On Mon, Feb 16, 2015 at 03:38:34PM +0300, Kirill Tkhai wrote:
> We shouldn't enqueue migrating tasks. Please, try this one instead ;)
Ha, we should amend that task-rq-lock loop for that. See below.
I've not yet tested; going to try and reconstruct a .config that
triggers the oops.
---
Subject: sched/dl: Prevent enqueue of a sleeping task in dl_task_timer()
From: Kirill Tkhai <tk...@yandex.ru>
Date: Mon, 16 Feb 2015 15:38:34 +0300
A deadline task may be throttled and dequeued at the same time.
This happens, when it becomes throttled in schedule(), which
is called to go to sleep:
current->state = TASK_INTERRUPTIBLE;
schedule()
deactivate_task()
dequeue_task_dl()
update_curr_dl()
start_dl_timer()
__dequeue_task_dl()
prev->on_rq = 0;
Later the timer fires, but the task is still dequeued:
dl_task_timer()
enqueue_task_dl() /* queues on dl_rq; on_rq remains 0 */
Someone wakes it up:
try_to_wake_up()
enqueue_dl_entity()
BUG_ON(on_dl_rq())
Patch fixes this problem, it prevents queueing !on_rq tasks
on dl_rq.
Also teach the rq-lock loop about TASK_ON_RQ_MIGRATING as per
cca26e8009d1 ("sched: Teach scheduler to understand
TASK_ON_RQ_MIGRATING state").
Fixes: 1019a359d3dc ("sched/deadline: Fix stale yield state")
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Juri Lelli <juri.le...@arm.com>
Reported-by: Fengguang Wu <fengguang...@intel.com>
Signed-off-by: Kirill Tkhai <ktk...@parallels.com>
[peterz: Wrote comment; fixed task-rq-lock loop]
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Link: http://lkml.kernel.org/r/1374601424090...@web4j.yandex.ru
---
kernel/sched/deadline.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -515,9 +515,8 @@ static enum hrtimer_restart dl_task_time
again:
rq = task_rq(p);
raw_spin_lock(&rq->lock);
-
- if (rq != task_rq(p)) {
- /* Task was moved, retrying. */
+ if (rq != task_rq(p) || task_on_rq_migrating(p)) {
+ /* Task was move{d,ing}, retry */
raw_spin_unlock(&rq->lock);
goto again;
}
@@ -541,6 +540,26 @@ static enum hrtimer_restart dl_task_time
sched_clock_tick();
update_rq_clock(rq);
+
+ /*
+ * If the throttle happened during sched-out; like:
+ *
+ * schedule()
+ * deactivate_task()
+ * dequeue_task_dl()
+ * update_curr_dl()
+ * start_dl_timer()
+ * __dequeue_task_dl()
+ * prev->on_rq = 0;
+ *
+ * We can be both throttled and !queued. Replenish the counter
+ * but do not enqueue -- wait for our wakeup to do that.
+ */
+ if (!task_on_rq_queued(p)) {
+ replenish_dl_entity(dl_se, dl_se);
+ goto unlock;
+ }
+
enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
if (dl_task(rq->curr))
check_preempt_curr_dl(rq, p, 0);
--
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