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

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Message-ID: <20161004201607.GQ16071@codeblueprint.co.uk>
Date:   Tue, 4 Oct 2016 21:16:07 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        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 Wed, 28 Sep, at 04:46:06AM, Vincent Guittot wrote:
> 
> ok so i'm a bit confused there
> my understand of your explanation above  is that now we left a small
> amount of load in runnable_load_avg after the dequeue so another cpu
> will be chosen. But this explanation seems to be the opposite of what
> Matt said in a previous email that:
> "The performance drop comes from the fact that enqueueing/dequeueing a
> task with load 1002 during fork() results in a zero runnable_load_avg,
> which signals to the load balancer that the CPU is idle, so the next
> time we fork() we'll pick the same CPU to enqueue on -- and the cycle
> continues."

Right, we want to avoid the performance drop, which we can do by
leaving a small amount of load in runnable_load_avg. I think Dietmar
and me are saying the same thing.