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.guit...@linaro.org>
> Cc: Mel Gorman <mgor...@techsingularity.net>
> Cc: Andi Kleen <a...@linux.intel.com>
> Cc: Tim Chen <tim.c.c...@linux.intel.com>
> Cc: Srinivas Pandruvada <srinivas.pandruv...@linux.intel.com>
> Cc: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Signed-off-by: Aubrey Li <aubrey...@linux.intel.com>
> ---
> 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;
>
>
