linux-kernel - [PATCH v4 04/15] sched/deadline: Prevent setting server as started if params couldn't be applied

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Message-ID: <20250617200523.1261231-5-joelagnelf@nvidia.com>
Date: Tue, 17 Jun 2025 16:05:07 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	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: Joel Fernandes <joelagnelf@...dia.com>,
	Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>
Subject: [PATCH v4 04/15] sched/deadline: Prevent setting server as started if params couldn't be applied

The following call trace fails to set dl_server_apply_params() as
dl_bw_cpus() is 0 during CPU onlining.

[   11.878356] ------------[ cut here ]------------
[   11.882592]  <TASK>
[   11.882685]  enqueue_task_scx+0x190/0x280
[   11.882802]  ttwu_do_activate+0xaa/0x2a0
[   11.882925]  try_to_wake_up+0x371/0x600
[   11.883047]  cpuhp_bringup_ap+0xd6/0x170
[   11.883172]  cpuhp_invoke_callback+0x142/0x540
[   11.883327]  _cpu_up+0x15b/0x270
[   11.883450]  cpu_up+0x52/0xb0
[   11.883576]  cpu_subsys_online+0x32/0x120
[   11.883704]  online_store+0x98/0x130
[   11.883824]  kernfs_fop_write_iter+0xeb/0x170
[   11.883972]  vfs_write+0x2c7/0x430
[   11.884091]  ksys_write+0x70/0xe0
[   11.884209]  do_syscall_64+0xd6/0x250
[   11.884327]  ? clear_bhb_loop+0x40/0x90
[   11.884443]  entry_SYSCALL_64_after_hwframe+0x77/0x7f

It is too early to start the server. Simply defer the starting of the
server to the next enqueue if dl_server_apply_params() returns an error.
In any case, we should not pretend like the server started and it does
seem to mess up with the sched_ext CPU hotplug test.

With this, the sched_ext hotplug test reliably passes.

Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
 kernel/sched/deadline.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ae15ec6294cf..d2eb31b45ba9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1656,8 +1656,8 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 		u64 runtime =  50 * NSEC_PER_MSEC;
 		u64 period = 1000 * NSEC_PER_MSEC;
 
-		dl_server_apply_params(dl_se, runtime, period, 1);
-
+		if (dl_server_apply_params(dl_se, runtime, period, 1))
+			return;
 		dl_se->dl_server = 1;
 		dl_se->dl_defer = 1;
 		setup_new_dl_entity(dl_se);
@@ -1674,7 +1674,7 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 
 void dl_server_stop(struct sched_dl_entity *dl_se)
 {
-	if (!dl_se->dl_runtime)
+	if (!dl_se->dl_runtime || !dl_se->dl_server_active)
 		return;
 
 	dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
-- 
2.43.0