sched_ext: Temporarily work around pick_task_scx() being called without balance_scx()

pick_task_scx() must be preceded by balance_scx() but there currently is a
bug where fair could say yes on balance() but no on pick_task(), which then
ends up calling pick_task_scx() without preceding balance_scx(). Work around
by dropping WARN_ON_ONCE() and ignoring cases which don't make sense.

This isn't great and can theoretically lead to stalls. However, for
switch_all cases, this happens only while a BPF scheduler is being loaded or
unloaded, and, for partial cases, fair will likely keep triggering this CPU.

This will be reverted once the fair behavior is fixed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>

kernel/sched/ext.c

@@ -2909,9 +2909,24 @@ static struct task_struct *pick_task_scx(struct rq *rq)
 	 * If balance_scx() is telling us to keep running @prev, replenish slice
 	 * if necessary and keep running @prev. Otherwise, pop the first one
 	 * from the local DSQ.
+	 *
+	 * WORKAROUND:
+	 *
+	 * %SCX_RQ_BAL_KEEP should be set iff $prev is on SCX as it must just
+	 * have gone through balance_scx(). Unfortunately, there currently is a
+	 * bug where fair could say yes on balance() but no on pick_task(),
+	 * which then ends up calling pick_task_scx() without preceding
+	 * balance_scx().
+	 *
+	 * For now, ignore cases where $prev is not on SCX. This isn't great and
+	 * can theoretically lead to stalls. However, for switch_all cases, this
+	 * happens only while a BPF scheduler is being loaded or unloaded, and,
+	 * for partial cases, fair will likely keep triggering this CPU.
+	 *
+	 * Once fair is fixed, restore WARN_ON_ONCE().
 	 */
 	if ((rq->scx.flags & SCX_RQ_BAL_KEEP) &&
-	    !WARN_ON_ONCE(prev->sched_class != &ext_sched_class)) {
+	    prev->sched_class == &ext_sched_class) {
 		p = prev;
 		if (!p->scx.slice)
 			p->scx.slice = SCX_SLICE_DFL;