[PATCH] mm: Add unlikely for current_order test

[PATCH] mm: Add unlikely for current_order test

[Zhang Yanfei]

From: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>

Since we have an unlikely for the "current_order >= pageblock_order / 2"
test above, adding an unlikely for this "current_order >= pageblock_order"
test seems more appropriate.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
---
 mm/page_alloc.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c3edb62..1b6d7de 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1071,7 +1071,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
 			rmv_page_order(page);
 
 			/* Take ownership for orders >= pageblock_order */
-			if (current_order >= pageblock_order &&
+			if (unlikely(current_order >= pageblock_order) &&
 			    !is_migrate_cma(migratetype))
 				change_pageblock_range(page, current_order,
 							start_migratetype);
-- 
1.7.1

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Re: [PATCH] mm: Add unlikely for current_order test

[David Rientjes]

On Sat, 15 Jun 2013, Zhang Yanfei wrote:

> From: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
> 
> Since we have an unlikely for the "current_order >= pageblock_order / 2"
> test above, adding an unlikely for this "current_order >= pageblock_order"
> test seems more appropriate.
> 

I don't understand the justification at all, current_order being unlikely 
greater than or equal to pageblock_order / 2 doesn't imply at all that 
it's unlikely that current_order is greater than or equal to 
pageblock_order.

> Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
> ---
>  mm/page_alloc.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c3edb62..1b6d7de 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1071,7 +1071,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  			rmv_page_order(page);
>  
>  			/* Take ownership for orders >= pageblock_order */
> -			if (current_order >= pageblock_order &&
> +			if (unlikely(current_order >= pageblock_order) &&
>  			    !is_migrate_cma(migratetype))
>  				change_pageblock_range(page, current_order,
>  							start_migratetype);

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Re: [PATCH] mm: Add unlikely for current_order test

[Zhang Yanfei]

Hi David,

On 06/17/2013 02:04 AM, David Rientjes wrote:
> On Sat, 15 Jun 2013, Zhang Yanfei wrote:
> 
>> From: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
>>
>> Since we have an unlikely for the "current_order >= pageblock_order / 2"
>> test above, adding an unlikely for this "current_order >= pageblock_order"
>> test seems more appropriate.
>>
> 
> I don't understand the justification at all, current_order being unlikely 
> greater than or equal to pageblock_order / 2 doesn't imply at all that 
> it's unlikely that current_order is greater than or equal to 
> pageblock_order.
> 

hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely,
why current_order is >= pageblock_order isn't unlikely. Or there are other
tips?

Actually, I am also a little confused about why current_order should be
unlikely greater than or equal to pageblock_order / 2. When borrowing pages
with other migrate_type, we always search from MAX_ORDER-1, which is greater
or equal to pageblock_order.

-- 
Thanks.
Zhang Yanfei

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Re: [PATCH] mm: Add unlikely for current_order test

[David Rientjes]

On Mon, 17 Jun 2013, Zhang Yanfei wrote:

> > I don't understand the justification at all, current_order being unlikely 
> > greater than or equal to pageblock_order / 2 doesn't imply at all that 
> > it's unlikely that current_order is greater than or equal to 
> > pageblock_order.
> > 
> 
> hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely,
> why current_order is >= pageblock_order isn't unlikely. Or there are other
> tips?
> 
> Actually, I am also a little confused about why current_order should be
> unlikely greater than or equal to pageblock_order / 2. When borrowing pages
> with other migrate_type, we always search from MAX_ORDER-1, which is greater
> or equal to pageblock_order.
> 

Look at what is being done in the function: current_order loops down from 
MAX_ORDER-1 to the order passed.  It is not at all "unlikely" that 
current_order is greater than pageblock_order, or pageblock_order / 2.

MAX_ORDER is typically 11 and pageblock_order is typically 9 on x86.  
Integer division truncates, so pageblock_order / 2 is 4.  For the first 
eight iterations, it's guaranteed that current_order >= pageblock_order / 
2 if it even gets that far!

So just remove the unlikely() entirely, it's completely bogus.

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Re: [PATCH] mm: Add unlikely for current_order test

[Zhang Yanfei]

On 06/18/2013 05:37 AM, David Rientjes wrote:
> On Mon, 17 Jun 2013, Zhang Yanfei wrote:
> 
>>> I don't understand the justification at all, current_order being unlikely 
>>> greater than or equal to pageblock_order / 2 doesn't imply at all that 
>>> it's unlikely that current_order is greater than or equal to 
>>> pageblock_order.
>>>
>>
>> hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely,
>> why current_order is >= pageblock_order isn't unlikely. Or there are other
>> tips?
>>
>> Actually, I am also a little confused about why current_order should be
>> unlikely greater than or equal to pageblock_order / 2. When borrowing pages
>> with other migrate_type, we always search from MAX_ORDER-1, which is greater
>> or equal to pageblock_order.
>>
> 
> Look at what is being done in the function: current_order loops down from 
> MAX_ORDER-1 to the order passed.  It is not at all "unlikely" that 
> current_order is greater than pageblock_order, or pageblock_order / 2.
> 
> MAX_ORDER is typically 11 and pageblock_order is typically 9 on x86.  
> Integer division truncates, so pageblock_order / 2 is 4.  For the first 
> eight iterations, it's guaranteed that current_order >= pageblock_order / 
> 2 if it even gets that far!
> 
> So just remove the unlikely() entirely, it's completely bogus.

I see. Thanks!

I will send another patch.

-- 
Thanks.
Zhang Yanfei

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