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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 cccb45d7c4
("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: cccb45d7c4 ("sched/deadline: Less agressive dl_server handling")
Reported-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: John Stultz <jstultz@google.com>
This commit is contained in:
Peter Zijlstra 2025-09-16 23:02:41 +02:00
parent f83ec76bf2
commit 4ae8d9aa9f
4 changed files with 2 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/sched.h
View File

@ -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

12
kernel/sched/deadline.c
View File

@ -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;
}

7
kernel/sched/fair.c
View File

@ -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);
}
/*

4
kernel/sched/sched.h
View File

@ -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);