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

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

[Kirill Tkhai]

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))
 			check_resched = 0;
 #endif /* CONFIG_SMP */
 		if (check_resched && p->prio < rq->curr->prio)

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

[Libo Chen]

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?

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.

So I don`t agree this patch.  


>  			check_resched = 0;
>  #endif /* CONFIG_SMP */
>  		if (check_resched && p->prio < rq->curr->prio)
> --
> To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

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

[Steven Rostedt]

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


>  		    /* 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)

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

[Steven Rostedt]

Peter,

I'm going through my inbox (over a year old), and found this patch from
Kirill. It looks fine to me. You can apply it with my

  Acked-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

[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)

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

[Nicholas Mc Guire]

On Wed, 12 Mar 2014, Steven Rostedt wrote:

> Peter,
> 
> I'm going through my inbox (over a year old), and found this patch from
> Kirill. It looks fine to me. You can apply it with my
> 
>   Acked-by: Steven Rostedt <rostedt@goodmis.org>
>
> -- Steve
> 
> [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)

would there not need to be a check for p->migrate_disable ?
push_rt_task() is not checking and so a high prio RT task 
preemting a low prio RT task in a migrate_disable() section
would actually push it off this cpu ? atleast I did not 
find why that would not happen.

thx!
hofrat

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

[Kirill Tkhai]

12.03.2014, 14:39, "Nicholas Mc Guire" <der.herr@hofr.at>:
> On Wed, 12 Mar 2014, Steven Rostedt wrote:
>
>>  Peter,
>>
>>  I'm going through my inbox (over a year old), and found this patch from
>>  Kirill. It looks fine to me. You can apply it with my
>>
>>    Acked-by: Steven Rostedt <rostedt@goodmis.org>
>>
>>  -- Steve
>>
>>  [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)
>
> would there not need to be a check for p->migrate_disable ?
> push_rt_task() is not checking and so a high prio RT task
> preemting a low prio RT task in a migrate_disable() section
> would actually push it off this cpu ? atleast I did not
> find why that would not happen.

Hi, Nicholas!

p is not rq->curr, so its p->migrate_disable state is already updated and
it can't be pushed (nr_cpus_allowed == 1 and it's not pushable).

(If I understand right, that you worry about this).

Kirill

> thx!
> hofrat

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

[Peter Zijlstra]

On Wed, Mar 12, 2014 at 06:18:33AM -0400, Steven Rostedt wrote:
> Peter,
> 
> I'm going through my inbox (over a year old), and found this patch from
> Kirill. It looks fine to me. You can apply it with my
> 

Thanks!

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

[Steven Rostedt]

On Wed, 12 Mar 2014 16:23:48 +0400
Kirill Tkhai <tkhai@yandex.ru> wrote:

> > would there not need to be a check for p->migrate_disable ?
> > push_rt_task() is not checking and so a high prio RT task
> > preemting a low prio RT task in a migrate_disable() section
> > would actually push it off this cpu ? atleast I did not
> > find why that would not happen.
> 
> Hi, Nicholas!
> 
> p is not rq->curr, so its p->migrate_disable state is already updated and
> it can't be pushed (nr_cpus_allowed == 1 and it's not pushable).
> 
> (If I understand right, that you worry about this).
> 

Correct. If p has migrate_disabled set and scheduled out, then when it
gets scheduled out, its cpu affinity gets set to only the current cpu
and nr_cpu_allowed to 1. No need to check here if p->migrate_disabled is
set.

-- Steve