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