Re: [PATCH 2/2] mm: page_alloc: Add unlikely for MAX_ORDER check

[Cody P Schafer <cody@linux.vnet.ibm.com>]

On 07/29/2013 05:36 PM, Cody P Schafer wrote:
> On 07/29/2013 03:45 PM, Dave Hansen wrote:
>> On 07/28/2013 07:48 AM, SeungHun Lee wrote:
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index b8475ed..e644cf5 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -2408,7 +2408,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned
>>> int order,
>>>        * be using allocators in order of preference for an area that is
>>>        * too large.
>>>        */
>>> -    if (order >= MAX_ORDER) {
>>> +    if (unlikely(order >= MAX_ORDER)) {
>>>           WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
>>>           return NULL;
>>>       }
>>
>> What problem is this patch solving?  I can see doing this in hot paths,
>> or places where the compiler is known to be generating bad or suboptimal
>> code.  but, this costs me 512 bytes of text size:
>>
>>   898384 Jul 29 15:40 mm/page_alloc.o.nothing
>>   898896 Jul 29 15:40 mm/page_alloc.o.unlikely

[...]
>
> -rw-rw-r-- 1 cody cody 2942208 Jul 29 17:33 normal/arch/x86/boot/bzImage
> -rw-rw-r-- 1 cody cody 2942208 Jul 29 17:33 unlikely/arch/x86/boot/bzImage

So I screwed this last one up and didn't reapply/unapply the patch, so 
they probably are actually different sizes. I'll run a build and check 
tomorrow.

>> I really don't think we should be adding these without having _concrete_
>> reasons for it.
>
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