[PATCH 3/3] sched/core: Indicate a sched_delayed task was migrated before wakeup

[K Prateek Nayak]

Since sched_delayed tasks remain on "rq->cfs_tasks" list even after
blocking, they can be migrated from one runqueue to another in a delayed
state by the load balancer. When they are eventually requeued or woken
up on the same CPU via the try_to_wake_up() path, the eventual
activation is clueless about the migration. This trips the PSI
accounting since, in terms of enqueue flags, PSI only considers the
following as a wakeup for PSI accounting:

    (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED)

This can lead inconsistent PSI task state splat similar to:

    psi: inconsistent task state! task=... cpu=... psi_flags=4 clear=. set=4 # Without partial fixup from this patch
    psi: inconsistent task state! task=... cpu=... psi_flags=0 clear=4 set=1 # With partial fixup from this patch

Tracking the PSI changes along with task's state revealed the following
series of events:

    psi_task_switch: psi_flags=4 clear=4 set=1 # sched_delayed is set to 1
    psi_dequeue:     psi_flags=1 clear=1 set=0 # dequeued for migration
    psi_enqueue:     psi_flags=0 clear=0 set=4 # enqueued after migration
    psi_enqueue:     psi_flags=4 clear=1 set=4 # wakeup after migration
    psi: inconsistent task state! task=... cpu=... psi_flags=4 clear=1 set=4

Moving psi_enqueue() to after "p->sched_class->enqueue_task()" and
skipping enqueue until the delayed task is actually woken up (referred
to partial fixup previously) changes the above scenario to the
following:

    psi_task_switch: psi_flags=4 clear=4 set=1 # sched_delayed is set to 1
    psi_dequeue:     psi_flags=1 clear=1 set=0 # dequeued for migration
    psi_enqueue:     psi_flags=0 clear=0 set=0 # enqueued after migration, sched delayed
    psi_enqueue:     psi_flags=0 clear=1 set=4 # wakeup of delayed task
    psi: inconsistent task state! task=... cpu=... psi_flags=0 clear=1 set=4

psi_enqueue() tries to clear the TSK_IOWAIT since it believes the task
has not migrated due to the lack of ENQUEUE_MIGRATED flag in case of a
requeue or a full wakeup on "p->wake_cpu", but in-fact TSK_IOWAIT was
cleared during dequeue for migration and was never set again.

Define "DELAYED_MIGRATED" and set it in "p->migration_flags" when a
delayed task is migrated. This flag is consumed when the delayed entity
is finally woken up, and psi_enqueue() is notified of the migration.

Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Closes: https://lore.kernel.org/lkml/20240830123458.3557-1-spasswolf@xxxxxx/
Closes: https://lore.kernel.org/all/cd67fbcd-d659-4822-bb90-7e8fbb40a856@xxxxxxxxxxxxx/
Link: https://lore.kernel.org/lkml/f82def74-a64a-4a05-c8d4-4eeb3e03d0c0@xxxxxxx/
Tested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
 kernel/sched/core.c  | 19 ++++++++++++++++++-
 kernel/sched/sched.h |  1 +
 kernel/sched/stats.h | 10 ++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 52be38021ebb..1a353fa69a54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2009,12 +2009,19 @@ unsigned long get_wchan(struct task_struct *p)
 
 void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 {
+	bool wakee_not_migrated = (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED);
+
 	if (!(flags & ENQUEUE_NOCLOCK))
 		update_rq_clock(rq);
 
 	if (!(flags & ENQUEUE_RESTORE)) {
 		sched_info_enqueue(rq, p);
-		psi_enqueue(p, (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED));
+
+		/* Notify PSI that the task was migrated in a delayed state before wakeup. */
+		if ((p->migration_flags & DELAYED_MIGRATED) && !task_on_rq_migrating(p)) {
+			wakee_not_migrated = false;
+			p->migration_flags &= ~DELAYED_MIGRATED;
+		}
 	}
 
 	p->sched_class->enqueue_task(rq, p, flags);
@@ -2023,6 +2030,8 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 	 * ->sched_delayed.
 	 */
 	uclamp_rq_inc(rq, p);
+	if (!(flags & ENQUEUE_RESTORE))
+		psi_enqueue(p, wakee_not_migrated);
 
 	if (sched_core_enabled(rq))
 		sched_core_enqueue(rq, p);
@@ -2042,6 +2051,14 @@ inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
 	if (!(flags & DEQUEUE_SAVE)) {
 		sched_info_dequeue(rq, p);
 		psi_dequeue(p, flags & DEQUEUE_SLEEP);
+
+		/*
+		 * Indicate that a sched_delayed task was migrated.
+		 * enqueue_task() needs this for correct accounting
+		 * when the delayed task eventually wakes up.
+		 */
+		if (p->se.sched_delayed && task_on_rq_migrating(p))
+			p->migration_flags |= DELAYED_MIGRATED;
 	}
 
 	/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b1c3588a8f00..2dc2c4cb4f5f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1326,6 +1326,7 @@ static inline int cpu_of(struct rq *rq)
 }
 
 #define MDF_PUSH		0x01
+#define DELAYED_MIGRATED	0x02 /* Task was migrated when in DELAYED_DEQUEUE state */
 
 static inline bool is_migration_disabled(struct task_struct *p)
 {
diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index 237780aa3c53..06a2c6d3ec1e 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -129,6 +129,13 @@ static inline void psi_enqueue(struct task_struct *p, bool wakeup)
 	if (static_branch_likely(&psi_disabled))
 		return;
 
+	/*
+	 * Delayed task is not ready to run yet!
+	 * Wait for a requeue before accounting.
+	 */
+	if (p->se.sched_delayed)
+		return;
+
 	if (p->in_memstall)
 		set |= TSK_MEMSTALL_RUNNING;
 
@@ -148,6 +155,9 @@ static inline void psi_dequeue(struct task_struct *p, bool sleep)
 	if (static_branch_likely(&psi_disabled))
 		return;
 
+	/* Delayed task can only be dequeued for migration. */
+	WARN_ON_ONCE(p->se.sched_delayed && sleep);
+
 	/*
 	 * A voluntary sleep is a dequeue followed by a task switch. To
 	 * avoid walking all ancestors twice, psi_task_switch() handles
-- 
2.34.1