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 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

I took a look at this on my system.

With gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5):

-rw-rw-r-- 1 cody cody 841160 Jul 29 16:47 unlikely/mm/page_alloc.o
-rw-rw-r-- 1 cody cody 840584 Jul 29 16:59 normal/mm/page_alloc.o

    text	   data	    bss	    dec	    hex	filename
   33799	   1414	    184	  35397	   8a45	unlikely/mm/page_alloc.o
   33799	   1414	    184	  35397	   8a45	normal/mm/page_alloc.o

Well, where are are those extra bytes coming from, then?

Using readelf -S + `git diff --no-index --word-diff` shows:
.debug_info      shrinks from 1e991 to 1e98f
.rela.debug_info shrinks from 33a80 to 33a68
.debug_loc         grows from 15e1d to 15ecb
.rela.debug_loc    grows from 26f40 to 270f0
.debug_line        grows from 038eb to 038ed
.debug_str       shrinks from 0adb6 to 0adb2

The sizes of all other sections are unchanged.

Also: comparing vmlinux sizes:
-rwxrwxr-x 1 cody cody 94121230 Jul 29 17:00 normal/vmlinux
-rwxrwxr-x 1 cody cody 94121294 Jul 29 16:51 unlikely/vmlinux

And the bzImage sizes:
-rw-rw-r-- 1 cody cody 2942240 Jul 29 16:51 unlikely/arch/x86/boot/bzImage
-rw-rw-r-- 1 cody cody 2942208 Jul 29 17:00 normal/arch/x86/boot/bzImage

I build this kernel with debug info built in though, what happens when 
it is removed?

-rwxrwxr-x 1 cody cody 16392454 Jul 29 17:33 normal/vmlinux
-rwxrwxr-x 1 cody cody 16392454 Jul 29 17:33 unlikely/vmlinux

-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, it looks like the only difference in size due to this patch is in 
the debug info.

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

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