sched/deadline: Always stop dl-server before changing parameters

JIRA: https://issues.redhat.com/browse/RHEL-110301

commit bb4700a
Author: Juri Lelli <juri.lelli@redhat.com>
Date:   Mon Jul 21 15:01:42 2025 +0200

    sched/deadline: Always stop dl-server before changing parameters

    Commit cccb45d ("sched/deadline: Less agressive dl_server
    handling") reduced dl-server overhead by delaying disabling servers only
    after there are no fair task around for a whole period, which means that
    deadline entities are not dequeued right away on a server stop event.
    However, the delay opens up a window in which a request for changing
    server parameters can break per-runqueue running_bw tracking, as
    reported by Yuri.

    Close the problematic window by unconditionally calling dl_server_stop()
    before applying the new parameters (ensuring deadline entities go
    through an actual dequeue).

    Fixes: cccb45d ("sched/deadline: Less agressive dl_server handling")
    Reported-by: Yuri Andriaccio <yurand2000@gmail.com>
    Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Valentin Schneider <vschneid@redhat.com>
    Link: https://lore.kernel.org/r/20250721-upstream-fix-dlserver-lessaggressive-b4-v1-1-4ebc10c87e40@redhat.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/debug.c

@@ -376,10 +376,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 			return  -EINVAL;
 		}
 
-		if (rq->cfs.h_nr_queued) {
-			update_rq_clock(rq);
-			dl_server_stop(&rq->fair_server);
-		}
+		update_rq_clock(rq);
+		dl_server_stop(&rq->fair_server);
 
 		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
 		if (retval)