup2 basic CPU usage accounting

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Date:   Tue, 29 Aug 2017 11:32:35 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc:     lizefan@...wei.com, hannes@...xchg.org, mingo@...hat.com,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        efault@....de, torvalds@...ux-foundation.org, guro@...com
Subject: Re: [PATCH 3/3] cgroup: Implement cgroup2 basic CPU usage accounting

On 08/29/2017 11:24 AM, Tejun Heo wrote:
> Hello, Peter.
>
> On Tue, Aug 29, 2017 at 04:32:52PM +0200, Peter Zijlstra wrote:
>> So I mostly like. On accounting it only adds to the immediate cgroup (if
>> it has a parent, aka !root).
>>
>> On update it does a DFS of all sub-groups and propagates the deltas up
>> to the requested group.
> ...
>> What I don't get is why you need cgroup_cpu_stat_updated(). That is, I
>> see you use it to keep the keep the DFS 'stack' up-to-date, but what I
>> don't see is why you'd need that.
> That is to make reading stats O(number of descendants which have been
> active since last read) instad of O(number of all descendants) as
> there can be a lot of not-too-active cgroups in a system.  Stat
> reading can be frequent, so the combination can get really bad.  By
> keeping the updated list separate, increasing read frequency decreases
> the cost of each read.
>
> Also, please note that a system may end up with a lot of cgroups
> without the user intending to.  memcg drains removed cgroups lazily
> and the number of draining cgroups can reach very high numbers if the
> system isn't under memory pressure.  The plan is to add basic stats
> for other resources too and keeping it scalable w.r.t. idle cgroups
> allows using the same mechanism for all resources.

I think it will be good to put the rationale behind this function as a
comment in the code.

Cheers,
Longman