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sched_rt: Avoid unnecessary dequeue and enqueue of pushable tasks in set_cpus_allowed_rt()

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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 (or not pushable). We are not insterested in its exact
values, 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.

Link: http://lkml.kernel.org/r/273741334120764@web83.yandex.ru

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tkhai Kirill <tkhai@yandex.ru>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Kirill Tkhai 2012-04-11 09:06:04 +04:00 committed by Steven Rostedt
parent 600e145882
commit 8d3d5ada56
1 changed files with 26 additions and 30 deletions
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56
kernel/sched/rt.c
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@ -1803,44 +1803,40 @@ static void task_woken_rt(struct rq *rq, struct task_struct *p)
static void set_cpus_allowed_rt(struct task_struct *p,
const struct cpumask *new_mask)
{
int weight = cpumask_weight(new_mask);
struct rq *rq;
int weight;
BUG_ON(!rt_task(p));
if (!p->on_rq)
return;
weight = cpumask_weight(new_mask);
/*
* Update the migration status of the RQ if we have an RT task
* which is running AND changing its weight value.
* Only update if the process changes its state from whether it
* can migrate or not.
*/
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))
return;
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);
rq = 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)) {
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--;
}
update_rt_migration(&rq->rt);
/*
* The process used to be able to migrate OR it can now migrate
*/
if (weight <= 1) {
if (!task_current(rq, p))
dequeue_pushable_task(rq, p);
BUG_ON(!rq->rt.rt_nr_migratory);
rq->rt.rt_nr_migratory--;
} else {
if (!task_current(rq, p))
enqueue_pushable_task(rq, p);
rq->rt.rt_nr_migratory++;
}
update_rt_migration(&rq->rt);
}
/* Assumes rq->lock is held */