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mm, pagealloc: micro-optimisation: save two branches on hot page allocation path 

This patch makes ALLOC_KSWAPD equal to __GFP_KSWAPD_RECLAIM (cast to int).

Thanks to that code like:

    if (gfp_mask & __GFP_KSWAPD_RECLAIM)
	    alloc_flags |= ALLOC_KSWAPD;

can be changed to:

    alloc_flags |= (__force int) (gfp_mask &__GFP_KSWAPD_RECLAIM);

Thanks to this one branch less is generated in the assembly.

In case of ALLOC_KSWAPD flag two branches are saved, first one in code
that always executes in the beginning of page allocation and the second
one in loop in page allocator slowpath.

Signed-off-by: Mateusz Nosek <mateusznosek0@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: http://lkml.kernel.org/r/20200304162118.14784-1-mateusznosek0@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mateusz Nosek 2020-04-01 21:09:47 -07:00 committed by Linus Torvalds
parent ee8eb9a5fe
commit 736838e964
2 changed files with 15 additions and 9 deletions
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2
mm/internal.h
View File

@ -555,7 +555,7 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
#else
#define ALLOC_NOFRAGMENT 0x0
#endif
#define ALLOC_KSWAPD 0x200 /* allow waking of kswapd */
#define ALLOC_KSWAPD 0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
enum ttu_flags;
struct tlbflush_unmap_batch;

22
mm/page_alloc.c
View File

@ -3536,10 +3536,13 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
static inline unsigned int
alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
{
unsigned int alloc_flags = 0;
unsigned int alloc_flags;
if (gfp_mask & __GFP_KSWAPD_RECLAIM)
alloc_flags |= ALLOC_KSWAPD;
/*
* __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
* to save a branch.
*/
alloc_flags = (__force int) (gfp_mask & __GFP_KSWAPD_RECLAIM);
#ifdef CONFIG_ZONE_DMA32
if (!zone)
@ -4175,8 +4178,13 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
{
unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
/* __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch. */
/*
* __GFP_HIGH is assumed to be the same as ALLOC_HIGH
* and __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
* to save two branches.
*/
BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_HIGH);
BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);
/*
* The caller may dip into page reserves a bit more if the caller
@ -4184,7 +4192,8 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
* policy or is asking for __GFP_HIGH memory. GFP_ATOMIC requests will
* set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_HIGH (__GFP_HIGH).
*/
alloc_flags |= (__force int) (gfp_mask & __GFP_HIGH);
alloc_flags |= (__force int)
(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
if (gfp_mask & __GFP_ATOMIC) {
/*
@ -4201,9 +4210,6 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
} else if (unlikely(rt_task(current)) && !in_interrupt())
alloc_flags |= ALLOC_HARDER;
if (gfp_mask & __GFP_KSWAPD_RECLAIM)
alloc_flags |= ALLOC_KSWAPD;
#ifdef CONFIG_CMA
if (gfpflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
alloc_flags |= ALLOC_CMA;