[RESEND,1/2] sched/rt: Check to push the task when changing its affinity

Message ID 1423012341-30265-1-git-send-email-xlpang@126.com
State New
Headers show

Commit Message

Xunlei Pang Feb. 4, 2015, 1:12 a.m.
From: Xunlei Pang <pang.xunlei@linaro.org>

We may suffer from extra rt overload rq due to the affinity,
so when the affinity of any runnable rt task is changed, we
should check to trigger balancing, otherwise it will cause
some unnecessary delayed real-time response. Unfortunately,
current RT global scheduler doesn't trigger anything.

For example: a 2-cpu system with two runnable FIFO tasks(same
rt_priority) bound on CPU0, let's name them rt1(running) and
rt2(runnable) respectively; CPU1 has no RTs. Then, someone sets
the affinity of rt2 to 0x3(i.e. CPU0 and CPU1), but after this,
rt2 still can't be scheduled until rt1 enters schedule(), this
definitely causes some/big response latency for rt2.

So, when doing set_cpus_allowed_rt(), if detecting such cases,
check to trigger a push behaviour.

Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
---
 kernel/sched/rt.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 59 insertions(+), 10 deletions(-)

Comments

pang.xunlei Feb. 4, 2015, 12:59 p.m. | #1
Hi Steve,

On 4 February 2015 at 11:14, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Wed,  4 Feb 2015 09:12:20 +0800
> Xunlei Pang <xlpang@126.com> wrote:
>
>> From: Xunlei Pang <pang.xunlei@linaro.org>
>>
>> +              */
>> +             cpumask_copy(&p->cpus_allowed, new_mask);
>> +             p->nr_cpus_allowed = new_weight;
>> +
>> +             if (task_running(rq, p) &&
>> +                 cpumask_test_cpu(task_cpu(p), new_mask) &&
>> +                 cpupri_find(&rq->rd->cpupri, p, NULL)) {
>
> Hmm, You called cpupri_find() which should also return a mask of the
> CPUs with the lowest priorities. I wonder if we could have utilize this
> information instead of doing it twice? Of course things could change by
> the time the task migrates.

We do this if the target task is running, so we can migrate it only
by resched_cur() or stop_one_cpu(), that's what I can think of now :)
I think resched_cur() would be better.

>
>> +                     /*
>> +                      * At this point, current task gets migratable most
>> +                      * likely due to the change of its affinity, let's
>> +                      * figure out if we can migrate it.
>> +                      *
>> +                      * Is there any task with the same priority as that
>> +                      * of current task? If found one, we should resched.
>> +                      * NOTE: The target may be unpushable.
>> +                      */
>> +                     if (p->prio == rq->rt.highest_prio.next) {
>> +                             /* One target just in pushable_tasks list. */
>> +                             requeue_task_rt(rq, p, 0);
>> +                             preempt_push = 1;
>> +                     } else if (rq->rt.rt_nr_total > 1) {
>> +                             struct task_struct *next;
>> +
>> +                             requeue_task_rt(rq, p, 0);
>> +                             /* peek only */
>> +                             next = _pick_next_task_rt(rq, 1);
>> +                             if (next != p && next->prio == p->prio)
>> +                                     preempt_push = 1;
>> +                     }
>
> I'm thinking it would be better just to send an IPI to the CPU that
> figures this out and pushes a task off of itself.

My thought is that we try the best not to disturb the running task,
actually using direct push_rt_tasks() here instead of IPI is sort of
similar to that logic in task_woken_rt().

>
>> +             } else if (!task_running(rq, p))
>> +                     direct_push = 1;
>> +     }
>>
>>       /*
>>        * Only update if the process changes its state from whether it
>>        * can migrate or not.
>>        */
>> -     if ((p->nr_cpus_allowed > 1) == (weight > 1))
>> -             return;
>> -
>> -     rq = task_rq(p);
>> +     if ((old_weight > 1) == (new_weight > 1))
>> +             goto out;
>>
>>       /*
>>        * The process used to be able to migrate OR it can now migrate
>>        */
>> -     if (weight <= 1) {
>> +     if (new_weight <= 1) {
>>               if (!task_current(rq, p))
>>                       dequeue_pushable_task(rq, p);
>>               BUG_ON(!rq->rt.rt_nr_migratory);
>> @@ -1919,6 +1961,13 @@ static void set_cpus_allowed_rt(struct task_struct *p,
>>       }
>>
>>       update_rt_migration(&rq->rt);
>> +
>> +out:
>> +     if (direct_push)
>> +             push_rt_tasks(rq);
>> +
>> +     if (preempt_push)
>> +             resched_curr(rq);
>
> I don't know. This just doesn't seem clean.
>

Thanks for your time, any of your suggestions would be helpful.

