Commit 51bf903b authored by Chengming Zhou's avatar Chengming Zhou Committed by Peter Zijlstra
Browse files

sched/fair: Optimize and simplify rq leaf_cfs_rq_list 



We notice the rq leaf_cfs_rq_list has two problems when do bugfix
backports and some test profiling.

1. cfs_rqs under throttled subtree could be added to the list, and
   make their fully decayed ancestors on the list, even though not needed.

2. #1 also make the leaf_cfs_rq_list management complex and error prone,
   this is the list of related bugfix so far:

   commit 31bc6aea ("sched/fair: Optimize update_blocked_averages()")
   commit fe61468b ("sched/fair: Fix enqueue_task_fair warning")
   commit b34cb07d ("sched/fair: Fix enqueue_task_fair() warning some more")
   commit 39f23ce0 ("sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list")
   commit 0258bdfa ("sched/fair: Fix unfairness caused by missing load decay")
   commit a7b359fc ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
   commit fdaba61e ("sched/fair: Ensure that the CFS parent is added after unthrottling")
   commit 2630cde2 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")

commit 31bc6aea ("sched/fair: Optimize update_blocked_averages()")
delete every cfs_rq under throttled subtree from rq->leaf_cfs_rq_list,
and delete the throttled_hierarchy() test in update_blocked_averages(),
which optimized update_blocked_averages().

But those later bugfix add cfs_rqs under throttled subtree back to
rq->leaf_cfs_rq_list again, with their fully decayed ancestors, for
the integrity of rq->leaf_cfs_rq_list.

This patch takes another method, skip all cfs_rqs under throttled
hierarchy when list_add_leaf_cfs_rq(), to completely make cfs_rqs
under throttled subtree off the leaf_cfs_rq_list.

So we don't need to consider throttled related things in
enqueue_entity(), unthrottle_cfs_rq() and enqueue_task_fair(),
which simplify the code a lot. Also optimize update_blocked_averages()
since cfs_rqs under throttled hierarchy and their ancestors
won't be on the leaf_cfs_rq_list.

Signed-off-by: default avatarChengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20220601021848.76943-1-zhouchengming@bytedance.com
parent f5b2eeb4

kernel/sched/fair.c

+28 −64
Original line number Diff line number Diff line
@@ -3179,6 +3179,8 @@ void reweight_task(struct task_struct *p, int prio) 
	load->inv_weight = sched_prio_to_wmult[prio];

}



static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);



#ifdef CONFIG_FAIR_GROUP_SCHED

#ifdef CONFIG_SMP

/*
@@ -3289,8 +3291,6 @@ static long calc_group_shares(struct cfs_rq *cfs_rq) 
}

#endif /* CONFIG_SMP */



static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);



/*

 * Recomputes the group entity based on the current state of its group

 * runqueue.
@@ -4403,16 +4403,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) 
		__enqueue_entity(cfs_rq, se);

	se->on_rq = 1;



	/*

	 * When bandwidth control is enabled, cfs might have been removed

	 * because of a parent been throttled but cfs->nr_running > 1. Try to

	 * add it unconditionally.

	 */

	if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())

		list_add_leaf_cfs_rq(cfs_rq);



	if (cfs_rq->nr_running == 1)

	if (cfs_rq->nr_running == 1) {

		check_enqueue_throttle(cfs_rq);

		if (!throttled_hierarchy(cfs_rq))

			list_add_leaf_cfs_rq(cfs_rq);

	}

}



static void __clear_buddies_last(struct sched_entity *se)
@@ -5027,11 +5022,18 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) 
	/* update hierarchical throttle state */

	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);



	/* Nothing to run but something to decay (on_list)? Complete the branch */

	if (!cfs_rq->load.weight) {

		if (cfs_rq->on_list)

			goto unthrottle_throttle;

		if (!cfs_rq->on_list)

			return;

		/*

		 * Nothing to run but something to decay (on_list)?

		 * Complete the branch.

		 */

		for_each_sched_entity(se) {

			if (list_add_leaf_cfs_rq(cfs_rq_of(se)))

				break;

		}

		goto unthrottle_throttle;

	}



	task_delta = cfs_rq->h_nr_running;
@@ -5069,31 +5071,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) 
		/* end evaluation on encountering a throttled cfs_rq */

		if (cfs_rq_throttled(qcfs_rq))

			goto unthrottle_throttle;



		/*

		 * One parent has been throttled and cfs_rq removed from the

		 * list. Add it back to not break the leaf list.

		 */

		if (throttled_hierarchy(qcfs_rq))

			list_add_leaf_cfs_rq(qcfs_rq);

	}



	/* At this point se is NULL and we are at root level*/

	add_nr_running(rq, task_delta);



unthrottle_throttle:

	/*

	 * The cfs_rq_throttled() breaks in the above iteration can result in

	 * incomplete leaf list maintenance, resulting in triggering the

	 * assertion below.

	 */

	for_each_sched_entity(se) {

		struct cfs_rq *qcfs_rq = cfs_rq_of(se);



		if (list_add_leaf_cfs_rq(qcfs_rq))

			break;

	}



	assert_list_leaf_cfs_rq(rq);



	/* Determine whether we need to wake up potentially idle CPU: */
@@ -5748,13 +5731,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) 
		/* end evaluation on encountering a throttled cfs_rq */

		if (cfs_rq_throttled(cfs_rq))

			goto enqueue_throttle;



               /*

                * One parent has been throttled and cfs_rq removed from the

                * list. Add it back to not break the leaf list.

                */

               if (throttled_hierarchy(cfs_rq))

                       list_add_leaf_cfs_rq(cfs_rq);

	}



	/* At this point se is NULL and we are at root level*/
@@ -5778,21 +5754,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) 
		update_overutilized_status(rq);



enqueue_throttle:

	if (cfs_bandwidth_used()) {

		/*

		 * When bandwidth control is enabled; the cfs_rq_throttled()

		 * breaks in the above iteration can result in incomplete

		 * leaf list maintenance, resulting in triggering the assertion

		 * below.

		 */

		for_each_sched_entity(se) {

			cfs_rq = cfs_rq_of(se);



			if (list_add_leaf_cfs_rq(cfs_rq))

				break;

		}

	}



	assert_list_leaf_cfs_rq(rq);



	hrtick_update(rq);
@@ -11316,9 +11277,13 @@ static inline bool vruntime_normalized(struct task_struct *p) 
 */

static void propagate_entity_cfs_rq(struct sched_entity *se)

{

	struct cfs_rq *cfs_rq;

	struct cfs_rq *cfs_rq = cfs_rq_of(se);



	list_add_leaf_cfs_rq(cfs_rq_of(se));

	if (cfs_rq_throttled(cfs_rq))

		return;



	if (!throttled_hierarchy(cfs_rq))

		list_add_leaf_cfs_rq(cfs_rq);



	/* Start to propagate at parent */

	se = se->parent;
@@ -11326,14 +11291,13 @@ static void propagate_entity_cfs_rq(struct sched_entity *se) 
	for_each_sched_entity(se) {

		cfs_rq = cfs_rq_of(se);



		if (!cfs_rq_throttled(cfs_rq)){

		update_load_avg(cfs_rq, se, UPDATE_TG);

			list_add_leaf_cfs_rq(cfs_rq);

			continue;

		}



		if (list_add_leaf_cfs_rq(cfs_rq))

		if (cfs_rq_throttled(cfs_rq))

			break;



		if (!throttled_hierarchy(cfs_rq))

			list_add_leaf_cfs_rq(cfs_rq);

	}

}

#else