Re: [sched/rt] Optimization of function pull_rt_task()

From: Kirill Tkhai
Date: Tue Dec 18 2012 - 05:43:35 EST




16.11.2012, 00:36, "Steven Rostedt" <rostedt@xxxxxxxxxxx>:
> Doing my INBOX maintenance (clean up), I've stumbled on this thread
> again. I'm not sure the changes here are hopeless.
>
> On Mon, 2012-06-04 at 13:27 +0800, Yong Zhang wrote:
>
>>  On Fri, Jun 01, 2012 at 08:45:16PM +0400, Kirill Tkhai wrote:
>>>  19.04.2012, 12:54, "Yong Zhang" <yong.zhang0@xxxxxxxxx>:
>>>>  On Wed, Apr 18, 2012 at 05:16:55PM -0400, Steven Rostedt wrote:
>>>>>  ?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 :-)
>>>>  Hmm... throttle doesn't handle the pushable list, so we may find a
>>>>  throttled task by pick_next_pushable_task().
>>>>
>>>>  Thanks,
>>>>  Yong
>>>  I don't complitelly understand throttle logic.
>>>
>>>  Is the source patch not-appliable the same reason?
>>  I guess so.
>>
>>  Your patch will change the semantic of pick_next_pushable_task().
>
> Looking at the original patch, I don't see how it changes the semantics
> (although mine may have). The original patch was:
>
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1729,7 +1729,7 @@ static int pull_rt_task(struct rq *this_rq)
>                 /*
>                  * Are there still pullable RT tasks?
>                  */
> -               if (src_rq->rt.rt_nr_running <= 1)
> +               if (!has_pushable_tasks(src_rq))
>                         goto skip;
>
>                 p = pick_next_highest_task_rt(src_rq, this_cpu);
>
> And I still don't see a problem with this. If a rq has no pushable
> tasks, then we shouldn't bother trying to pull from it (no task can
> migrate).
>
> Thus, the original patch, I believe should be applied without question.
>
> Now, about my patch, the one that made pick_next_highest_task_rt into
> just:
>
> static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
> {
>        struct plist_head *head = &rq->rt.pushable_tasks;
>        struct task_struct *next;
>
>        plist_for_each_entry(next, head, pushable_tasks) {
>               if (pick_rt_task(rq, next, cpu))
>                       return next;
>        }
>
>        return NULL;
> }
>
> You said could pick a task from a throttled rq. I'm not sure that is
> different than what we have now. As the current
> pick_next_highest_task_rt() just does a loop over the leaf_rt_rqs which
> includes throttled rqs. That's because a throttled rq will not dequeue
> the rt_rq from the leaf_rt_rq list if the rt_rq has rt_nr_running != 0.

Yes, there is no connection between logic of pushable tasks and throttling at the moment.
These activities are independent. ( I tried to connect them at the patch:
http://lkml.indiana.edu/hypermail/linux/kernel/1211.2/03750.html )

I think, there is no problem.

Kirill

>
> I'm still thinking about adding both patches.
>
> -- Steve
>
>>  Thanks,
>>  Yong
>>>  Kirill
>>>>>  ?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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