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>

diff --git a/mm/internal.h b/mm/internal.h
index 9fb2b8c..129659f 100644 (file)
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -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;

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fb0b3ff..2dedf5a 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -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;