From 4ae8d9aa9f9dc7137ea5e564d79c5aa5af1bc45c Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 16 Sep 2025 23:02:41 +0200 Subject: 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 Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz --- include/linux/sched.h | 1 - 1 file changed, 1 deletion(-) (limited to 'include') diff --git a/include/linux/sched.h b/include/linux/sched.h index f8188b833350..f89313b150e6 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 -- cgit v1.2.3