sched: Guarantee task priority in pick_next_task()
Michael spotted that the idle_balance() push down created a task
priority problem.
Previously, when we called idle_balance() before pick_next_task() it
wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
task slipped in.
Similarly for pre_schedule(), rt pre-schedule could have a dl task
slip in.
But by pulling it into the pick_next_task() loop, we'll not try a
higher task priority again.
Cure this by creating a re-start condition in pick_next_task(); and
triggering this from pick_next_task_{rt,fair}().
It also fixes a live-lock where we get stuck in pick_next_task_fair()
due to idle_balance() seeing !0 nr_running but there not actually
being any fair tasks about.
Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Fixes: 38033c37fa
("sched: Push down pre_schedule() and idle_balance()")
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20140224121218.GR15586@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
parent
06d50c65b1
commit
37e117c07b
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@ -2586,25 +2586,29 @@ static inline void schedule_debug(struct task_struct *prev)
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static inline struct task_struct *
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pick_next_task(struct rq *rq, struct task_struct *prev)
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{
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const struct sched_class *class;
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const struct sched_class *class = &fair_sched_class;
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struct task_struct *p;
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/*
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* Optimization: we know that if all tasks are in
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* the fair class we can call that function directly:
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*/
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if (likely(prev->sched_class == &fair_sched_class &&
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if (likely(prev->sched_class == class &&
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rq->nr_running == rq->cfs.h_nr_running)) {
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p = fair_sched_class.pick_next_task(rq, prev);
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if (likely(p))
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if (likely(p && p != RETRY_TASK))
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return p;
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}
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again:
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for_each_class(class) {
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p = class->pick_next_task(rq, prev);
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if (p)
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if (p) {
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if (unlikely(p == RETRY_TASK))
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goto again;
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return p;
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}
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}
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BUG(); /* the idle class will always have a runnable task */
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}
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@ -4686,6 +4686,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev)
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struct cfs_rq *cfs_rq = &rq->cfs;
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struct sched_entity *se;
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struct task_struct *p;
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int new_tasks;
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again:
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#ifdef CONFIG_FAIR_GROUP_SCHED
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@ -4784,7 +4785,17 @@ simple:
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return p;
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idle:
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if (idle_balance(rq)) /* drops rq->lock */
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/*
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* Because idle_balance() releases (and re-acquires) rq->lock, it is
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* possible for any higher priority task to appear. In that case we
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* must re-start the pick_next_entity() loop.
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*/
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new_tasks = idle_balance(rq);
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if (rq->nr_running != rq->cfs.h_nr_running)
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return RETRY_TASK;
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if (new_tasks)
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goto again;
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return NULL;
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@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
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struct task_struct *p;
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struct rt_rq *rt_rq = &rq->rt;
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if (need_pull_rt_task(rq, prev))
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if (need_pull_rt_task(rq, prev)) {
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pull_rt_task(rq);
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/*
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* pull_rt_task() can drop (and re-acquire) rq->lock; this
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* means a dl task can slip in, in which case we need to
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* re-start task selection.
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*/
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if (unlikely(rq->dl.dl_nr_running))
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return RETRY_TASK;
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}
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if (!rt_rq->rt_nr_running)
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return NULL;
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@ -1091,6 +1091,8 @@ static const u32 prio_to_wmult[40] = {
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#define DEQUEUE_SLEEP 1
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#define RETRY_TASK ((void *)-1UL)
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struct sched_class {
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const struct sched_class *next;
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@ -1105,6 +1107,9 @@ struct sched_class {
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* It is the responsibility of the pick_next_task() method that will
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* return the next task to call put_prev_task() on the @prev task or
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* something equivalent.
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*
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* May return RETRY_TASK when it finds a higher prio class has runnable
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* tasks.
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*/
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struct task_struct * (*pick_next_task) (struct rq *rq,
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struct task_struct *prev);
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