sched/deadline: Fix the intention to re-evalute tick dependency for offline CPU

The dl task will be replenished after dl task timer fire and start a
new period. It will be enqueued and to re-evaluate its dependency on
the tick in order to restart it. However, if the CPU is hot-unplugged,
irq_work_queue will splash since the target CPU is offline.

As a result we get:

    WARNING: CPU: 2 PID: 0 at kernel/irq_work.c:69 irq_work_queue_on+0xad/0xe0
    Call Trace:
     dump_stack+0x99/0xd0
     __warn+0xd1/0xf0
     warn_slowpath_null+0x1d/0x20
     irq_work_queue_on+0xad/0xe0
     tick_nohz_full_kick_cpu+0x44/0x50
     tick_nohz_dep_set_cpu+0x74/0xb0
     enqueue_task_dl+0x226/0x480
     activate_task+0x5c/0xa0
     dl_task_timer+0x19b/0x2c0
     ? push_dl_task.part.31+0x190/0x190

This can be triggered by hot-unplugging the full dynticks CPU which dl
task is running on.

We enqueue the dl task on the offline CPU, because we need to do
replenish for start_dl_timer(). So, as Juri pointed out, we would
need to do is calling replenish_dl_entity() directly, instead of
enqueue_task_dl(). pi_se shouldn't be a problem as the task shouldn't
be boosted if it was throttled.

This patch fixes it by avoiding the whole enqueue+dequeue+enqueue story, by
first migrating (set_task_cpu()) and then doing 1 enqueue.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luca Abeni <luca.abeni@unitn.it>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1472639264-3932-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 18fb0b8..0c75bc6 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -243,10 +243,8 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
 static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p)
 {
        struct rq *later_rq = NULL;
-       bool fallback = false;
 
        later_rq = find_lock_later_rq(p, rq);
-
        if (!later_rq) {
                int cpu;
 
@@ -254,7 +252,6 @@ static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p
                 * If we cannot preempt any rq, fall back to pick any
                 * online cpu.
                 */
-               fallback = true;
                cpu = cpumask_any_and(cpu_active_mask, tsk_cpus_allowed(p));
                if (cpu >= nr_cpu_ids) {
                        /*
@@ -274,16 +271,7 @@ static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p
                double_lock_balance(rq, later_rq);
        }
 
-       /*
-        * By now the task is replenished and enqueued; migrate it.
-        */
-       deactivate_task(rq, p, 0);
        set_task_cpu(p, later_rq->cpu);
-       activate_task(later_rq, p, 0);
-
-       if (!fallback)
-               resched_curr(later_rq);
-
        double_unlock_balance(later_rq, rq);
 
        return later_rq;
@@ -641,29 +629,31 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
                goto unlock;
        }
 
-       enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
-       if (dl_task(rq->curr))
-               check_preempt_curr_dl(rq, p, 0);
-       else
-               resched_curr(rq);
-
 #ifdef CONFIG_SMP
-       /*
-        * Perform balancing operations here; after the replenishments.  We
-        * cannot drop rq->lock before this, otherwise the assertion in
-        * start_dl_timer() about not missing updates is not true.
-        *
-        * If we find that the rq the task was on is no longer available, we
-        * need to select a new rq.
-        *
-        * XXX figure out if select_task_rq_dl() deals with offline cpus.
-        */
        if (unlikely(!rq->online)) {
+               /*
+                * If the runqueue is no longer available, migrate the
+                * task elsewhere. This necessarily changes rq.
+                */
                lockdep_unpin_lock(&rq->lock, rf.cookie);
                rq = dl_task_offline_migration(rq, p);
                rf.cookie = lockdep_pin_lock(&rq->lock);
+
+               /*
+                * Now that the task has been migrated to the new RQ and we
+                * have that locked, proceed as normal and enqueue the task
+                * there.
+                */
        }
+#endif
 
+       enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
+       if (dl_task(rq->curr))
+               check_preempt_curr_dl(rq, p, 0);
+       else
+               resched_curr(rq);
+
+#ifdef CONFIG_SMP
        /*
         * Queueing this task back might have overloaded rq, check if we need
         * to kick someone away.