[tip: sched/core] sched: Teach dequeue_task() about special task states

["tip-bot2 for Peter Zijlstra" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     e1459a50ba31831efdfc35278023d959e4ba775b
Gitweb:        https://git.kernel.org/tip/e1459a50ba31831efdfc35278023d959e4ba775b
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Mon, 01 Jul 2024 21:38:11 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 17 Aug 2024 11:06:44 +02:00

sched: Teach dequeue_task() about special task states

Since special task states must not suffer spurious wakeups, and the
proposed delayed dequeue can cause exactly these (under some boundary
conditions), propagate this knowledge into dequeue_task() such that it
can do the right thing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105030.110439521@infradead.org
---
 kernel/sched/core.c  | 7 ++++++-
 kernel/sched/sched.h | 3 ++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 80e639e..868b71b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6530,11 +6530,16 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 		if (signal_pending_state(prev_state, prev)) {
 			WRITE_ONCE(prev->__state, TASK_RUNNING);
 		} else {
+			int flags = DEQUEUE_NOCLOCK;
+
 			prev->sched_contributes_to_load =
 				(prev_state & TASK_UNINTERRUPTIBLE) &&
 				!(prev_state & TASK_NOLOAD) &&
 				!(prev_state & TASK_FROZEN);
 
+			if (unlikely(is_special_task_state(prev_state)))
+				flags |= DEQUEUE_SPECIAL;
+
 			/*
 			 * __schedule()			ttwu()
 			 *   prev_state = prev->state;    if (p->on_rq && ...)
@@ -6546,7 +6551,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 			 *
 			 * After this, schedule() must not care about p->state any more.
 			 */
-			block_task(rq, prev, DEQUEUE_NOCLOCK);
+			block_task(rq, prev, flags);
 		}
 		switch_count = &prev->nvcsw;
 	}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ffca977..263b4de 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2248,10 +2248,11 @@ extern const u32		sched_prio_to_wmult[40];
  *
  */
 
-#define DEQUEUE_SLEEP		0x01
+#define DEQUEUE_SLEEP		0x01 /* Matches ENQUEUE_WAKEUP */
 #define DEQUEUE_SAVE		0x02 /* Matches ENQUEUE_RESTORE */
 #define DEQUEUE_MOVE		0x04 /* Matches ENQUEUE_MOVE */
 #define DEQUEUE_NOCLOCK		0x08 /* Matches ENQUEUE_NOCLOCK */
+#define DEQUEUE_SPECIAL		0x10
 #define DEQUEUE_MIGRATING	0x100 /* Matches ENQUEUE_MIGRATING */
 #define DEQUEUE_DELAYED		0x200 /* Matches ENQUEUE_DELAYED */