Commit 4c6c4e38 authored by Kirill Tkhai's avatar Kirill Tkhai Committed by Ingo Molnar
Browse files

sched/core: Fix endless loop in pick_next_task() 



1) Single cpu machine case.

When rq has only RT tasks, but no one of them can be picked
because of throttling, we enter in endless loop.

pick_next_task_{dl,rt} return NULL.

In pick_next_task_fair() we permanently go to retry

	if (rq->nr_running != rq->cfs.h_nr_running)
		return RETRY_TASK;

(rq->nr_running is not being decremented when rt_rq becomes
throttled).

No chances to unthrottle any rt_rq or to wake fair here,
because of rq is locked permanently and interrupts are
disabled.

2) In case of SMP this can cause a hang too. Although we unlock
   rq in idle_balance(), interrupts are still disabled.

The solution is to check for available tasks in DL and RT
classes instead of checking for sum.

Signed-off-by: default avatarKirill Tkhai <ktkhai@parallels.com>
Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1394098321.19290.11.camel@tkhai


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e4aa358b

kernel/sched/fair.c

+3 −1
Original line number Diff line number Diff line
@@ -6728,7 +6728,9 @@ static int idle_balance(struct rq *this_rq) 


out:

	/* Is there a task of a high priority class? */

	if (this_rq->nr_running != this_rq->cfs.h_nr_running)

	if (this_rq->nr_running != this_rq->cfs.h_nr_running &&

	    (this_rq->dl.dl_nr_running ||

	     (this_rq->rt.rt_nr_running && !rt_rq_throttled(&this_rq->rt))))

		pulled_task = -1;



	if (pulled_task) {

kernel/sched/rt.c

+0 −10
Original line number Diff line number Diff line
@@ -470,11 +470,6 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq) 
		dequeue_rt_entity(rt_se);

}



static inline int rt_rq_throttled(struct rt_rq *rt_rq)

{

	return rt_rq->rt_throttled && !rt_rq->rt_nr_boosted;

}



static int rt_se_boosted(struct sched_rt_entity *rt_se)

{

	struct rt_rq *rt_rq = group_rt_rq(rt_se);
@@ -545,11 +540,6 @@ static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq) 
{

}



static inline int rt_rq_throttled(struct rt_rq *rt_rq)

{

	return rt_rq->rt_throttled;

}



static inline const struct cpumask *sched_rt_period_mask(void)

{

	return cpu_online_mask;

kernel/sched/sched.h

+12 −0
Original line number Diff line number Diff line
@@ -423,6 +423,18 @@ struct rt_rq { 
#endif

};



#ifdef CONFIG_RT_GROUP_SCHED

static inline int rt_rq_throttled(struct rt_rq *rt_rq)

{

	return rt_rq->rt_throttled && !rt_rq->rt_nr_boosted;

}

#else

static inline int rt_rq_throttled(struct rt_rq *rt_rq)

{

	return rt_rq->rt_throttled;

}

#endif



/* Deadline class' related fields in a runqueue */

struct dl_rq {

	/* runqueue is an rbtree, ordered by deadline */

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