Re: [PATCH 3/4] sched/fair: minimize concurrent LBs between domain level

[Valentin Schneider <valentin.schneider@arm.com>]

On 14/09/20 11:03, Vincent Guittot wrote:
> sched domains tend to trigger simultaneously the load balance loop but
> the larger domains often need more time to collect statistics. This
> slowness makes the larger domain trying to detach tasks from a rq whereas
> tasks already migrated somewhere else at a sub-domain level. This is not
> a real problem for idle LB because the period of smaller domains will
> increase with its CPUs being busy and this will let time for higher ones
> to pulled tasks. But this becomes a problem when all CPUs are already busy
> because all domains stay synced when they trigger their LB.
>
> A simple way to minimize simultaneous LB of all domains is to decrement the
> the busy interval by 1 jiffies. Because of the busy_factor, the interval of
> larger domain will not be a multiple of smaller ones anymore.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 765be8273292..7d7eefd8e2d4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9780,6 +9780,9 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)
>
>       /* scale ms to jiffies */
>       interval = msecs_to_jiffies(interval);

A comment here would be nice, I think. What about:

/*
 * Reduce likelihood of (busy) balancing at higher domains racing with
 * balancing at lower domains by preventing their balancing periods from being
 * multiples of each other.
 */

> +	if (cpu_busy)
> +		interval -= 1;
> +
>       interval = clamp(interval, 1UL, max_load_balance_interval);
>
>       return interval;