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Date: Wed, 17 May 2017 16:20:10 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
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        Morten Rasmussen <morten.rasmussen@arm.com>,
        Ben Segall <bsegall@google.com>, Yuyang Du <yuyang.du@intel.com>
Subject: Re: [RFC][PATCH 03/14] sched/fair: Remove se->load.weight from
 se->avg.load_sum
Message-ID: <20170517142010.pooe6iipiv43ubul@hirez.programming.kicks-ass.net>
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On Wed, May 17, 2017 at 11:50:45AM +0200, Vincent Guittot wrote:
> Le Wednesday 17 May 2017 à 09:04:47 (+0200), Vincent Guittot a écrit :

> > I wonder if there is a problem with this new way to compute se's
> > load_avg and cfs_rq's load_avg when a task changes is nice prio before
> > migrating to another CPU.
> > 
> > se load_avg is now: runnable x current weight
> > but cfs_rq load_avg keeps the history of the previous weight of the se
> > When we detach se, we will remove an up to date se's load_avg from
> > cfs_rq which doesn't have the up to date load_avg in its own load_avg.
> > So if se's prio decreases just before migrating, some load_avg stays
> > in prev cfs_rq and if se's prio increases, we will remove too much
> > load_avg and possibly make the cfs_rq load_avg null whereas other
> > tasks are running.
> > 
> > Thought ?
> > 
> > I'm able to reproduce the problem with a simple rt-app use case (after
> > adding a new feature in rt-app)
> > 
> > Vincent
> >
> 
> The hack below fixes the problem I mentioned above. It applies on task what is
> done when updating group_entity's weight

Right. I didn't think people much used nice so I skipped it for now. Yes
your patch looks about right for that.

I'll include it, thanks!