linux-kernel - [PATCH] sched/deadline: Always stop dl-server before changing parameters

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Message-Id: <20250721-upstream-fix-dlserver-lessaggressive-b4-v1-1-4ebc10c87e40@redhat.com>
Date: Mon, 21 Jul 2025 15:01:42 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
 Vincent Guittot <vincent.guittot@...aro.org>, 
 Dietmar Eggemann <dietmar.eggemann@....com>, 
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, 
 Marcel Ziswiler <marcel.ziswiler@...ethink.co.uk>, 
 Yuri Andriaccio <yurand2000@...il.com>, Juri Lelli <juri.lelli@...hat.com>
Subject: [PATCH] sched/deadline: Always stop dl-server before changing
 parameters

Commit cccb45d7c4295 ("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: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling")
Reported-by: Yuri Andriaccio <yurand2000@...il.com>
Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/sched/debug.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 3f06ab84d53f0..02e16b70a7901 100644
--- a/kernel/sched/debug.c
+++ b/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)

---
base-commit: ed0272f0675f31642c3d445a596b544de9db405b
change-id: 20250721-upstream-fix-dlserver-lessaggressive-b4-c5af242d3083

Best regards,
--  
Juri Lelli <juri.lelli@...hat.com>