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Message-ID: <175817861820.709179.10538516755307778527.tip-bot2@tip-bot2>
Date: Thu, 18 Sep 2025 06:56:58 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: John Stultz <jstultz@...gle.com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/deadline: Fix dl_server getting stuck
The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: 077e1e2e0015e5ba6538d1c5299fb299a3a92d60
Gitweb: https://git.kernel.org/tip/077e1e2e0015e5ba6538d1c5299fb299a3a92d60
Author: Peter Zijlstra <peterz@...radead.org>
AuthorDate: Tue, 16 Sep 2025 23:02:41 +02:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 18 Sep 2025 08:50:05 +02:00
sched/deadline: Fix dl_server getting stuck
John found it was easy to hit lockup warnings when running locktorture
on a 2 CPU VM, which he bisected down to: commit cccb45d7c429
("sched/deadline: Less agressive dl_server handling").
While debugging it seems there is a chance where we end up with the
dl_server dequeued, with dl_se->dl_server_active. This causes
dl_server_start() to return without enqueueing the dl_server, thus it
fails to run when RT tasks starve the cpu.
When this happens, dl_server_timer() catches the
'!dl_se->server_has_tasks(dl_se)' case, which then calls
replenish_dl_entity() and dl_server_stopped() and finally return
HRTIMER_NO_RESTART.
This ends in no new timer and also no enqueue, leaving the dl_server
'dead', allowing starvation.
What should have happened is for the bandwidth timer to start the
zero-laxity timer, which in turn would enqueue the dl_server and cause
dl_se->server_pick_task() to be called -- which will stop the
dl_server if no fair tasks are observed for a whole period.
IOW, it is totally irrelevant if there are fair tasks at the moment of
bandwidth refresh.
This removes all dl_se->server_has_tasks() users, so remove the whole
thing.
Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling")
Reported-by: John Stultz <jstultz@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: John Stultz <jstultz@...gle.com>
---
include/linux/sched.h | 1 -
kernel/sched/deadline.c | 12 +-----------
kernel/sched/fair.c | 7 +------
kernel/sched/sched.h | 4 ----
4 files changed, 2 insertions(+), 22 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f8188b8..f89313b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -733,7 +733,6 @@ struct sched_dl_entity {
* runnable task.
*/
struct rq *rq;
- dl_server_has_tasks_f server_has_tasks;
dl_server_pick_f server_pick_task;
#ifdef CONFIG_RT_MUTEXES
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index f253012..5a5080b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -875,7 +875,7 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
*/
if (dl_se->dl_defer && !dl_se->dl_defer_running &&
dl_time_before(rq_clock(dl_se->rq), dl_se->deadline - dl_se->runtime)) {
- if (!is_dl_boosted(dl_se) && dl_se->server_has_tasks(dl_se)) {
+ if (!is_dl_boosted(dl_se)) {
/*
* Set dl_se->dl_defer_armed and dl_throttled variables to
@@ -1152,8 +1152,6 @@ static void __push_dl_task(struct rq *rq, struct rq_flags *rf)
/* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
-static bool dl_server_stopped(struct sched_dl_entity *dl_se);
-
static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
{
struct rq *rq = rq_of_dl_se(dl_se);
@@ -1171,12 +1169,6 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_
if (!dl_se->dl_runtime)
return HRTIMER_NORESTART;
- if (!dl_se->server_has_tasks(dl_se)) {
- replenish_dl_entity(dl_se);
- dl_server_stopped(dl_se);
- return HRTIMER_NORESTART;
- }
-
if (dl_se->dl_defer_armed) {
/*
* First check if the server could consume runtime in background.
@@ -1625,11 +1617,9 @@ static bool dl_server_stopped(struct sched_dl_entity *dl_se)
}
void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
- dl_server_has_tasks_f has_tasks,
dl_server_pick_f pick_task)
{
dl_se->rq = rq;
- dl_se->server_has_tasks = has_tasks;
dl_se->server_pick_task = pick_task;
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c4d91e8..59d7dc9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8859,11 +8859,6 @@ static struct task_struct *__pick_next_task_fair(struct rq *rq, struct task_stru
return pick_next_task_fair(rq, prev, NULL);
}
-static bool fair_server_has_tasks(struct sched_dl_entity *dl_se)
-{
- return !!dl_se->rq->cfs.nr_queued;
-}
-
static struct task_struct *fair_server_pick_task(struct sched_dl_entity *dl_se)
{
return pick_task_fair(dl_se->rq);
@@ -8875,7 +8870,7 @@ void fair_server_init(struct rq *rq)
init_dl_entity(dl_se);
- dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick_task);
+ dl_server_init(dl_se, rq, fair_server_pick_task);
}
/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index be9745d..f10d627 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -365,9 +365,6 @@ extern s64 dl_scaled_delta_exec(struct rq *rq, struct sched_dl_entity *dl_se, s6
*
* dl_se::rq -- runqueue we belong to.
*
- * dl_se::server_has_tasks() -- used on bandwidth enforcement; we 'stop' the
- * server when it runs out of tasks to run.
- *
* dl_se::server_pick() -- nested pick_next_task(); we yield the period if this
* returns NULL.
*
@@ -383,7 +380,6 @@ extern void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec);
extern void dl_server_start(struct sched_dl_entity *dl_se);
extern void dl_server_stop(struct sched_dl_entity *dl_se);
extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
- dl_server_has_tasks_f has_tasks,
dl_server_pick_f pick_task);
extern void sched_init_dl_servers(void);
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