linux-kernel - Re: [PATCH 4/4] sched/fair: reduce busy load balance interval

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Date:   Tue, 15 Sep 2020 20:04:56 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/fair: reduce busy load balance interval


On 14/09/20 11:03, Vincent Guittot wrote:
> The busy_factor, which increases load balance interval when a cpu is busy,
> is set to 32 by default. This value generates some huge LB interval on
> large system like the THX2 made of 2 node x 28 cores x 4 threads.
> For such system, the interval increases from 112ms to 3584ms at MC level.
> And from 228ms to 7168ms at NUMA level.
>
> Even on smaller system, a lower busy factor has shown improvement on the
> fair distribution of the running time so let reduce it for all.
>

ISTR you mentioned taking this one step further and making
(interval * busy_factor) scale logarithmically with the number of CPUs to
avoid reaching outrageous numbers. Did you experiment with that already?

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/topology.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 1a84b778755d..a8477c9e8569 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1336,7 +1336,7 @@ sd_init(struct sched_domain_topology_level *tl,
>       *sd = (struct sched_domain){
>               .min_interval		= sd_weight,
>               .max_interval		= 2*sd_weight,
> -		.busy_factor		= 32,
> +		.busy_factor		= 16,
>               .imbalance_pct		= 117,
>
>               .cache_nice_tries	= 0,