The following commit has been merged into the sched/core branch of tip:
Commit-ID: acb4decc1e900468d51b33c5f1ee445278e716a7
Gitweb: https://git.kernel.org/tip/acb4decc1e900468d51b33c5f1ee445278e716a7
Author: Aubrey Li <aubrey.li@xxxxxxxxx>
AuthorDate: Wed, 24 Feb 2021 16:15:49 +08:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 23 Mar 2021 16:01:59 +01:00
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@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/1614154549-116078-1-git-send-email-aubrey.li@xxxxxxxxx
---
kernel/sched/fair.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index aaa0dfa..6d73bdb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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;
|