On 6/13/2025 1:35 PM, Joel Fernandes wrote:
>
>
> On 6/13/2025 1:17 AM, Joel Fernandes wrote:
>> sched_ext tasks currently are starved by RT hoggers especially since RT
>> throttling was replaced by deadline servers to boost only CFS tasks. Several
>> users in the community have reported issues with RT stalling sched_ext tasks.
>> Add a sched_ext deadline server as well so that sched_ext tasks are also
>> boosted and do not suffer starvation.
>>
>> A kselftest is also provided to verify the starvation issues are now fixed.
>>
>> Btw, there is still something funky going on with CPU hotplug and the
>> relinquish patch. Sometimes the sched_ext's hotplug self-test locks up
>> (./runner -t hotplug). Reverting that patch fixes it, so I am suspecting
>> something is off in dl_server_remove_params() when it is being called on
>> offline CPUs.
>
> I think I got somewhere here with this sched_ext hotplug test but still not
> there yet. Juri, Andrea, Tejun, can you take a look at the below when you get a
> chance?
The following patch makes the sched_ext hotplug test reliably pass for me now.
Thoughts?
From: Joel Fernandes <joelagnelf@xxxxxxxxxx>
Subject: [PATCH] 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 in the below path.
[ 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 seems 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@xxxxxxxxxx>
---
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 f0cd1dbca4b8..8dd0c6d71489 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1657,8 +1657,8 @@ 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);
@@ -1675,7 +1675,7 @@ 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);
