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Message-ID: <Ztpjt5Pz9pJliblL@slm.duckdns.org>
Date: Thu, 5 Sep 2024 16:06:47 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH sched/core] sched: Warn if a sched_class says yes on
balance() but no on pick_task()
If a sched_class says yes on balance() but no on pick_task(),
__pick_next_task() will end up descending to lower sched classes which
haven't been balanced. This can lead to sub-optimal scheduling decisions
and, especially for sched_ext, stalls. Detect the condition and warn.
Signed-off-by: Tejun Heo <tj@...nel.org>
---
This triggers immedately on the current tip/sched/core. Best to apply after
fixing the existing issue.
Thanks.
kernel/sched/core.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5893,8 +5893,8 @@ static inline void schedule_debug(struct
schedstat_inc(this_rq()->sched_count);
}
-static void prev_balance(struct rq *rq, struct task_struct *prev,
- struct rq_flags *rf)
+static const struct sched_class *
+prev_balance(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
const struct sched_class *start_class = prev->sched_class;
const struct sched_class *class;
@@ -5919,8 +5919,10 @@ static void prev_balance(struct rq *rq,
*/
for_active_class_range(class, start_class, &idle_sched_class) {
if (class->balance && class->balance(rq, prev, rf))
- break;
+ return class;
}
+
+ return &idle_sched_class;
}
/*
@@ -5929,7 +5931,7 @@ static void prev_balance(struct rq *rq,
static inline struct task_struct *
__pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
{
- const struct sched_class *class;
+ const struct sched_class *class, *balanced;
struct task_struct *p;
rq->dl_server = NULL;
@@ -5960,23 +5962,29 @@ __pick_next_task(struct rq *rq, struct t
}
restart:
- prev_balance(rq, prev, rf);
+ balanced = prev_balance(rq, prev, rf);
for_each_active_class(class) {
if (class->pick_next_task) {
p = class->pick_next_task(rq, prev);
if (p)
- return p;
+ goto picked;
} else {
p = class->pick_task(rq);
if (p) {
put_prev_set_next_task(rq, prev, p);
- return p;
+ goto picked;
}
}
}
BUG(); /* The idle class should always have a runnable task. */
+
+picked:
+ WARN_ONCE(sched_class_above(balanced, class),
+ "%ps->balance() returned true but no task, %ps picked instead\n",
+ balanced, class);
+ return p;
}
#ifdef CONFIG_SCHED_CORE
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