From: Tejun Heo <tj@kernel.org>
Date: Fri, 6 Sep 2024 18:17:09 +0000 (-1000)
Subject: sched_ext: Temporarily work around pick_task_scx() being called without balance_scx()
X-Git-Tag: v6.12-rc1~111^2~18
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=da330f5e4c197980ce4d343b1e557c9f33282770;p=thirdparty%2Fkernel%2Flinux.git

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 bba9d805dc2b8..bd8cb37b4b2eb 100644
--- 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;