linux-kernel - [PATCH 20/24] sched/fair: Avoid re-setting virtual deadline on migrations

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Message-Id: <20240727105030.625119246@infradead.org>
Date: Sat, 27 Jul 2024 12:27:52 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...hat.com,
 peterz@...radead.org,
 juri.lelli@...hat.com,
 vincent.guittot@...aro.org,
 dietmar.eggemann@....com,
 rostedt@...dmis.org,
 bsegall@...gle.com,
 mgorman@...e.de,
 vschneid@...hat.com,
 linux-kernel@...r.kernel.org
Cc: kprateek.nayak@....com,
 wuyun.abel@...edance.com,
 youssefesmat@...omium.org,
 tglx@...utronix.de,
 efault@....de
Subject: [PATCH 20/24] 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: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/sched.h   |    6 ++++--
 kernel/sched/fair.c     |   23 ++++++++++++++++++-----
 kernel/sched/features.h |    4 ++++
 3 files changed, 26 insertions(+), 7 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -542,8 +542,10 @@ 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;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5270,6 +5270,12 @@ place_entity(struct cfs_rq *cfs_rq, stru
 
 	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
@@ -5382,6 +5388,8 @@ static __always_inline void return_cfs_r
 static bool
 dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
+	bool sleep = flags & DEQUEUE_SLEEP;
+
 	if (flags & DEQUEUE_DELAYED) {
 		/*
 		 * DEQUEUE_DELAYED is typically called from pick_next_entity()
@@ -5391,19 +5399,18 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
 		SCHED_WARN_ON(!se->sched_delayed);
 		se->sched_delayed = 0;
 	} 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);
 		update_curr(cfs_rq);
 
-		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;
@@ -5434,6 +5441,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
 	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;
@@ -13024,6 +13036,7 @@ static void switched_from_fair(struct rq
 	 * over the excursion into the new class.
 	 */
 	p->se.vlag = 0;
+	p->se.rel_deadline = 0;
 }
 
 static void switched_to_fair(struct rq *rq, struct task_struct *p)
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -10,6 +10,10 @@ SCHED_FEAT(PLACE_LAG, true)
  */
 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.
  */