[PATCH 03/13] sched_ext: Skip tasks with stale task_rq in bypass_lb_cpu()

[Tejun Heo <tj@kernel.org>]

bypass_lb_cpu() transfers tasks between per-CPU bypass DSQs without
migrating them - task_cpu() only updates when the donee later consumes the
task via move_remote_task_to_local_dsq(). If the LB timer fires again before
consumption and the new DSQ becomes a donor, @p is still on the previous CPU
and task_rq(@p) != donor_rq. @p can't be moved without its own rq locked.

Skip such tasks.

Fixes: 95d1df610cdc ("sched_ext: Implement load balancer for bypass mode")
Cc: stable@vger.kernel.org # v6.19+
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 89170a0e5779..62b4139a4cc8 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5002,6 +5002,15 @@ static u32 bypass_lb_cpu(struct scx_sched *sch, s32 donor,
 		if (cpumask_empty(donee_mask))
 			break;
 
+		/*
+		 * If an earlier pass placed @p on @donor_dsq from a different
+		 * CPU and the donee hasn't consumed it yet, @p is still on the
+		 * previous CPU and task_rq(@p) != @donor_rq. @p can't be moved
+		 * without its rq locked. Skip.
+		 */
+		if (task_rq(p) != donor_rq)
+			continue;
+
 		donee = cpumask_any_and_distribute(donee_mask, p->cpus_ptr);
 		if (donee >= nr_cpu_ids)
 			continue;
-- 
2.53.0