sched: pull RT tasks from overloaded runqueues

This patch adds the algorithm to pull tasks from RT overloaded runqueues.

When a pull RT is initiated, all overloaded runqueues are examined for
a RT task that is higher in prio than the highest prio task queued on the
target runqueue. If another runqueue holds a RT task that is of higher
prio than the highest prio task on the target runqueue is found it is pulled
to the target runqueue.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 176 insertions, 11 deletions

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 547f858b0752..bacb32039e95 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -179,8 +179,17 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
 static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);
 
+static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
+{
+        if (!task_running(rq, p) &&
+            (cpu < 0 || cpu_isset(cpu, p->cpus_allowed)))
+                return 1;
+        return 0;
+}
+
 /* Return the second highest RT task, NULL otherwise */
-static struct task_struct *pick_next_highest_task_rt(struct rq *rq)
+static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
+                                                     int cpu)
 {
         struct rt_prio_array *array = &rq->rt.active;
         struct task_struct *next;
@@ -199,26 +208,36 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq)
         }
 
         queue = array->queue + idx;
+        BUG_ON(list_empty(queue));
+
         next = list_entry(queue->next, struct task_struct, run_list);
-        if (unlikely(next != rq->curr))
-                return next;
+        if (unlikely(pick_rt_task(rq, next, cpu)))
+                goto out;
 
         if (queue->next->next != queue) {
                 /* same prio task */
                 next = list_entry(queue->next->next, struct task_struct, run_list);
-                return next;
+                if (pick_rt_task(rq, next, cpu))
+                        goto out;
         }
 
+ retry:
         /* slower, but more flexible */
         idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
-        if (unlikely(idx >= MAX_RT_PRIO)) {
-                WARN_ON(1); /* rt_nr_running was 2 and above! */
+        if (unlikely(idx >= MAX_RT_PRIO))
                 return NULL;
-        }
 
         queue = array->queue + idx;
-        next = list_entry(queue->next, struct task_struct, run_list);
+        BUG_ON(list_empty(queue));
+
+        list_for_each_entry(next, queue, run_list) {
+                if (pick_rt_task(rq, next, cpu))
+                        goto out;
+        }
+
+        goto retry;
 
+ out:
         return next;
 }
 
@@ -305,13 +324,15 @@ static int push_rt_task(struct rq *this_rq)
 
         assert_spin_locked(&this_rq->lock);
 
-        next_task = pick_next_highest_task_rt(this_rq);
+        next_task = pick_next_highest_task_rt(this_rq, -1);
         if (!next_task)
                 return 0;
 
  retry:
-        if (unlikely(next_task == this_rq->curr))
+        if (unlikely(next_task == this_rq->curr)) {
+                WARN_ON(1);
                 return 0;
+        }
 
         /*
          * It's possible that the next_task slipped in of
@@ -335,7 +356,7 @@ static int push_rt_task(struct rq *this_rq)
                  * so it is possible that next_task has changed.
                  * If it has, then try again.
                  */
-                task = pick_next_highest_task_rt(this_rq);
+                task = pick_next_highest_task_rt(this_rq, -1);
                 if (unlikely(task != next_task) && task && paranoid--) {
                         put_task_struct(next_task);
                         next_task = task;
@@ -378,6 +399,149 @@ static void push_rt_tasks(struct rq *rq)
                 ;
 }
 
