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Date: Wed, 24 Aug 2016 03:09:10 +0800
From: Yuyang Du <yuyang.du@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Benjamin Segall <bsegall@google.com>, Paul Turner <pjt@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Mike Galbraith <umgwanakikbuti@gmail.com>
Subject: Re: [PATCH v1 00/10] Optimize sched avgs computation and implement
 flat util hierarchy
Message-ID: <20160823190910.GE3273@intel.com>
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On Tue, Aug 23, 2016 at 04:13:41PM +0200, Peter Zijlstra wrote:
> 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
> > 
> > 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.
> 
> Because while we migrate the load_avg on /root/level2, we do not
> propagate the load_avg up the hierarchy?
> 
> And always propagating everyrthing up will indeed also fix the
> utilization issue.

Yes, but for util it's actually irrespective to the number of
hirarchical levels, just propagating directly to the top cfs_rq
or simply rq will do. In other words, it's flat :)
 
> Of course, doing that propagation has its costs..
> 
> Didn't you post a patch doing just this a while ago?