[PATCH v2] sched_ext: Reset task fields on exit of an SCX_TASK_READY task

[Rahad Bhuiya <rahadbhuiya2021@gmail.com>]

Tasks in SCX_TASK_READY are attached to a scheduler but outside the ext
class, such as RT/DL tasks, or non-SCHED_EXT tasks under a partial switch.
The scheduler's BPF code can still update their dsq_vtime and slice (for
instance, via ops.cgroup_move() as done by scx_flatcg).

Because __scx_disable_and_exit_task() currently breaks out of the state
switch for SCX_TASK_READY without calling scx_reset_task_fields(), these
fields are not cleared when the task leaves the scheduler. If the task is
subsequently moved to another cgroup/sub-scheduler and later switches to
SCHED_NORMAL, the new scheduler's ops.enable() sees stale dsq_vtime from
the previous scheduler's clock.

Call scx_reset_task_fields() in the SCX_TASK_READY case of
__scx_disable_and_exit_task() to ensure SCX-managed fields are properly
reset when leaving the scheduler.

Fixes: 5771e79e461e ("sched_ext: Reset dsq_vtime and slice when a task leaves SCX")
Signed-off-by: Rahad Bhuiya <rahadbhuiya2021@gmail.com>
---
v2:
- Rewrote the commit description to describe the cgroup sub-sched / RT task
  migration case (per Tejun Heo).
- Updated Fixes: tag to 5771e79e461e ("sched_ext: Reset dsq_vtime and slice
  when a task leaves SCX").
- Targeted sched_ext tree instead of bpf-next.

 kernel/sched/ext/ext.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index f74ffb3b1aca..b57a7989f304 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4728,6 +4728,7 @@ void __scx_disable_and_exit_task(struct scx_sched *sch, struct task_struct *p)
 		args.cancelled = true;
 		break;
 	case SCX_TASK_READY:
+		scx_reset_task_fields(task_rq(p), p);
 		break;
 	case SCX_TASK_ENABLED:
 		scx_disable_task(sch, p);
-- 
2.55.0.windows.3