linux-kernel - [tip: sched/urgent] sched/deadline: Ensure get_prio

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Message-ID: <176851086404.510.18122560875480513196.tip-bot2@tip-bot2>
Date: Thu, 15 Jan 2026 21:01:04 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: K Prateek Nayak <kprateek.nayak@....com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: sched/urgent] sched/deadline: Ensure get_prio_dl() is up-to-date

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

Commit-ID:     375410bb9a403009a44af3cc7f087090da076e09
Gitweb:        https://git.kernel.org/tip/375410bb9a403009a44af3cc7f087090da076e09
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Tue, 06 Jan 2026 11:41:13 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 15 Jan 2026 21:57:52 +01:00

sched/deadline: Ensure get_prio_dl() is up-to-date

Pratheek tripped a WARN and noted the following issue:

> Inspecting the set of events that led to the warning being triggered
> showed the following:
>
>     systemd-1  [008] dN.31 ...: do_set_cpus_allowed: set_cpus_allowed begin!
>
>     systemd-1  [008] dN.31 ...: sched_change_begin: Begin!
>     systemd-1  [008] dN.31 ...: sched_change_begin: Before dequeue_task()!
>     systemd-1  [008] dN.31 ...: update_curr_dl_se: update_curr_dl_se: ENQUEUE_REPLENISH
>     systemd-1  [008] dN.31 ...: enqueue_dl_entity: enqueue_dl_entity: ENQUEUE_REPLENISH
>     systemd-1  [008] dN.31 ...: replenish_dl_entity: Replenish before: 14815760217
>     systemd-1  [008] dN.31 ...: replenish_dl_entity: Replenish after: 14816960047
>     systemd-1  [008] dN.31 ...: sched_change_begin: Before put_prev_task()!
>
>     systemd-1  [008] dN.31 ...: sched_change_end: Before enqueue_task()!
>     systemd-1  [008] dN.31 ...: sched_change_end: Before put_prev_task()!
>     systemd-1  [008] dN.31 ...: prio_changed_dl: Queuing pull task on prio change: 14815760217 -> 14816960047
>     systemd-1  [008] dN.31 ...: prio_changed_dl: Queuing balance callback!
>     systemd-1  [008] dN.31 ...: sched_change_end: End!
>
>     systemd-1  [008] dN.31 ...: do_set_cpus_allowed: set_cpus_allowed end!
>     systemd-1  [008] dN.21 ...: __schedule: Woops! Balance callback found!
>
> 1. sched_change_begin() from guard(sched_change) in
>    do_set_cpus_allowed() stashes the priority, which for the deadline
>    task, is "p->dl.deadline".
> 2. The dequeue of the deadline task replenishes the deadline.
> 3. The task is enqueued back after guard's scope ends and since there is
>    no *_CLASS flags set, sched_change_end() calls
>    dl_sched_class->prio_changed() which compares the deadline.
> 4. Since deadline was moved on dequeue, prio_changed_dl() sees the value
>    differ from the stashed value and queues a balance pull callback.
> 5. do_set_cpus_allowed() finishes and drops the rq_lock without doing a
>    do_balance_callbacks().
> 6. Grabbing the rq_lock() at subsequent __schedule() triggers the
>    warning since the balance pull callback was never executed before
>    dropping the lock.

Meaning get_prio_dl() ought to update current and return an up-to-date
value.

Fixes: 6455ad5346c9 ("sched: Move sched_class::prio_changed() into the change pattern")
Reported-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
Tested-by: K Prateek Nayak <kprateek.nayak@....com>
Link: https://patch.msgid.link/20260106104113.GX3707891@noisy.programming.kicks-ass.net
---
 kernel/sched/deadline.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index b5c19b1..b7acf74 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3296,6 +3296,12 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
 
 static u64 get_prio_dl(struct rq *rq, struct task_struct *p)
 {
+	/*
+	 * Make sure to update current so we don't return a stale value.
+	 */
+	if (task_current_donor(rq, p))
+		update_curr_dl(rq);
+
 	return p->dl.deadline;
 }