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

From: tip-bot for Wanpeng Li
Date: Mon Sep 05 2016 - 07:56:57 EST


Commit-ID: 61c7aca695b6fabe85d0fc424fe8ae2f66f267dd
Gitweb: http://git.kernel.org/tip/61c7aca695b6fabe85d0fc424fe8ae2f66f267dd
Author: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
AuthorDate: Wed, 31 Aug 2016 18:27:44 +0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 5 Sep 2016 13:29:45 +0200

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@xxxxxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Luca Abeni <luca.abeni@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1472639264-3932-1-git-send-email-wanpeng.li@xxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
kernel/sched/deadline.c | 46 ++++++++++++++++++----------------------------
1 file changed, 18 insertions(+), 28 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 18fb0b8..0c75bc6 100644
--- 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.