[PATCH 1/2] sched_ext: Fix the running task keeping its protection into bypass

[Tejun Heo]

Protection ends when the scheduler enters bypass, so that bypass scheduling
can cut every task's slice. scx_bypass() clears it for the queued tasks by
cycling them, and strips the running task's in the same walk over
rq->scx.runnable_list. The running task is never on that list.
set_next_task_scx() takes it off and put_prev_task_scx() puts it back, so
the strip never runs. A protected running task then keeps its slice into
bypass, where the tick's attempts to cut it are refused.

Strip the running task's protection outside the walk.

Fixes: 9cfc6ab34a31 ("sched_ext: Add SCX_TASK_PROTECTED")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 2c213153bdab..d6689bf8e769 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6318,6 +6318,16 @@ void scx_bypass(struct scx_sched *sch, bool bypass)
 			continue;
 		}
 
+		/*
+		 * Bypass trumps protection. Cycling the queued tasks below
+		 * clears theirs. The current task isn't on the runnable list,
+		 * so strip its protection here.
+		 */
+		p = rq->curr;
+		if (bypass && p->sched_class == &ext_sched_class &&
+		    scx_is_descendant(scx_task_sched(p), sch))
+			scx_task_slice_ended(rq, p);
+
 		/*
 		 * The use of list_for_each_entry_safe_reverse() is required
 		 * because each task is going to be removed from and added back
@@ -6330,13 +6340,6 @@ void scx_bypass(struct scx_sched *sch, bool bypass)
 			if (!scx_is_descendant(scx_task_sched(p), sch))
 				continue;
 
-			/*
-			 * Bypass trumps protection. Cycling clears for queued
-			 * tasks but current task needs explicit stripping.
-			 */
-			if (bypass && task_current(rq, p))
-				scx_task_slice_ended(rq, p);
-
 			/* cycling deq/enq is enough, see the function comment */
 			scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
 				/* nothing */ ;
-- 
2.56.0