sched: rework of "prioritize non-migratable tasks over migratable ones"

regarding this commit: 45c01e824991b2dd0a332e19efc4901acb31209f

I think we can do it simpler. Please take a look at the patch below.

Instead of having 2 separate arrays (which is + ~800 bytes on x86_32 and
twice so on x86_64), let's add "exclusive" (the ones that are bound to
this CPU) tasks to the head of the queue and "shared" ones -- to the
end.

In case of a few newly woken up "exclusive" tasks, they are 'stacked'
(not queued as now), meaning that a task {i+1} is being placed in front
of the previously woken up task {i}. But I don't think that this
behavior may cause any realistic problems.

There are a couple of changes on top of this one.

(1) in check_preempt_curr_rt()

I don't think there is a need for the "pick_next_rt_entity(rq, &rq->rt)
!= &rq->curr->rt" check.

enqueue_task_rt(p) and check_preempt_curr_rt() are always called one
after another with rq->lock being held so the following check
"p->rt.nr_cpus_allowed == 1 && rq->curr->rt.nr_cpus_allowed != 1" should
be enough (well, just its left part) to guarantee that 'p' has been
queued in front of the 'curr'.

(2) in set_cpus_allowed_rt()

I don't thinks there is a need for requeue_task_rt() here.

Perhaps, the only case when 'requeue' (+ reschedule) might be useful is
as follows:

i) weight == 1 && cpu_isset(task_cpu(p), *new_mask)

i.e. a task is being bound to this CPU);

ii) 'p' != rq->curr

but here, 'p' has already been on this CPU for a while and was not
migrated. i.e. it's possible that 'rq->curr' would not have high chances
to be migrated right at this particular moment (although, has chance in
a bit longer term), should we allow it to be preempted.

Anyway, I think we should not perhaps make it more complex trying to
address some rare corner cases. For instance, that's why a single queue
approach would be preferable. Unless I'm missing something obvious, this
approach gives us similar functionality at lower cost.

Verified only compilation-wise.

(Almost)-Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 11 insertions, 39 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 554de4009803..cc1d558406f8 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -153,8 +153,7 @@ static inline int task_has_rt_policy(struct task_struct *p)
  */
 struct rt_prio_array {
         DECLARE_BITMAP(bitmap, MAX_RT_PRIO+1); /* include 1 bit for delimiter */
-        struct list_head xqueue[MAX_RT_PRIO]; /* exclusive queue */
-        struct list_head squeue[MAX_RT_PRIO];  /* shared queue */
+        struct list_head queue[MAX_RT_PRIO];
 };
 
 struct rt_bandwidth {
@@ -7620,8 +7619,7 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
 
         array = &rt_rq->active;
         for (i = 0; i < MAX_RT_PRIO; i++) {
-                INIT_LIST_HEAD(array->xqueue + i);
-                INIT_LIST_HEAD(array->squeue + i);
+                INIT_LIST_HEAD(array->queue + i);
                 __clear_bit(i, array->bitmap);
         }
         /* delimiter for bitsearch: */
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 8ae3416e0bb4..f721b52acd8d 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -576,16 +576,15 @@ static void enqueue_rt_entity(struct sched_rt_entity *rt_se)
         struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
         struct rt_prio_array *array = &rt_rq->active;
         struct rt_rq *group_rq = group_rt_rq(rt_se);
+        struct list_head *queue = array->queue + rt_se_prio(rt_se);
 
         if (group_rq && rt_rq_throttled(group_rq))
                 return;
 
         if (rt_se->nr_cpus_allowed == 1)
-                list_add_tail(&rt_se->run_list,
-                              array->xqueue + rt_se_prio(rt_se));
+                list_add(&rt_se->run_list, queue);
         else
-                list_add_tail(&rt_se->run_list,
-                              array->squeue + rt_se_prio(rt_se));
+                list_add_tail(&rt_se->run_list, queue);
 
         __set_bit(rt_se_prio(rt_se), array->bitmap);
 
@@ -598,8 +597,7 @@ static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
         struct rt_prio_array *array = &rt_rq->active;
 
         list_del_init(&rt_se->run_list);
-        if (list_empty(array->squeue + rt_se_prio(rt_se))
-            && list_empty(array->xqueue + rt_se_prio(rt_se)))
+        if (list_empty(array->queue + rt_se_prio(rt_se)))
                 __clear_bit(rt_se_prio(rt_se), array->bitmap);
 
         dec_rt_tasks(rt_se, rt_rq);
@@ -666,11 +664,6 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
 /*
  * Put task to the end of the run list without the overhead of dequeue
  * followed by enqueue.
- *
- * Note: We always enqueue the task to the shared-queue, regardless of its
- * previous position w.r.t. exclusive vs shared.  This is so that exclusive RR
- * tasks fairly round-robin with all tasks on the runqueue, not just other
- * exclusive tasks.
  */
 static
 void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
@@ -678,7 +671,7 @@ void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
         struct rt_prio_array *array = &rt_rq->active;
 
         list_del_init(&rt_se->run_list);
-        list_add_tail(&rt_se->run_list, array->squeue + rt_se_prio(rt_se));
+        list_add_tail(&rt_se->run_list, array->queue + rt_se_prio(rt_se));
 }
 
 static void requeue_task_rt(struct rq *rq, struct task_struct *p)
@@ -736,9 +729,6 @@ static int select_task_rq_rt(struct task_struct *p, int sync)
 }
 #endif /* CONFIG_SMP */
 
-static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
-                                                   struct rt_rq *rt_rq);
-
 /*
  * Preempt the current task with a newly woken task if needed:
  */
@@ -764,8 +754,7 @@ static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
          */
         if((p->prio == rq->curr->prio)
            && p->rt.nr_cpus_allowed == 1
-           && rq->curr->rt.nr_cpus_allowed != 1
-           && pick_next_rt_entity(rq, &rq->rt) != &rq->curr->rt) {
+           && rq->curr->rt.nr_cpus_allowed != 1) {
                 cpumask_t mask;
 
                 if (cpupri_find(&rq->rd->cpupri, rq->curr, &mask))
@@ -789,15 +778,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
         idx = sched_find_first_bit(array->bitmap);
         BUG_ON(idx >= MAX_RT_PRIO);
 
-        queue = array->xqueue + idx;
-        if (!list_empty(queue))
-                next = list_entry(queue->next, struct sched_rt_entity,
-                                  run_list);
-        else {
-                queue = array->squeue + idx;
-                next = list_entry(queue->next, struct sched_rt_entity,
-                                  run_list);
-        }
+        queue = array->queue + idx;
+        next = list_entry(queue->next, struct sched_rt_entity, run_list);
 
         return next;
 }
@@ -867,7 +849,7 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
                         continue;
                 if (next && next->prio < idx)
                         continue;
-                list_for_each_entry(rt_se, array->squeue + idx, run_list) {
+                list_for_each_entry(rt_se, array->queue + idx, run_list) {
                         struct task_struct *p = rt_task_of(rt_se);
                         if (pick_rt_task(rq, p, cpu)) {
                                 next = p;
@@ -1249,14 +1231,6 @@ static void set_cpus_allowed_rt(struct task_struct *p,
                 }
 
                 update_rt_migration(rq);
-
-                if (unlikely(weight == 1 || p->rt.nr_cpus_allowed == 1))
-                        /*
-                         * If either the new or old weight is a "1", we need
-                         * to requeue to properly move between shared and
-                         * exclusive queues.
-                         */
-                        requeue_task_rt(rq, p);
         }
 
         p->cpus_allowed    = *new_mask;