Regards,
Xunlei
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Patch

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index f4d4b07..4dacb6e 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1428,7 +1428,7 @@  static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
 	return next;
 }
 
-static struct task_struct *_pick_next_task_rt(struct rq *rq)
+static struct task_struct *_pick_next_task_rt(struct rq *rq, int peek_only)
 {
 	struct sched_rt_entity *rt_se;
 	struct task_struct *p;
@@ -1441,7 +1441,8 @@  static struct task_struct *_pick_next_task_rt(struct rq *rq)
 	} while (rt_rq);
 
 	p = rt_task_of(rt_se);
-	p->se.exec_start = rq_clock_task(rq);
+	if (!peek_only)
+		p->se.exec_start = rq_clock_task(rq);
 
 	return p;
 }
@@ -1476,7 +1477,7 @@  pick_next_task_rt(struct rq *rq, struct task_struct *prev)
 
 	put_prev_task(rq, prev);
 
-	p = _pick_next_task_rt(rq);
+	p = _pick_next_task_rt(rq, 0);
 
 	/* The running task is never eligible for pushing */
 	dequeue_pushable_task(rq, p);
@@ -1886,28 +1887,69 @@  static void set_cpus_allowed_rt(struct task_struct *p,
 				const struct cpumask *new_mask)
 {
 	struct rq *rq;
-	int weight;
+	int old_weight, new_weight;
+	int preempt_push = 0, direct_push = 0;
 
 	BUG_ON(!rt_task(p));
 
 	if (!task_on_rq_queued(p))
 		return;
 
-	weight = cpumask_weight(new_mask);
+	old_weight = p->nr_cpus_allowed;
+	new_weight = cpumask_weight(new_mask);
+
+	rq = task_rq(p);
+
+	if (new_weight > 1 &&
+	    rt_task(rq->curr) &&
+	    !test_tsk_need_resched(rq->curr)) {
+		/*
+		 * Set new mask information to prepare pushing.
+		 * It's safe to do this here.
+		 */
+		cpumask_copy(&p->cpus_allowed, new_mask);
+		p->nr_cpus_allowed = new_weight;
+
+		if (task_running(rq, p) &&
+		    cpumask_test_cpu(task_cpu(p), new_mask) &&
+		    cpupri_find(&rq->rd->cpupri, p, NULL)) {
+			/*
+			 * At this point, current task gets migratable most
+			 * likely due to the change of its affinity, let's
+			 * figure out if we can migrate it.
+			 *
+			 * Is there any task with the same priority as that
+			 * of current task? If found one, we should resched.
+			 * NOTE: The target may be unpushable.
+			 */
+			if (p->prio == rq->rt.highest_prio.next) {
+				/* One target just in pushable_tasks list. */
+				requeue_task_rt(rq, p, 0);
+				preempt_push = 1;
+			} else if (rq->rt.rt_nr_total > 1) {
+				struct task_struct *next;
+
+				requeue_task_rt(rq, p, 0);
+				/* peek only */
+				next = _pick_next_task_rt(rq, 1);
+				if (next != p && next->prio == p->prio)
+					preempt_push = 1;
+			}
+		} else if (!task_running(rq, p))
+			direct_push = 1;
+	}
 
 	/*
 	 * Only update if the process changes its state from whether it
 	 * can migrate or not.
 	 */
-	if ((p->nr_cpus_allowed > 1) == (weight > 1))
-		return;
-
-	rq = task_rq(p);
+	if ((old_weight > 1) == (new_weight > 1))
+		goto out;
 
 	/*
 	 * The process used to be able to migrate OR it can now migrate
 	 */
-	if (weight <= 1) {
+	if (new_weight <= 1) {
 		if (!task_current(rq, p))
 			dequeue_pushable_task(rq, p);
 		BUG_ON(!rq->rt.rt_nr_migratory);
@@ -1919,6 +1961,13 @@  static void set_cpus_allowed_rt(struct task_struct *p,
 	}
 
 	update_rt_migration(&rq->rt);
+
+out:
+	if (direct_push)
+		push_rt_tasks(rq);
+
+	if (preempt_push)
+		resched_curr(rq);
 }
 
 /* Assumes rq->lock is held */