linux-kernel - Re: [sched/rt] Optimization of function pull_rt

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Date:	Wed, 18 Apr 2012 17:16:55 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [sched/rt] Optimization of function pull_rt_task()

On Wed, 2012-04-18 at 14:32 -0400, Steven Rostedt wrote:
> On Mon, 2012-04-16 at 12:06 -0400, Steven Rostedt wrote:
> > On Sun, 2012-04-15 at 23:45 +0400, Kirill Tkhai wrote:
> > > The condition (src_rq->rt.rt_nr_running) is weak because it doesn't
> > > consider the cases when src_rq has only processes bound to it (when
> > > single cpu is allowed). It may be running kernel thread like
> > > migration/x etc.
> > > 
> > > So it's better to use more stronger condition which is able to exclude
> > > above conditions. The function has_pushable_tasks() complitely does
> > > this. A task may be pullable for another cpu rq only if he is pushable
> > > for his own queue.
> > 
> > I considered this before, and for some reason I never did the change.
> > I'll have to think about it. It seems like this would be the obvious
> > case, but I think there was something not so obvious that caused issues.
> > But I don't remember what it was.
> > 
> > I'll have to rethink this again.
> 
> I can't find anything wrong with this change. Maybe things change, or I
> was thinking of another change.
> 
> I'll apply it and start running my tests against it.

Not only does this seem to work fine, I took it one step further :-)

Peter, do you see anything wrong with this patch?

-- Steve

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 61e3086..b44fd1b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1416,39 +1416,15 @@ static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 /* Return the second highest RT task, NULL otherwise */
 static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
 {
-	struct task_struct *next = NULL;
-	struct sched_rt_entity *rt_se;
-	struct rt_prio_array *array;
-	struct rt_rq *rt_rq;
-	int idx;
+	struct plist_head *head = &rq->rt.pushable_tasks;
+	struct task_struct *next;
 
-	for_each_leaf_rt_rq(rt_rq, rq) {
-		array = &rt_rq->active;
-		idx = sched_find_first_bit(array->bitmap);
-next_idx:
-		if (idx >= MAX_RT_PRIO)
-			continue;
-		if (next && next->prio <= idx)
-			continue;
-		list_for_each_entry(rt_se, array->queue + idx, run_list) {
-			struct task_struct *p;
-
-			if (!rt_entity_is_task(rt_se))
-				continue;
-
-			p = rt_task_of(rt_se);
-			if (pick_rt_task(rq, p, cpu)) {
-				next = p;
-				break;
-			}
-		}
-		if (!next) {
-			idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
-			goto next_idx;
-		}
+	plist_for_each_entry(next, head, pushable_tasks) {
+		if (pick_rt_task(rq, next, cpu))
+			return next;
 	}
 
-	return next;
+	return NULL;
 }
 
 static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);


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