Commit 736838e9 authored by Mateusz Nosek's avatar Mateusz Nosek Committed by Linus Torvalds
Browse files

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: default avatarMateusz Nosek <mateusznosek0@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Acked-by: default avatarMel Gorman <mgorman@techsingularity.net>
Link: http://lkml.kernel.org/r/20200304162118.14784-1-mateusznosek0@gmail.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ee8eb9a5

mm/internal.h

+1 −1
Original line number Diff line number Diff line
@@ -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;

mm/page_alloc.c

+14 −8
Original line number Diff line number Diff line
@@ -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;

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