diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2025-09-16 23:02:41 +0200 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2025-09-25 09:51:50 +0200 |
| commit | 4ae8d9aa9f9dc7137ea5e564d79c5aa5af1bc45c (patch) | |
| tree | ee3a49e04d70d79beb5ac3bb93e39adde655af30 /kernel/sched/sched.h | |
| parent | f83ec76bf285bea5727f478a68b894f5543ca76e (diff) | |
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@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>
Diffstat (limited to 'kernel/sched/sched.h')
| -rw-r--r-- | kernel/sched/sched.h | 4 |
1 files changed, 0 insertions, 4 deletions
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index be9745d104f7..f10d6277dca1 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); |