Re: [PATCH]sched_rt.c: Avoid unnecessary dequeue and enqueue of pushable tasks in set_cpus_allowed_rt()

Tkhai Kirill
2011-12-20T20:36:17Z

20.12.2011, 21:44, "Oleg Nesterov" <oleg@redhat.com>:
> On 12/02, Kirill Tkhai wrote:
>
>>  Migration status depends on a difference of weight from 0 and 1. If
>>  weight > 1 (<= 1) and old weight <= 1 (> 1) then task becomes pushable
>>  (not pushable). We are not insterested in exact values of it, is it 3 or
>>  4, for example.
>>
>>  Now if we are changing affinity from a set of 3 cpus to a set of 4, the
>>  task will be dequeued and enqueued sequentially without important
>>  difference in comparison with initial state. The only difference is in
>>  internal representation of plist queue of pushable tasks and the fact
>>  that the task may won't be the first in a sequence of the same priority
>>  tasks. But it seems to me it gives nothing.
>
> Looks reasonable, although I can't say I really understand this code.
> Add Gregory.
>
>>  Signed-off-by: Tkhai Kirill <tkhai@yandex.ru>
>>
>>  --- kernel/sched_rt.c.orig 2011-12-02 00:29:11.970243145 +0400
>>  +++ kernel/sched_rt.c 2011-12-02 00:37:43.622846606 +0400
>
> please use -p1
>

Sorry, this time I'm sending "git diffed" output.

>>  @@ -1572,43 +1572,37 @@ static void set_cpus_allowed_rt(struct t
>>                                   const struct cpumask *new_mask)
>>   {
>>           int weight = cpumask_weight(new_mask);
>>  + struct rq *rq;
>>
>>           BUG_ON(!rt_task(p));
>>
>>           /*
>>  - * Update the migration status of the RQ if we have an RT task
>>  - * which is running AND changing its weight value.
>>  + * Just exit if it's not necessary to change migration status
>>            */
>>  - if (p->on_rq && (weight != p->rt.nr_cpus_allowed)) {
>>  - struct rq *rq = task_rq(p);
>>  + if ((p->rt.nr_cpus_allowed <= 1 && weight <= 1)
>>  + || (p->rt.nr_cpus_allowed > 1 && weight > 1))
>>  + return;
>
> Subjective, but may be
>
>         if ((p->rt.nr_cpus_allowed > 1) != (weight > 1))
>                 return;
>
> looks more understandable?

Yes, thanks.

---

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 3640ebb..4467f4d 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1774,43 +1774,36 @@ static void set_cpus_allowed_rt(struct task_struct *p,
const struct cpumask *new_mask)
{
int weight = cpumask_weight(new_mask);
+ struct rq *rq;

BUG_ON(!rt_task(p));

/*
- * Update the migration status of the RQ if we have an RT task
- * which is running AND changing its weight value.
+ * Just exit if it's not necessary to change migration status
*/
- if (p->on_rq && (weight != p->rt.nr_cpus_allowed)) {
- struct rq *rq = task_rq(p);
-
- if (!task_current(rq, p)) {
- /*
- * Make sure we dequeue this task from the pushable list
- * before going further. It will either remain off of
- * the list because we are no longer pushable, or it
- * will be requeued.
- */
- if (p->rt.nr_cpus_allowed > 1)
- dequeue_pushable_task(rq, p);
-
- /*
- * Requeue if our weight is changing and still > 1
- */
- if (weight > 1)
- enqueue_pushable_task(rq, p);
+ if ((p->rt.nr_cpus_allowed > 1) != (weight > 1))
+ return;

- }
+ if (!p->on_rq)
+ return;

- if ((p->rt.nr_cpus_allowed <= 1) && (weight > 1)) {
- rq->rt.rt_nr_migratory++;
- } else if ((p->rt.nr_cpus_allowed > 1) && (weight <= 1)) {
- BUG_ON(!rq->rt.rt_nr_migratory);
- rq->rt.rt_nr_migratory--;
- }
+ rq = task_rq(p);

- update_rt_migration(&rq->rt);
+ /*
+ * Several cpus were allowed but now it's not so OR vice versa