Commit 8039e96f authored by Vineeth Pillai's avatar Vineeth Pillai Committed by Peter Zijlstra
Browse files

sched/fair: Fix forced idle sibling starvation corner case 



If there is only one long running local task and the sibling is
forced idle, it  might not get a chance to run until a schedule
event happens on any cpu in the core.

So we check for this condition during a tick to see if a sibling
is starved and then give it a chance to schedule.

Signed-off-by: default avatarVineeth Pillai <viremana@linux.microsoft.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarDon Hiatt <dhiatt@digitalocean.com>
Tested-by: default avatarHongyu Ning <hongyu.ning@linux.intel.com>
Tested-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210422123308.617407840@infradead.org
parent 539f6512

kernel/sched/core.c

+8 −7
Original line number Diff line number Diff line
@@ -5459,16 +5459,15 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) 


	/* reset state */

	rq->core->core_cookie = 0UL;

	if (rq->core->core_forceidle) {

		need_sync = true;

		rq->core->core_forceidle = false;

	}

	for_each_cpu(i, smt_mask) {

		struct rq *rq_i = cpu_rq(i);



		rq_i->core_pick = NULL;



		if (rq_i->core_forceidle) {

			need_sync = true;

			rq_i->core_forceidle = false;

		}



		if (i != cpu)

			update_rq_clock(rq_i);

	}
@@ -5588,8 +5587,10 @@ next_class:; 
		if (!rq_i->core_pick)

			continue;



		if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running)

			rq_i->core_forceidle = true;

		if (is_task_rq_idle(rq_i->core_pick) && rq_i->nr_running &&

		    !rq_i->core->core_forceidle) {

			rq_i->core->core_forceidle = true;

		}



		if (i == cpu) {

			rq_i->core_pick = NULL;

kernel/sched/fair.c

+40 −0
Original line number Diff line number Diff line
@@ -10767,6 +10767,44 @@ static void rq_offline_fair(struct rq *rq) 


#endif /* CONFIG_SMP */



#ifdef CONFIG_SCHED_CORE

static inline bool

__entity_slice_used(struct sched_entity *se, int min_nr_tasks)

{

	u64 slice = sched_slice(cfs_rq_of(se), se);

	u64 rtime = se->sum_exec_runtime - se->prev_sum_exec_runtime;



	return (rtime * min_nr_tasks > slice);

}



#define MIN_NR_TASKS_DURING_FORCEIDLE	2

static inline void task_tick_core(struct rq *rq, struct task_struct *curr)

{

	if (!sched_core_enabled(rq))

		return;



	/*

	 * If runqueue has only one task which used up its slice and

	 * if the sibling is forced idle, then trigger schedule to

	 * give forced idle task a chance.

	 *

	 * sched_slice() considers only this active rq and it gets the

	 * whole slice. But during force idle, we have siblings acting

	 * like a single runqueue and hence we need to consider runnable

	 * tasks on this cpu and the forced idle cpu. Ideally, we should

	 * go through the forced idle rq, but that would be a perf hit.

	 * We can assume that the forced idle cpu has atleast

	 * MIN_NR_TASKS_DURING_FORCEIDLE - 1 tasks and use that to check

	 * if we need to give up the cpu.

	 */

	if (rq->core->core_forceidle && rq->cfs.nr_running == 1 &&

	    __entity_slice_used(&curr->se, MIN_NR_TASKS_DURING_FORCEIDLE))

		resched_curr(rq);

}

#else

static inline void task_tick_core(struct rq *rq, struct task_struct *curr) {}

#endif



/*

 * scheduler tick hitting a task of our scheduling class.

 *
@@ -10790,6 +10828,8 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued) 


	update_misfit_status(curr, rq);

	update_overutilized_status(task_rq(curr));



	task_tick_core(rq, curr);

}



/*

kernel/sched/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -1083,12 +1083,12 @@ struct rq { 
	unsigned int		core_enabled;

	unsigned int		core_sched_seq;

	struct rb_root		core_tree;

	unsigned char		core_forceidle;



	/* shared state */

	unsigned int		core_task_seq;

	unsigned int		core_pick_seq;

	unsigned long		core_cookie;

	unsigned char		core_forceidle;

#endif

};