Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

[Kirill Tkhai]

> From: Libo Chen <libo.chen@huawei.com>
> 
> On 2013-1-29 4:23, Kirill Tkhai wrote:
> 
>> Just switched pinned task is not able to be pushed. If the rq had had
>> several RT tasks before they have already been considered as candidates
>> to be pushed (or pulled).
>>
>> Signed-off-by: Kirill V Tkhai <tkhai@yandex.ru>
>> CC: Steven Rostedt <rostedt@goodmis.org>
>> CC: Ingo Molnar <mingo@kernel.org>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> CC: linux-rt-users <linux-rt-users@vger.kernel.org>
>> ---
>> kernel/sched/rt.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 4e8f0f4..5f7d92b 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -1925,9 +1925,9 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>> */
>> if (p->on_rq && rq->curr != p) {
>> #ifdef CONFIG_SMP
>> - if (rq->rt.overloaded && push_rt_task(rq) &&
>> + if (rq->rt.overloaded && p->nr_cpus_allowed > 1 &&
>> /* Don't resched if we changed runqueues */
>> - rq != task_rq(p))
>> + push_rt_task(rq) && rq != task_rq(p))
> 
> I think you worry about it was excess to call push_rt_task, since the task of p->nr_cpus_allowed=1 can`t be pushed. is that right?

Sure.

> 
> The task of p->nr_cpus_allowed =1 would`t be added to pushable_tasks list （see the enqueue_task_rt()）and this push_rt_task() need to push other tasks when rt.overloaded.

If rq has already had 2 or more pushable tasks and we try to add a pinned task then call of push_rt_task will just waste a time.

> 
> So I don`t agree this patch.
> 
>> check_resched = 0;
>> #endif /* CONFIG_SMP */
>> if (check_resched && p->prio < rq->curr->prio)
>> --
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 -- 

Re: Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

[Steven Rostedt]

On Tue, 2013-01-29 at 13:13 +0400, Kirill Tkhai wrote:

> > 
> > The task of p->nr_cpus_allowed =1 would`t be added to pushable_tasks list （see the enqueue_task_rt()）and this push_rt_task() need to push other tasks when rt.overloaded.
> 
> If rq has already had 2 or more pushable tasks and we try to add a pinned task then call of push_rt_task will just waste a time.
> 

Correct, I just have some minor comments about the patch.

-- Steve

Re:[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

[Kirill Tkhai]

> On Tue, 2013-01-29 at 00:23 +0400, Kirill Tkhai wrote:
> 
>> Just switched pinned task is not able to be pushed. If the rq had had
>> several RT tasks before they have already been considered as candidates
>> to be pushed (or pulled).
> 
> Thanks, but I have one minor nit.
> 
>> Signed-off-by: Kirill V Tkhai <tkhai@yandex.ru>
>> CC: Steven Rostedt <rostedt@goodmis.org>
>> CC: Ingo Molnar <mingo@kernel.org>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> CC: linux-rt-users <linux-rt-users@vger.kernel.org>
>> ---
>> kernel/sched/rt.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 4e8f0f4..5f7d92b 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -1925,9 +1925,9 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>> */
>> if (p->on_rq && rq->curr != p) {
>> #ifdef CONFIG_SMP
>> - if (rq->rt.overloaded && push_rt_task(rq) &&
>> + if (rq->rt.overloaded && p->nr_cpus_allowed > 1 &&
> 
> I would swap the order here. That is, check p->nr_cpus_allowed before
> rq->rt.overloaded. The order doesn't really matter, and even though
> rq->rt.overloaded is probably more likely to be 0, it is used by other
> CPUs. And as a micro-optimization, I rather read something that is used
> by only one CPU than to read something that may be modified by many
> CPUs.
> 
> -- Steve
> 

I corrected. Thanks.

[PATCH]sched/rt: Do not try to push tasks if pinned task switches to RT

Just switched pinned task is not able to be pushed. If the rq had had
several RT tasks before they have already been considered as candidates
to be pushed (or pulled).

Signed-off-by: Kirill V Tkhai <tkhai@yandex.ru>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: linux-rt-users <linux-rt-users@vger.kernel.org>
---
 kernel/sched/rt.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4e8f0f4..5aab032 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1925,9 +1925,9 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
 	 */
 	if (p->on_rq && rq->curr != p) {
 #ifdef CONFIG_SMP
-		if (rq->rt.overloaded && push_rt_task(rq) &&
+		if (p->nr_cpus_allowed > 1 && rq->rt.overloaded &&
 		    /* Don't resched if we changed runqueues */
-		    rq != task_rq(p))
+		    push_rt_task(rq) && rq != task_rq(p))
 			check_resched = 0;
 #endif /* CONFIG_SMP */
 		if (check_resched && p->prio < rq->curr->prio)