[PATCH v2] sched/deadline: Compare against the donor in prio_changed_dl()

[Zhan Xusheng]

prio_changed_dl() dispatches on task_current_donor(), so its else branch
asks whether p should preempt the current scheduling context.  Under proxy
execution that is rq->donor; rq->curr is the execution context and may be
a mutex owner running on the donor's parameters.  dl_task() tests
dl_prio(p->prio), so dl_task(rq->curr) says nothing about what the
runqueue is currently scheduling.

commit af0c8b2bf67b ("sched: Split scheduler and execution contexts")
converted the identical else branch of prio_changed_rt() to rq->donor.
Its hunk for deadline.c enters prio_changed_dl() and converts
task_current() to task_current_donor(), but leaves the comparison below
it on rq->curr.

Fixes: af0c8b2bf67b ("sched: Split scheduler and execution contexts")
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
---
Changes since v1:

 - Dropped the claim that the donor test implies the curr test.  It does
   not: with a non-DL donor and a DL rq->curr the donor test is true while
   the curr test can be false, so the current code can also skip a
   reschedule, not only add one.
 - Dropped the suggestion to convert dl_server_timer() and
   dl_server_start().  Yuri Andriaccio's "sched/deadline: Do not access
   dl_se->rq directly" (RFC v6 03/25) rewrites both of those lines,
   turning dl_se->rq into a local rq while keeping ->curr, so a donor
   conversion there is better done on top of that series than against
   it.  prio_changed_dl() is not touched by it.

Built with CONFIG_SCHED_PROXY_EXEC=y and =n.  With =n the two rq members
are an anonymous union and kernel/sched/build_policy.o is byte-identical
to the unpatched tree.

v1: https://patch.msgid.link/20260924121213.106673-1-zhanxusheng@xxxxxxxxxx

 kernel/sched/deadline.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0663c00c41c0..f27aa930420f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3631,8 +3631,8 @@ static void prio_changed_dl(struct rq *rq, struct task_struct *p, u64 old_deadli
 		 *
 		 * Otherwise, if p was given an earlier deadline, reschedule.
 		 */
-		if (!dl_task(rq->curr) ||
-		    dl_time_before(p->dl.deadline, rq->curr->dl.deadline))
+		if (!dl_task(rq->donor) ||
+		    dl_time_before(p->dl.deadline, rq->donor->dl.deadline))
 			resched_curr(rq);
 	}
 }

base-commit: a90ee4305c4a5df72c11b31dacfdc76e00fcf78a
-- 
2.43.0