[PATCH] sched_ext: Rebuild fair weight when disabling BPF scheduler

[Zicheng Qu]

From: Zicheng Qu <quzicheng@xxxxxxxxxx>

When a BPF scheduler is disabled, scx_root_disable() switches tasks
from ext_sched_class back to fair_sched_class directly. This does not
go through __setscheduler_params(), so p->se.load is not rebuilt for
tasks returning to fair.

For example, after enabling a sched_ext BPF scheduler and creating
CPU-bound tasks with different nice values, disabling the BPF scheduler
can leave them running under fair with stale p->se.load. They may then
split CPU time according to the stale weight instead of their current
nice weights.

Rebuild the fair load weight when scx_root_disable() switches a task
from ext_sched_class to fair_sched_class. Use set_load_weight(p, false)
so CFS gets a native load_weight derived from the task's current
policy/static_prio before the task is enqueued on fair.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Zicheng Qu <quzicheng@xxxxxxxxxx>
---
 kernel/sched/ext.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 65631e577ee9..e5b8509ce7ee 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5967,6 +5967,10 @@ static void scx_root_disable(struct scx_sched *sch)
 
 		scoped_guard (sched_change, p, queue_flags) {
 			p->sched_class = new_class;
+
+			if (old_class == &ext_sched_class &&
+			    new_class == &fair_sched_class)
+				set_load_weight(p, false);
 		}
 
 		scx_disable_and_exit_task(scx_task_sched(p), p);
-- 
2.43.0