Commit e5e678e4 authored by Rik van Riel's avatar Rik van Riel Committed by Peter Zijlstra
Browse files

sched,fair: Skip newidle_balance if a wakeup is pending 



The try_to_wake_up function has an optimization where it can queue
a task for wakeup on its previous CPU, if the task is still in the
middle of going to sleep inside schedule().

Once schedule() re-enables IRQs, the task will be woken up with an
IPI, and placed back on the runqueue.

If we have such a wakeup pending, there is no need to search other
CPUs for runnable tasks. Just skip (or bail out early from) newidle
balancing, and run the just woken up task.

For a memcache like workload test, this reduces total CPU use by
about 2%, proportionally split between user and system time,
and p99 and p95 application response time by 10% on average.
The schedstats run_delay number shows a similar improvement.

Signed-off-by: default avatarRik van Riel <riel@surriel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Acked-by: default avatarMel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210422130236.0bb353df@imladris.surriel.com
parent 2b8ca1a9

kernel/sched/fair.c

+10 −1
Original line number Diff line number Diff line
@@ -10592,6 +10592,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) 
	u64 curr_cost = 0;



	update_misfit_status(NULL, this_rq);



	/*

	 * There is a task waiting to run. No need to search for one.

	 * Return 0; the task will be enqueued when switching to idle.

	 */

	if (this_rq->ttwu_pending)

		return 0;



	/*

	 * We must set idle_stamp _before_ calling idle_balance(), such that we

	 * measure the duration of idle_balance() as idle time.
@@ -10657,7 +10665,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) 
		 * Stop searching for tasks to pull if there are

		 * now runnable tasks on this rq.

		 */

		if (pulled_task || this_rq->nr_running > 0)

		if (pulled_task || this_rq->nr_running > 0 ||

		    this_rq->ttwu_pending)

			break;

	}

	rcu_read_unlock();