[tip: sched/urgent] sched/deadline: Fix this_bw on class change of a dying task

[tip-bot2 for Nickolai Zeldovich]

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

Commit-ID:     0c93e25ef11a9dee43f6076bd1f09771dc2b167e
Gitweb:        https://git.kernel.org/tip/0c93e25ef11a9dee43f6076bd1f09771dc2b167e
Author:        Nickolai Zeldovich <nickolai@xxxxxxxxxxxxx>
AuthorDate:    Sun, 25 Oct 2026 08:46:57 -04:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 06 Nov 2026 10:00:50 +01:00

sched/deadline: Fix this_bw on class change of a dying task

Moving a SCHED_DEADLINE task out of the class with sched_setscheduler(2)
after it stored TASK_DEAD in do_task_dead(), but before __schedule()
dequeued it, subtracts its bandwidth from the runqueue's this_bw twice.
this_bw then stays short until reboot: __sub_rq_bw() warns about the
underflow or about running_bw > this_bw, and GRUB reclaim for
SCHED_FLAG_RECLAIM tasks on that CPU is miscomputed.

The class change dequeues the still queued task with DEQUEUE_SAVE, which
removes its bandwidth from running_bw and this_bw. switched_from_dl()
then calls task_non_contending(), which, if the 0-lag time has passed,
removes it from this_bw again because the task is TASK_DEAD. That
removal is meant for the dying task's own DEQUEUE_SLEEP dequeue, which
has removed nothing before it.

Do the TASK_DEAD removal only on that dequeue path.

Reproduced under QEMU on x86_64 and riscv64 by moving exiting deadline
tasks to SCHED_OTHER in a loop; with this patch there is no warning.

Fixes: 8fd27231c330 ("sched/deadline: Track the "total rq utilization" too")
Signed-off-by: Nickolai Zeldovich <nickolai@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Tested-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Link: https://patch.msgid.link/20261025124657.836229-1-nickolai@xxxxxxxxxxxxx
---
 kernel/sched/deadline.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index c0ebdcd..dfae086 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -444,7 +444,7 @@ static void task_non_contending(struct sched_dl_entity *dl_se, bool dl_task)
 			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)
+				if (dl_task && READ_ONCE(p->__state) == TASK_DEAD)
 					sub_rq_bw(dl_se, &rq->dl);
 				raw_spin_lock(&dl_b->lock);
 				__dl_sub(dl_b, dl_se->dl_bw, dl_bw_cpus(task_cpu(p)));