linux-kernel - [tip: sched/core] sched/deadline: Prepare for switched

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Message-ID: <176060721804.709179.14529626426508592907.tip-bot2@tip-bot2>
Date: Thu, 16 Oct 2025 09:33:38 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/deadline: Prepare for switched_from() change

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

Commit-ID:     5e42d4c123ba9b89ce19b3aa7e22b7684cbfa49c
Gitweb:        https://git.kernel.org/tip/5e42d4c123ba9b89ce19b3aa7e22b7684cbfa49c
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 09 Oct 2025 16:09:25 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 16 Oct 2025 11:13:51 +02:00

sched/deadline: Prepare for switched_from() change

Prepare for the sched_class::switch*() methods getting folded into the
change pattern. As a result of that, the location of switched_from
will change slightly. SCHED_DEADLINE is affected by this change in
location:

  OLD                              NEW
				|
				|  switching_from()
  dequeue_task();		|  dequeue_task()
  put_prev_task();		|  put_prev_task()
				|  switched_from()
				|
  ... change task ...		|  ... change task ...
				|
  switching_to();		|  switching_to()
  enqueue_task();		|  enqueue_task()
  set_next_task();		|  set_next_task()
  prev_class->switched_from()	|
  switched_to()			|  switched_to()
				|

Notably, where switched_from() was called *after* the change to the
task, it will get called before it. Specifically, switched_from_dl()
uses dl_task(p) which uses p->prio; which is changed when switching
class (it might be the reason to switch class in case of PI).

When switched_from_dl() gets called, the task will have left the
deadline class and dl_task() must be false, while when doing
dequeue_dl_entity() the task must be a dl_task(), otherwise we'd have
called a different dequeue method.

Reported-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/deadline.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 933bd1f..fd147a7 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -405,7 +405,7 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se);
  * up, and checks if the task is still in the "ACTIVE non contending"
  * state or not (in the second case, it updates running_bw).
  */
-static void task_non_contending(struct sched_dl_entity *dl_se)
+static void task_non_contending(struct sched_dl_entity *dl_se, bool dl_task)
 {
 	struct hrtimer *timer = &dl_se->inactive_timer;
 	struct rq *rq = rq_of_dl_se(dl_se);
@@ -444,10 +444,10 @@ static void task_non_contending(struct sched_dl_entity *dl_se)
 		} else {
 			struct task_struct *p = dl_task_of(dl_se);
 
-			if (dl_task(p))
+			if (dl_task)
 				sub_running_bw(dl_se, dl_rq);
 
-			if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
+			if (!dl_task || READ_ONCE(p->__state) == TASK_DEAD) {
 				struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
 
 				if (READ_ONCE(p->__state) == TASK_DEAD)
@@ -2045,7 +2045,7 @@ static void dequeue_dl_entity(struct sched_dl_entity *dl_se, int flags)
 	 * or "inactive")
 	 */
 	if (flags & DEQUEUE_SLEEP)
-		task_non_contending(dl_se);
+		task_non_contending(dl_se, true);
 }
 
 static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
@@ -2970,7 +2970,7 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 	 * will reset the task parameters.
 	 */
 	if (task_on_rq_queued(p) && p->dl.dl_runtime)
-		task_non_contending(&p->dl);
+		task_non_contending(&p->dl, false);
 
 	/*
 	 * In case a task is setscheduled out from SCHED_DEADLINE we need to