sched/fair: Optimize dequeue_task_fair()

While looking at enqueue_task_fair and dequeue_task_fair, it occurred
to me that dequeue_task_fair can also be optimized as Vincent described
in commit 7d148be69e3a ("sched/fair: Optimize enqueue_task_fair()").

When encountering throttled cfs_rq, dequeue_throttle label can ensure
se not to be NULL, and rq->nr_running remains unchanged, so we can also
skip the early balance check.

Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/701eef9a40de93dcf5fe7063fd607bca5db38e05.1592287263.git.rocking@linux.alibaba.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a63f400..b9b9f19 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5624,14 +5624,14 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 
        }
 
-dequeue_throttle:
-       if (!se)
-               sub_nr_running(rq, 1);
+       /* At this point se is NULL and we are at root level*/
+       sub_nr_running(rq, 1);
 
        /* balance early to pull high priority tasks */
        if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
                rq->next_balance = jiffies;
 
+dequeue_throttle:
        util_est_dequeue(&rq->cfs, p, task_sleep);
        hrtick_update(rq);
 }