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>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f8188b8333503cb252366779b0117f0e74d11218..f89313b150e6cad915be3e4d9b4d5f2c99e0c29b 100644 (file)
--- 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 f25301267e4714802d7e12f47ef480036223d6ed..5a5080b3a670e5f10bf3a4ae94634040515fcc3e 100644 (file)
--- 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 b173a059315c2e076ef1d8347f8816d9c2c33937..8ce56a8d507f98ecb5164f101fdbaa8c616a68cd 100644 (file)
--- 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 be9745d104f751d71787cb777c5235dce9f31aa1..f10d6277dca16da73d689483fe26ac90eb6b9352 100644 (file)
--- 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);