linux-kernel - Re: [PATCH] sched/fair: fix load_balance redo for null imbalance

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Message-ID: <20180907113748.GV24106@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Sep 2018 13:37:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, jhugo@...eaurora.org
Subject: Re: [PATCH] sched/fair: fix load_balance redo for null imbalance

On Fri, Sep 07, 2018 at 09:51:04AM +0200, Vincent Guittot wrote:
> It can happen that load_balance finds a busiest group and then a busiest rq
> but the calculated imbalance is in fact null.

Cute. Does that happen often?

> If the calculated imbalance is null, it's useless to try to find a busiest
> rq as no task will be migrated and we can return immediately.
> 
> This situation can happen with heterogeneous system or smp system when RT
> tasks are decreasing the capacity of some CPUs.

Is it the result of one of those "force_balance" conditions in
find_busiest_group() ? Should we not fix that to then return NULL
instead?