linux-kernel - Re: [PATCH v1 00/10] Optimize sched avgs computation and implement flat util hierarchy

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Date:   Wed, 24 Aug 2016 03:01:12 +0800
From:   Yuyang Du <yuyang.du@...el.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH v1 00/10] Optimize sched avgs computation and implement
 flat util hierarchy

Hi Vincent,

On Tue, Aug 23, 2016 at 03:28:19PM +0200, Vincent Guittot wrote:
> I still wonder if using a flat util hierarchy is the right solution to
> solve this problem with utilization and task group. I have noticed
> exact same issues with load that generates weird task placement
> decision and i think that we should probably try to solve both wrong
> behavior with same mechanism. but this is not possible with flat
> hierarchy for load

I agree both util and load have the same hierarchical propagation
problem.

But util and load are different with respect to task group distribution
among CPUs and along hierarchical structure. Util is "fundamentally"
flat (CPU's util = tasks' util), so it's pretty natural as well as
simple to implement a flat hierarchy util. And because of that, I
feel util propagating up the hierarchical structure seems unnecessary.

It might be better to have a converged mechanism to solve both, but
it shouldn't be necessary. Right?

> Let me take an example.
> TA is a always running task on CPU1 in group /root/level1/
> TB wakes up on CPU0 and moves TA into group /root/level2/
> Even if TA stays on CPU1, runnable_load_avg of CPU1 root cfs rq will become 0.
> Then, TB forks a new task TC. TC will probably be schedule on CPU1
> because its root cfs_rq's runnable_load_avg is null and CPU1 is the
> next CPU after CPU0
> 
> Similar behavior can happen when TA migrates
> 
> Beside flat utilization consideration, i'm going to have a look at the

Many thanks.

Yuyang