[PATCH 1/3] sched_ext: Blame the DSQ's owning scheduler for a runnable stall

[Tejun Heo]

check_rq_for_timeouts() blames a runnable stall on the task's owner. Under
a sub-scheduler hierarchy the stalled task can be sitting on a DSQ that a
different scheduler has to drain, e.g. an ancestor's bypass DSQ while the
owner is bypassing. The drainer then escapes blame while the owner is
exited, and when the owner's exit is already claimed, nothing actionable is
reported at all.

Blame the DSQ's owning scheduler instead. The local DSQ is consumed by the
cpu itself and keeps blame on the owner. Detection keeps the owner's timeout
and single-scheduler behavior is unchanged.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index b730eac4b13f..96389ec31ab8 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -3391,8 +3391,19 @@ static bool check_rq_for_timeouts(struct rq *rq)
 
 		if (unlikely(time_after(jiffies,
 					last_runnable + READ_ONCE(sch->watchdog_timeout)))) {
+			struct scx_dispatch_q *dsq = READ_ONCE(p->scx.dsq);
 			u32 dur_ms = jiffies_to_msecs(jiffies - last_runnable);
 
+			/*
+			 * A task can be stuck on a DSQ that a sched other than
+			 * its owner is responsible for draining, e.g. an
+			 * ancestor's bypass DSQ while the owner is bypassing.
+			 * Blame the drainer. The local DSQ is consumed by the
+			 * cpu itself and keeps blame on the owner.
+			 */
+			if (dsq && dsq->sched && dsq->id != SCX_DSQ_LOCAL)
+				sch = dsq->sched;
+
 			__scx_exit(sch, SCX_EXIT_ERROR_STALL, 0, cpu_of(rq),
 				   "%s[%d] failed to run for %u.%03us",
 				   p->comm, p->pid, dur_ms / 1000,
-- 
2.55.0