[PATCH sched_ext/for-7.1-fixes] sched_ext: Reset dsq_vtime and slice when a task leaves SCX

[Andrea Righi]

When a task switches out of the sched_ext class, p->scx.dsq_vtime and
p->scx.slice keep whatever values they last held. On enable, slice is
reset to slice_dfl, but dsq_vtime is owned by the BPF scheduler and is
never cleared by the core, so a task that leaves SCX and later returns
carries a stale dsq_vtime across the round-trip.

The stale values are also visible to other SCX schedulers if they
inspects the scx fields of non-SCX tasks.

Zero dsq_vtime and slice in switched_from_scx() after scx_disable_task()
so the fields are reset to the same baseline a freshly forked task has.
The reset is done after the disable callback so the BPF scheduler can
still observe the task's final values.

Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/ext.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 8e88a25bc602f..915d5ece5d277 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3942,10 +3942,11 @@ static void switched_from_scx(struct rq *rq, struct task_struct *p)
 	 * scx_disable_task() would WARN on the non-%ENABLED state and trigger a
 	 * NONE -> READY validation failure.
 	 */
-	if (scx_get_task_state(p) == SCX_TASK_NONE)
-		return;
+	if (scx_get_task_state(p) != SCX_TASK_NONE)
+		scx_disable_task(scx_task_sched(p), p);
 
-	scx_disable_task(scx_task_sched(p), p);
+	p->scx.dsq_vtime = 0;
+	p->scx.slice = 0;
 }
 
 static void switched_to_scx(struct rq *rq, struct task_struct *p) {}
-- 
2.54.0