linux-kernel - Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

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Message-ID: <20160927192404.GC16071@codeblueprint.co.uk>
Date:   Tue, 27 Sep 2016 20:24:04 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Yuyang Du <yuyang.du@...el.com>
Subject: Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

On Tue, 27 Sep, at 02:48:31PM, Dietmar Eggemann wrote:
> 
> I think Matt is talking about the fact that the cfs->runnable_load_avg
> value is 0 once the hackbench task is initially dequeued.
 
Yes.

> Without this patch the value of se->avg.load_avg (e.g. both times 1002)
> is exactly the same when we add it to cfs_rq->runnable_load_avg in
> enqueue_entity_load_avg() and when we subtract it in
> dequeue_entity_load_avg(). That's because the initial runtime is short
> (~250us on my hikey board).
> 
> With this patch we add 1024 and subtract ~1002 which lets
> cfs_rq->runnable_load_avg still have a small positive value. This
> favours that for the next hackbench task another cpu will be chosen in
> (load-based) fork-balance.
 
Bingo, that's exactly it. Sorry if i was unclear.