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: 38033c37faab ("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>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a8a73b8..cde573d 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct task_struct *prev)
 static inline struct task_struct *
 pick_next_task(struct rq *rq, struct task_struct *prev)
 {
-       const struct sched_class *class;
+       const struct sched_class *class = &fair_sched_class;
        struct task_struct *p;
 
        /*
         * Optimization: we know that if all tasks are in
         * the fair class we can call that function directly:
         */
-       if (likely(prev->sched_class == &fair_sched_class &&
+       if (likely(prev->sched_class == class &&
                   rq->nr_running == rq->cfs.h_nr_running)) {
                p = fair_sched_class.pick_next_task(rq, prev);
-               if (likely(p))
+               if (likely(p && p != RETRY_TASK))
                        return p;
        }
 
+again:
        for_each_class(class) {
                p = class->pick_next_task(rq, prev);
-               if (p)
+               if (p) {
+                       if (unlikely(p == RETRY_TASK))
+                               goto again;
                        return p;
+               }
        }
 
        BUG(); /* the idle class will always have a runnable task */

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index be4f7d9..16042b5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4686,6 +4686,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev)
        struct cfs_rq *cfs_rq = &rq->cfs;
        struct sched_entity *se;
        struct task_struct *p;
+       int new_tasks;
 
 again:
 #ifdef CONFIG_FAIR_GROUP_SCHED
@@ -4784,7 +4785,17 @@ simple:
        return p;
 
 idle:
-       if (idle_balance(rq)) /* drops rq->lock */
+       /*
+        * Because idle_balance() releases (and re-acquires) rq->lock, it is
+        * possible for any higher priority task to appear. In that case we
+        * must re-start the pick_next_entity() loop.
+        */
+       new_tasks = idle_balance(rq);
+
+       if (rq->nr_running != rq->cfs.h_nr_running)
+               return RETRY_TASK;
+
+       if (new_tasks)
                goto again;
 
        return NULL;

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4d4b386..398b3f9 100644 (file)
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
        struct task_struct *p;
        struct rt_rq *rt_rq = &rq->rt;
 
-       if (need_pull_rt_task(rq, prev))
+       if (need_pull_rt_task(rq, prev)) {
                pull_rt_task(rq);
+               /*
+                * pull_rt_task() can drop (and re-acquire) rq->lock; this
+                * means a dl task can slip in, in which case we need to
+                * re-start task selection.
+                */
+               if (unlikely(rq->dl.dl_nr_running))
+                       return RETRY_TASK;
+       }
 
        if (!rt_rq->rt_nr_running)
                return NULL;

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 046084e..1929deb 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1091,6 +1091,8 @@ static const u32 prio_to_wmult[40] = {
 
 #define DEQUEUE_SLEEP          1
 
+#define RETRY_TASK             ((void *)-1UL)
+
 struct sched_class {
        const struct sched_class *next;
 
@@ -1105,6 +1107,9 @@ struct sched_class {
         * It is the responsibility of the pick_next_task() method that will
         * return the next task to call put_prev_task() on the @prev task or
         * something equivalent.
+        *
+        * May return RETRY_TASK when it finds a higher prio class has runnable
+        * tasks.
         */
        struct task_struct * (*pick_next_task) (struct rq *rq,
                                                struct task_struct *prev);