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

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Date:   Tue, 29 Aug 2017 17:59:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     lizefan@...wei.com, hannes@...xchg.org, mingo@...hat.com,
        longman@...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 Tue, Aug 29, 2017 at 08:24:27AM -0700, 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.

Hmm, and when its never read, we only do the O(depth) thing _once_,
right?