[PATCH v5 04/14] sched/deadline: Prevent setting server as started if params couldn't be applied

Joel Fernandes
2025-06-20T20:32:55Z

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@nvidia.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