[PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Byungchul Park]

Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
task == next_task. Remove a redundant condition and make code simpler.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 kernel/sched/deadline.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 27737f3..ad8d577 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
 		 * then possible that next_task has migrated.
 		 */
 		task = pick_next_pushable_dl_task(rq);
-		if (task_cpu(next_task) == rq->cpu && task == next_task) {
+		if (task == next_task) {
 			/*
 			 * The task is still there. We don't try
 			 * again, some other cpu will pull it when ready.
-- 
1.9.1

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Byungchul Park]

On Wed, Feb 15, 2017 at 02:11:51PM +0900, Byungchul Park wrote:
> Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> task == next_task. Remove a redundant condition and make code simpler.

Furthermore, unnecessary one branch and unnecessary two LOAD operations
in 'if' statement can be avoided.

> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  kernel/sched/deadline.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 27737f3..ad8d577 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
>  		 * then possible that next_task has migrated.
>  		 */
>  		task = pick_next_pushable_dl_task(rq);
> -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> +		if (task == next_task) {
>  			/*
>  			 * The task is still there. We don't try
>  			 * again, some other cpu will pull it when ready.
> -- 
> 1.9.1

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Juri Lelli]

[+Steve, Luca]

Hi,

On 15/02/17 14:11, Byungchul Park wrote:
> Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> task == next_task. Remove a redundant condition and make code simpler.
> 
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  kernel/sched/deadline.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 27737f3..ad8d577 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
>  		 * then possible that next_task has migrated.
>  		 */
>  		task = pick_next_pushable_dl_task(rq);
> -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> +		if (task == next_task) {

Seems a sensible optimization to me. Actually, we are doing the same for
rt.c; Steve, Peter, do you think we should optimize that as well?

Thanks,

- Juri

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Peter Zijlstra]

On Wed, Feb 15, 2017 at 10:47:49AM +0000, Juri Lelli wrote:
> [+Steve, Luca]
> 
> Hi,
> 
> On 15/02/17 14:11, Byungchul Park wrote:
> > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > task == next_task. Remove a redundant condition and make code simpler.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > ---
> >  kernel/sched/deadline.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 27737f3..ad8d577 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> >  		 * then possible that next_task has migrated.
> >  		 */
> >  		task = pick_next_pushable_dl_task(rq);
> > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > +		if (task == next_task) {
> 
> Seems a sensible optimization to me. Actually, we are doing the same for
> rt.c; Steve, Peter, do you think we should optimize that as well?
> 

If correct (and I've not spend brain cycles on that) yes. We should keep
this push-pull muck synced between rt and deadline as much as possible.

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Steven Rostedt]

On Wed, 15 Feb 2017 10:47:49 +0000
Juri Lelli <juri.lelli@arm.com> wrote:

> [+Steve, Luca]
> 
> Hi,
> 
> On 15/02/17 14:11, Byungchul Park wrote:
> > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > task == next_task. Remove a redundant condition and make code simpler.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > ---
> >  kernel/sched/deadline.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 27737f3..ad8d577 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> >  		 * then possible that next_task has migrated.
> >  		 */
> >  		task = pick_next_pushable_dl_task(rq);
> > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > +		if (task == next_task) {  
> 
> Seems a sensible optimization to me. Actually, we are doing the same for
> rt.c; Steve, Peter, do you think we should optimize that as well?
> 

Are we doing the same for push_rt_task()? I don't see it, and I don't
see it in tip/sched/core either. What I have is:

  if (task_cpu(next_task) == rq->cpu && task == next_task) {

But that said, I believe this patch is correct, and we should change
rt.c as well.


  task = pick_next_pushable_dl_task(rq);

Which has:

  BUG_ON(rq->cpu != task_cpu(task))

when it returns a task other than NULL. Which means that task_cpu(task)
must be rq->cpu. Then if task == next_task, then task_cpu(next_task)
must be rq->cpu as well.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Mind fixing rt.c if it hasn't been fixed already.

-- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Juri Lelli]

On 15/02/17 09:25, Steven Rostedt wrote:
> On Wed, 15 Feb 2017 10:47:49 +0000
> Juri Lelli <juri.lelli@arm.com> wrote:
> 
> > [+Steve, Luca]
> > 
> > Hi,
> > 
> > On 15/02/17 14:11, Byungchul Park wrote:
> > > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > > task == next_task. Remove a redundant condition and make code simpler.
> > > 
> > > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > > ---
> > >  kernel/sched/deadline.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 27737f3..ad8d577 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> > >  		 * then possible that next_task has migrated.
> > >  		 */
> > >  		task = pick_next_pushable_dl_task(rq);
> > > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > > +		if (task == next_task) {  
> > 
> > Seems a sensible optimization to me. Actually, we are doing the same for
> > rt.c; Steve, Peter, do you think we should optimize that as well?
> > 
> 
> Are we doing the same for push_rt_task()? I don't see it, and I don't
> see it in tip/sched/core either. What I have is:
> 
>   if (task_cpu(next_task) == rq->cpu && task == next_task) {

Sorry, bad wording on my side. I meant the are currently checking the
same conditions both for DL and for RT, and we should probably optimize
RT as well if we are going to take this patch.

> 
> But that said, I believe this patch is correct, and we should change
> rt.c as well.
> 

That's what I meant. :)

> 
>   task = pick_next_pushable_dl_task(rq);
> 
> Which has:
> 
>   BUG_ON(rq->cpu != task_cpu(task))
> 
> when it returns a task other than NULL. Which means that task_cpu(task)
> must be rq->cpu. Then if task == next_task, then task_cpu(next_task)
> must be rq->cpu as well.

Right.

> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> 

You can also add mine

Reviewed-by: Juri Lelli <juri.lelli@arm.com>

> Mind fixing rt.c if it hasn't been fixed already.
> 
> -- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Steven Rostedt]

On Wed, 15 Feb 2017 14:45:03 +0000
Juri Lelli <juri.lelli@arm.com> wrote:

> > 
> >   task = pick_next_pushable_dl_task(rq);
> > 
> > Which has:
> > 
> >   BUG_ON(rq->cpu != task_cpu(task))
> > 
> > when it returns a task other than NULL. Which means that task_cpu(task)
> > must be rq->cpu. Then if task == next_task, then task_cpu(next_task)
> > must be rq->cpu as well.  
> 
> Right.

Perhaps my comment should also be added in the change log.

-- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Byungchul Park]

On Wed, Feb 15, 2017 at 02:45:03PM +0000, Juri Lelli wrote:
> On 15/02/17 09:25, Steven Rostedt wrote:
> > On Wed, 15 Feb 2017 10:47:49 +0000
> > Juri Lelli <juri.lelli@arm.com> wrote:
> > 
> > > [+Steve, Luca]
> > > 
> > > Hi,
> > > 
> > > On 15/02/17 14:11, Byungchul Park wrote:
> > > > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > > > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > > > task == next_task. Remove a redundant condition and make code simpler.
> > > > 
> > > > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> > > > ---
> > > >  kernel/sched/deadline.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > > index 27737f3..ad8d577 100644
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> > > >  		 * then possible that next_task has migrated.
> > > >  		 */
> > > >  		task = pick_next_pushable_dl_task(rq);
> > > > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > > > +		if (task == next_task) {  
> > > 
> > > Seems a sensible optimization to me. Actually, we are doing the same for
> > > rt.c; Steve, Peter, do you think we should optimize that as well?
> > > 
> > 
> > Are we doing the same for push_rt_task()? I don't see it, and I don't
> > see it in tip/sched/core either. What I have is:
> > 
> >   if (task_cpu(next_task) == rq->cpu && task == next_task) {
> 
> Sorry, bad wording on my side. I meant the are currently checking the
> same conditions both for DL and for RT, and we should probably optimize
> RT as well if we are going to take this patch.
> 
> > 
> > But that said, I believe this patch is correct, and we should change
> > rt.c as well.
> > 
> 
> That's what I meant. :)
> 
> > 
> >   task = pick_next_pushable_dl_task(rq);
> > 
> > Which has:
> > 
> >   BUG_ON(rq->cpu != task_cpu(task))
> > 
> > when it returns a task other than NULL. Which means that task_cpu(task)
> > must be rq->cpu. Then if task == next_task, then task_cpu(next_task)
> > must be rq->cpu as well.
> 
> Right.
> 
> > 
> > Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > 
> 
> You can also add mine
> 
> Reviewed-by: Juri Lelli <juri.lelli@arm.com>

