Commit 82e9d045 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

sched/fair: Avoid re-setting virtual deadline on 'migrations' 



During OSPM24 Youssef noted that migrations are re-setting the virtual
deadline. Notably everything that does a dequeue-enqueue, like setting
nice, changing preferred numa-node, and a myriad of other random crap,
will cause this to happen.

This shouldn't be. Preserve the relative virtual deadline across such
dequeue/enqueue cycles.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <vschneid@redhat.com>
Tested-by: default avatarValentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105030.625119246@infradead.org
parent fc1892be

include/linux/sched.h

+4 −2
Original line number Diff line number Diff line
@@ -544,8 +544,10 @@ struct sched_entity { 
	u64				min_vruntime;



	struct list_head		group_node;

	unsigned int			on_rq;

	unsigned int			sched_delayed;

	unsigned char			on_rq;

	unsigned char			sched_delayed;

	unsigned char			rel_deadline;

					/* hole */



	u64				exec_start;

	u64				sum_exec_runtime;

kernel/sched/fair.c

+18 −5
Original line number Diff line number Diff line
@@ -5270,6 +5270,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) 


	se->vruntime = vruntime - lag;



	if (sched_feat(PLACE_REL_DEADLINE) && se->rel_deadline) {

		se->deadline += se->vruntime;

		se->rel_deadline = 0;

		return;

	}



	/*

	 * When joining the competition; the existing tasks will be,

	 * on average, halfway through their slice, as such start tasks
@@ -5382,23 +5388,24 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq); 
static bool

dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)

{

	bool sleep = flags & DEQUEUE_SLEEP;



	update_curr(cfs_rq);



	if (flags & DEQUEUE_DELAYED) {

		SCHED_WARN_ON(!se->sched_delayed);

	} else {

		bool sleep = flags & DEQUEUE_SLEEP;



		bool delay = sleep;

		/*

		 * DELAY_DEQUEUE relies on spurious wakeups, special task

		 * states must not suffer spurious wakeups, excempt them.

		 */

		if (flags & DEQUEUE_SPECIAL)

			sleep = false;

			delay = false;



		SCHED_WARN_ON(sleep && se->sched_delayed);

		SCHED_WARN_ON(delay && se->sched_delayed);



		if (sched_feat(DELAY_DEQUEUE) && sleep &&

		if (sched_feat(DELAY_DEQUEUE) && delay &&

		    !entity_eligible(cfs_rq, se)) {

			if (cfs_rq->next == se)

				cfs_rq->next = NULL;
@@ -5429,6 +5436,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) 
	clear_buddies(cfs_rq, se);



	update_entity_lag(cfs_rq, se);

	if (sched_feat(PLACE_REL_DEADLINE) && !sleep) {

		se->deadline -= se->vruntime;

		se->rel_deadline = 1;

	}



	if (se != cfs_rq->curr)

		__dequeue_entity(cfs_rq, se);

	se->on_rq = 0;
@@ -12992,6 +13004,7 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p) 
	if (p->se.sched_delayed) {

		dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP);

		p->se.sched_delayed = 0;

		p->se.rel_deadline = 0;

		if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)

			p->se.vlag = 0;

	}

kernel/sched/features.h

+4 −0
Original line number Diff line number Diff line
@@ -9,6 +9,10 @@ SCHED_FEAT(PLACE_LAG, true) 
 * Give new tasks half a slice to ease into the competition.

 */

SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)

/*

 * Preserve relative virtual deadline on 'migration'.

 */

SCHED_FEAT(PLACE_REL_DEADLINE, true)

/*

 * Inhibit (wakeup) preemption until the current task has either matched the

 * 0-lag point or until is has exhausted it's slice.