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Commit 91732a27 authored by Nicolas Saenz Julienne's avatar Nicolas Saenz Julienne Committed by Greg Kroah-Hartman
Browse files

nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt() 



[ Upstream commit 5c66d1b9b30f737fcef85a0b75bfe0590e16b62a ]

dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having
called sched_update_tick_dependency() preventing it from re-enabling the
tick on systems that no longer have pending SCHED_RT tasks but have
multiple runnable SCHED_OTHER tasks:

  dequeue_task_rt()
    dequeue_rt_entity()
      dequeue_rt_stack()
        dequeue_top_rt_rq()
	  sub_nr_running()	// decrements rq->nr_running
	    sched_update_tick_dependency()
	      sched_can_stop_tick()	// checks rq->rt.rt_nr_running,
	      ...
        __dequeue_rt_entity()
          dec_rt_tasks()	// decrements rq->rt.rt_nr_running
	  ...

Every other scheduler class performs the operation in the opposite
order, and sched_update_tick_dependency() expects the values to be
updated as such. So avoid the misbehaviour by inverting the order in
which the above operations are performed in the RT scheduler.

Fixes: 76d92ac3 ("sched: Migrate sched to use new tick dependency mask model")
Signed-off-by: default avatarNicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <vschneid@redhat.com>
Reviewed-by: default avatarPhil Auld <pauld@redhat.com>
Link: https://lore.kernel.org/r/20220628092259.330171-1-nsaenzju@redhat.com


Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 35f9e861

kernel/sched/rt.c

+9 −6
Original line number Diff line number Diff line
@@ -437,7 +437,7 @@ static inline void rt_queue_push_tasks(struct rq *rq) 
#endif /* CONFIG_SMP */



static void enqueue_top_rt_rq(struct rt_rq *rt_rq);

static void dequeue_top_rt_rq(struct rt_rq *rt_rq);

static void dequeue_top_rt_rq(struct rt_rq *rt_rq, unsigned int count);



static inline int on_rt_rq(struct sched_rt_entity *rt_se)

{
@@ -519,7 +519,7 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq) 
	rt_se = rt_rq->tg->rt_se[cpu];



	if (!rt_se) {

		dequeue_top_rt_rq(rt_rq);

		dequeue_top_rt_rq(rt_rq, rt_rq->rt_nr_running);

		/* Kick cpufreq (see the comment in kernel/sched/sched.h). */

		cpufreq_update_util(rq_of_rt_rq(rt_rq), 0);

	}
@@ -605,7 +605,7 @@ static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq) 


static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)

{

	dequeue_top_rt_rq(rt_rq);

	dequeue_top_rt_rq(rt_rq, rt_rq->rt_nr_running);

}



static inline int rt_rq_throttled(struct rt_rq *rt_rq)
@@ -1004,7 +1004,7 @@ static void update_curr_rt(struct rq *rq) 
}



static void

dequeue_top_rt_rq(struct rt_rq *rt_rq)

dequeue_top_rt_rq(struct rt_rq *rt_rq, unsigned int count)

{

	struct rq *rq = rq_of_rt_rq(rt_rq);
@@ -1015,7 +1015,7 @@ dequeue_top_rt_rq(struct rt_rq *rt_rq) 


	BUG_ON(!rq->nr_running);



	sub_nr_running(rq, rt_rq->rt_nr_running);

	sub_nr_running(rq, count);

	rt_rq->rt_queued = 0;



}
@@ -1294,18 +1294,21 @@ static void __dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flag 
static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)

{

	struct sched_rt_entity *back = NULL;

	unsigned int rt_nr_running;



	for_each_sched_rt_entity(rt_se) {

		rt_se->back = back;

		back = rt_se;

	}



	dequeue_top_rt_rq(rt_rq_of_se(back));

	rt_nr_running = rt_rq_of_se(back)->rt_nr_running;



	for (rt_se = back; rt_se; rt_se = rt_se->back) {

		if (on_rt_rq(rt_se))

			__dequeue_rt_entity(rt_se, flags);

	}



	dequeue_top_rt_rq(rt_rq_of_se(back), rt_nr_running);

}



static void enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)