[PATCH] sched: remove redundant check in select_task_rq_fair

[PATCH] sched: remove redundant check in select_task_rq_fair

[Nikunj A. Dadhania]

When balancing for wakeup affinity, a redundant check can removed.

Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
---
 kernel/sched_fair.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 433491c..b9e5701 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1753,7 +1753,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 	}
 
 	if (affine_sd) {
-		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
+		if (wake_affine(affine_sd, p, sync))
 			prev_cpu = cpu;
 
 		new_cpu = select_idle_sibling(p, prev_cpu);

Re: [PATCH] sched: remove redundant check in select_task_rq_fair

[Peter Zijlstra]

On Tue, 2011-06-07 at 15:18 +0530, Nikunj A. Dadhania wrote:
> When balancing for wakeup affinity, a redundant check can removed.

Yes, tempting, but no, now you've got an extra call to wake_affine(),
which is more expensive.

> Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
> ---
>  kernel/sched_fair.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 433491c..b9e5701 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1753,7 +1753,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  	}
>  
>  	if (affine_sd) {
> -		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
> +		if (wake_affine(affine_sd, p, sync))
>  			prev_cpu = cpu;
>  
>  		new_cpu = select_idle_sibling(p, prev_cpu);
> 

Re: [PATCH] sched: remove redundant check in select_task_rq_fair

[Nikunj A. Dadhania]

On Tue, 07 Jun 2011 11:53:34 +0200, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, 2011-06-07 at 15:18 +0530, Nikunj A. Dadhania wrote:
> > When balancing for wakeup affinity, a redundant check can removed.
> 
> Yes, tempting, but no, now you've got an extra call to wake_affine(),
> which is more expensive.
Ah, got that. How about this, it is more readable. Other options would
be to add a comment.

Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
---
 kernel/sched_fair.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 433491c..354e26b 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1753,7 +1753,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
 	}
 
 	if (affine_sd) {
-		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
+		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
 			prev_cpu = cpu;

Re: [PATCH] sched: remove redundant check in select_task_rq_fair

[Peter Zijlstra]

On Tue, 2011-06-07 at 15:34 +0530, Nikunj A. Dadhania wrote:
> On Tue, 07 Jun 2011 11:53:34 +0200, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, 2011-06-07 at 15:18 +0530, Nikunj A. Dadhania wrote:
> > > When balancing for wakeup affinity, a redundant check can removed.
> > 
> > Yes, tempting, but no, now you've got an extra call to wake_affine(),
> > which is more expensive.

> Ah, got that. How about this, it is more readable. Other options would
> be to add a comment.

> -		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
> +		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
>  			prev_cpu = cpu;

I guess that depends on which way your head is wired and how strong your
boolean algebra is.. Suresh, Mike any preference? I don't much care
either way.

Re: [PATCH] sched: remove redundant check in select_task_rq_fair

[Mike Galbraith]

On Tue, 2011-06-07 at 12:09 +0200, Peter Zijlstra wrote:
> On Tue, 2011-06-07 at 15:34 +0530, Nikunj A. Dadhania wrote:
> > On Tue, 07 Jun 2011 11:53:34 +0200, Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Tue, 2011-06-07 at 15:18 +0530, Nikunj A. Dadhania wrote:
> > > > When balancing for wakeup affinity, a redundant check can removed.
> > > 
> > > Yes, tempting, but no, now you've got an extra call to wake_affine(),
> > > which is more expensive.
> 
> > Ah, got that. How about this, it is more readable. Other options would
> > be to add a comment.
> 
> > -		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
> > +		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> >  			prev_cpu = cpu;
> 
> I guess that depends on which way your head is wired and how strong your
> boolean algebra is.. Suresh, Mike any preference? I don't much care
> either way.

Me either.

	-Mike