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>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index bba9d805dc2b88bb3925f5d6792823c5a080a738..bd8cb37b4b2ebca305f3b94b05f6af0be0eb18aa 100644 (file)
--- a/kernel/sched/ext.c
+++ b/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;