+static int pull_rt_task(struct rq *this_rq)
+{
+        struct task_struct *next;
+        struct task_struct *p;
+        struct rq *src_rq;
+        cpumask_t *rto_cpumask;
+        int this_cpu = this_rq->cpu;
+        int cpu;
+        int ret = 0;
+
+        assert_spin_locked(&this_rq->lock);
+
+        /*
+         * If cpusets are used, and we have overlapping
+         * run queue cpusets, then this algorithm may not catch all.
+         * This is just the price you pay on trying to keep
+         * dirtying caches down on large SMP machines.
+         */
+        if (likely(!rt_overloaded()))
+                return 0;
+
+        next = pick_next_task_rt(this_rq);
+
+        rto_cpumask = rt_overload();
+
+        for_each_cpu_mask(cpu, *rto_cpumask) {
+                if (this_cpu == cpu)
+                        continue;
+
+                src_rq = cpu_rq(cpu);
+                if (unlikely(src_rq->rt.rt_nr_running <= 1)) {
+                        /*
+                         * It is possible that overlapping cpusets
+                         * will miss clearing a non overloaded runqueue.
+                         * Clear it now.
+                         */
+                        if (double_lock_balance(this_rq, src_rq)) {
+                                /* unlocked our runqueue lock */
+                                struct task_struct *old_next = next;
+                                next = pick_next_task_rt(this_rq);
+                                if (next != old_next)
+                                        ret = 1;
+                        }
+                        if (likely(src_rq->rt.rt_nr_running <= 1))
+                                /*
+                                 * Small chance that this_rq->curr changed
+                                 * but it's really harmless here.
+                                 */
+                                rt_clear_overload(this_rq);
+                        else
+                                /*
+                                 * Heh, the src_rq is now overloaded, since
+                                 * we already have the src_rq lock, go straight
+                                 * to pulling tasks from it.
+                                 */
+                                goto try_pulling;
+                        spin_unlock(&src_rq->lock);
+                        continue;
+                }
+
+                /*
+                 * We can potentially drop this_rq's lock in
+                 * double_lock_balance, and another CPU could
+                 * steal our next task - hence we must cause
+                 * the caller to recalculate the next task
+                 * in that case:
+                 */
+                if (double_lock_balance(this_rq, src_rq)) {
+                        struct task_struct *old_next = next;
+                        next = pick_next_task_rt(this_rq);
+                        if (next != old_next)
+                                ret = 1;
+                }
+
+                /*
+                 * Are there still pullable RT tasks?
+                 */
+                if (src_rq->rt.rt_nr_running <= 1) {
+                        spin_unlock(&src_rq->lock);
+                        continue;
+                }
+
+ try_pulling:
+                p = pick_next_highest_task_rt(src_rq, this_cpu);
+
+                /*
+                 * Do we have an RT task that preempts
+                 * the to-be-scheduled task?
+                 */
+                if (p && (!next || (p->prio < next->prio))) {
+                        WARN_ON(p == src_rq->curr);
+                        WARN_ON(!p->se.on_rq);
+
+                        /*
+                         * There's a chance that p is higher in priority
+                         * than what's currently running on its cpu.
+                         * This is just that p is wakeing up and hasn't
+                         * had a chance to schedule. We only pull
+                         * p if it is lower in priority than the
+                         * current task on the run queue or
+                         * this_rq next task is lower in prio than
+                         * the current task on that rq.
+                         */
+                        if (p->prio < src_rq->curr->prio ||
+                            (next && next->prio < src_rq->curr->prio))
+                                goto bail;
+
+                        ret = 1;
+
+                        deactivate_task(src_rq, p, 0);
+                        set_task_cpu(p, this_cpu);
+                        activate_task(this_rq, p, 0);
+                        /*
+                         * We continue with the search, just in
+                         * case there's an even higher prio task
+                         * in another runqueue. (low likelyhood
+                         * but possible)
+                         */
+
+                        /*
+                         * Update next so that we won't pick a task
+                         * on another cpu with a priority lower (or equal)
+                         * than the one we just picked.
+                         */
+                        next = p;
+
+                }
+ bail:
+                spin_unlock(&src_rq->lock);
+        }
+
+        return ret;
+}
+
+static void schedule_balance_rt(struct rq *rq,
+                                struct task_struct *prev)
+{
+        /* Try to pull RT tasks here if we lower this rq's prio */
+        if (unlikely(rt_task(prev)) &&
+            rq->rt.highest_prio > prev->prio)
+                pull_rt_task(rq);
+}
+
 static void schedule_tail_balance_rt(struct rq *rq)
 {
         /*
@@ -500,6 +664,7 @@ move_one_task_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
 }
 #else /* CONFIG_SMP */
 # define schedule_tail_balance_rt(rq)   do { } while (0)
+# define schedule_balance_rt(rq, prev)  do { } while (0)
 #endif /* CONFIG_SMP */
 
 static void task_tick_rt(struct rq *rq, struct task_struct *p)