[PATCH sched_ext/for-6.12] sched_ext: Temporarily work around pick_task_scx() being called without balance_scx()

Tejun Heo
2024-09-06T18:16:34Z

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>
---
Applying a simpler workaround to sched_ext/for-6.12 for the time being.

Thanks.

kernel/sched/ext.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2909,9 +2909,24 @@ static struct task_struct *pick_task_scx
* 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;