Juri and steven, thank you very much for reviewing it.

I'm not sure and familiar with... Should I add your 'reviewed by' into
my patches by myself?

> 
> > Mind fixing rt.c if it hasn't been fixed already.
> > 
> > -- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Steven Rostedt]

On Thu, 16 Feb 2017 11:17:58 +0900
Byungchul Park <byungchul.park@lge.com> wrote:


> Juri and steven, thank you very much for reviewing it.
> 
> I'm not sure and familiar with... Should I add your 'reviewed by' into
> my patches by myself?
> 

No, it's the maintainer's job to add these tags when they take your
patch. If you were to resend the patch because of a minor change
(something a reviewer told you to make but still gave you their
reviewed-by tag), then you could add it. Or if you have a patch series
and resend a new series, you can add the reviewed-by tags to the
patches that were not changed. But if you were to change a patch, then
you need to ask the reviewer to give their tag again, because they need
to review the changes made before their tag should go on it.

Make sense?

-- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Steven Rostedt]

On Wed, 15 Feb 2017 21:31:50 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 16 Feb 2017 11:17:58 +0900
> Byungchul Park <byungchul.park@lge.com> wrote:
> 
> 
> > Juri and steven, thank you very much for reviewing it.
> > 
> > I'm not sure and familiar with... Should I add your 'reviewed by' into
> > my patches by myself?
> >   
> 
> No, it's the maintainer's job to add these tags when they take your
> patch. If you were to resend the patch because of a minor change
> (something a reviewer told you to make but still gave you their
> reviewed-by tag), then you could add it. Or if you have a patch series
> and resend a new series, you can add the reviewed-by tags to the
> patches that were not changed. But if you were to change a patch, then
> you need to ask the reviewer to give their tag again, because they need
> to review the changes made before their tag should go on it.
> 

I just noticed that you sent a v2 of the patch we reviewed. Since you
didn't change it, you could have added our review-by tags. That makes
it a bit easier for maintainers to know.

-- Steve

Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl_task()

[Byungchul Park]

On Wed, Feb 15, 2017 at 09:33:35PM -0500, Steven Rostedt wrote:
> On Wed, 15 Feb 2017 21:31:50 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Thu, 16 Feb 2017 11:17:58 +0900
> > Byungchul Park <byungchul.park@lge.com> wrote:
> > 
> > 
> > > Juri and steven, thank you very much for reviewing it.
> > > 
> > > I'm not sure and familiar with... Should I add your 'reviewed by' into
> > > my patches by myself?
> > >   
> > 
> > No, it's the maintainer's job to add these tags when they take your
> > patch. If you were to resend the patch because of a minor change
> > (something a reviewer told you to make but still gave you their
> > reviewed-by tag), then you could add it. Or if you have a patch series
> > and resend a new series, you can add the reviewed-by tags to the
> > patches that were not changed. But if you were to change a patch, then
> > you need to ask the reviewer to give their tag again, because they need
> > to review the changes made before their tag should go on it.
> > 
> 
> I just noticed that you sent a v2 of the patch we reviewed. Since you
> didn't change it, you could have added our review-by tags. That makes
> it a bit easier for maintainers to know.

I got it! Thank you very much for your kind explanation.

> 
> -- Steve