[PATCH] mm: percpu: micro-optimize round-to-even

[PATCH] mm: percpu: micro-optimize round-to-even

[Rasmus Villemoes]

This change shaves a few bytes off the generated code.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 mm/percpu.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 2ddf9a9..978097f 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -720,8 +720,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved)
 	if (unlikely(align < 2))
 		align = 2;
 
-	if (unlikely(size & 1))
-		size++;
+	size += size & 1;
 
 	if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE)) {
 		WARN(true, "illegal size (%zu) or align (%zu) for "
-- 
1.9.2

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Tejun Heo]

On Thu, Jun 19, 2014 at 12:02:29PM +0200, Rasmus Villemoes wrote:
> This change shaves a few bytes off the generated code.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  mm/percpu.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 2ddf9a9..978097f 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -720,8 +720,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved)
>  	if (unlikely(align < 2))
>  		align = 2;
>  
> -	if (unlikely(size & 1))
> -		size++;
> +	size += size & 1;

I'm not gonna apply this.  This isn't that hot a path.  It's not
worthwhile to micro optimize code like this.

Thanks.

-- 
tejun

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Tejun Heo]

On Thu, Jun 19, 2014 at 09:25:36AM -0400, Tejun Heo wrote:
> On Thu, Jun 19, 2014 at 12:02:29PM +0200, Rasmus Villemoes wrote:
> > This change shaves a few bytes off the generated code.
> > 
> > Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > ---
> >  mm/percpu.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 2ddf9a9..978097f 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -720,8 +720,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved)
> >  	if (unlikely(align < 2))
> >  		align = 2;
> >  
> > -	if (unlikely(size & 1))
> > -		size++;
> > +	size += size & 1;
> 
> I'm not gonna apply this.  This isn't that hot a path.  It's not
> worthwhile to micro optimize code like this.

Another thing is that it isn't even clear whether the micro
optimization is even actually better given that predicted branches are
extremely cheap and this one is extremely predictable.  So, again,
let's please leave it to the compiler.

Thanks.

-- 
tejun

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Christoph Lameter]

On Thu, 19 Jun 2014, Tejun Heo wrote:

> On Thu, Jun 19, 2014 at 12:02:29PM +0200, Rasmus Villemoes wrote:
> > This change shaves a few bytes off the generated code.
> >
> > Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > ---
> >  mm/percpu.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 2ddf9a9..978097f 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -720,8 +720,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved)
> >  	if (unlikely(align < 2))
> >  		align = 2;
> >
> > -	if (unlikely(size & 1))
> > -		size++;
> > +	size += size & 1;
>
> I'm not gonna apply this.  This isn't that hot a path.  It's not
> worthwhile to micro optimize code like this.

Dont we have an ALIGN() macro for this?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Tejun Heo]

On Thu, Jun 19, 2014 at 09:29:52AM -0500, Christoph Lameter wrote:
> > > -	if (unlikely(size & 1))
> > > -		size++;
> > > +	size += size & 1;
> >
> > I'm not gonna apply this.  This isn't that hot a path.  It's not
> > worthwhile to micro optimize code like this.
> 
> Dont we have an ALIGN() macro for this?

Indeed, a patch?

-- 
tejun

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Christoph Lameter]

On Thu, 19 Jun 2014, Tejun Heo wrote:

> Indeed, a patch?

Subject: percpu: Use ALIGN macro instead of hand coding alignment calculation

Signed-off-by: Christoph Lameter <cl@linux.com>

Index: linux/mm/percpu.c
===================================================================
--- linux.orig/mm/percpu.c	2014-06-04 13:43:12.541466633 -0500
+++ linux/mm/percpu.c	2014-06-19 09:56:10.458023912 -0500
@@ -720,8 +720,7 @@ static void __percpu *pcpu_alloc(size_t
 	if (unlikely(align < 2))
 		align = 2;

-	if (unlikely(size & 1))
-		size++;
+	size = ALIGN(size, 2);

 	if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE)) {
 		WARN(true, "illegal size (%zu) or align (%zu) for "

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: percpu: micro-optimize round-to-even

[Tejun Heo]

On Thu, Jun 19, 2014 at 09:59:18AM -0500, Christoph Lameter wrote:
> On Thu, 19 Jun 2014, Tejun Heo wrote:
> 
> > Indeed, a patch?
> 
> Subject: percpu: Use ALIGN macro instead of hand coding alignment calculation
> 
> Signed-off-by: Christoph Lameter <cl@linux.com>

Applied to percpu/for-3.17.  Thanks.

-- 
tejun

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>