linux-kernel - Re: [PATCH] sched/fair : Improve update_sd_pick

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Message-ID: <20200108114310.GD2844@hirez.programming.kicks-ass.net>
Date:   Wed, 8 Jan 2020 12:43:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair : Improve update_sd_pick_busiest for spare
 capacity case

On Fri, Dec 20, 2019 at 12:04:53PM +0100, Vincent Guittot wrote:
> Similarly to calculate_imbalance() and find_busiest_group(), using the
> number of idle CPUs when there is only 1 CPU in the group is not efficient
> because we can't make a difference between a CPU running 1 task and a CPU
> running dozens of small tasks competing for the same CPU but not enough
> to overload it. More generally speaking, we should use the number of
> running tasks when there is the same number of idle CPUs in a group instead
> of blindly select the 1st one.
> 
> When the groups have spare capacity and the same number of idle CPUs, we
> compare the number of running tasks to select the busiest group.
> 
Thanks!