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sched/fair: Reduce long-tail newly idle balance cost 

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.

Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1614154549-116078-1-git-send-email-aubrey.li@intel.com
This commit is contained in:
Aubrey Li 2021-02-24 16:15:49 +08:00 committed by Peter Zijlstra
parent c8987ae5af
commit acb4decc1e
1 changed files with 9 additions and 0 deletions
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9
kernel/sched/fair.c
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@ -7687,6 +7687,15 @@ 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;