Re: [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

[Li, Aubrey]

On 2021/2/24 16:15, Aubrey Li wrote:
> A long-tail load balance cost is observed on the newly idle path,
> this is caused by a race window between the first nr_running check
> of the busiest runqueue and its nr_running recheck in detach_tasks.
> 
> Before the busiest runqueue is locked, the tasks on the busiest
> runqueue could be pulled by other CPUs and nr_running of the busiest
> runqueu becomes 1 or even 0 if the running task becomes idle, this
> causes detach_tasks breaks with LBF_ALL_PINNED flag set, and triggers
> load_balance redo at the same sched_domain level.
> 
> In order to find the new busiest sched_group and CPU, load balance will
> recompute and update the various load statistics, which eventually leads
> to the long-tail load balance cost.
> 
> This patch clears LBF_ALL_PINNED flag for this race condition, and hence
> reduces the long-tail cost of newly idle balance.

Ping...

> 
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04a3ce2..5c67804 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7675,6 +7675,15 @@ static int detach_tasks(struct lb_env *env)
>  
>  	lockdep_assert_held(&env->src_rq->lock);
>  
> +	/*
> +	 * Source run queue has been emptied by another CPU, clear
> +	 * LBF_ALL_PINNED flag as we will not test any task.
> +	 */
> +	if (env->src_rq->nr_running <= 1) {
> +		env->flags &= ~LBF_ALL_PINNED;
> +		return 0;
> +	}
> +
>  	if (env->imbalance <= 0)
>  		return 0;
>  
>