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mm/page_alloc: use try_cmpxchg in set_pfnblock_flags_mask 

Use try_cmpxchg instead of cmpxchg in set_pfnblock_flags_mask.  x86
CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after cmpxchg (and related move instruction in front of cmpxchg). 
The main loop improves from:

    1c5d:	48 89 c2             	mov    %rax,%rdx
    1c60:	48 89 c1             	mov    %rax,%rcx
    1c63:	48 21 fa             	and    %rdi,%rdx
    1c66:	4c 09 c2             	or     %r8,%rdx
    1c69:	f0 48 0f b1 16       	lock cmpxchg %rdx,(%rsi)
    1c6e:	48 39 c1             	cmp    %rax,%rcx
    1c71:	75 ea                	jne    1c5d <...>

to:

    1c60:	48 89 ca             	mov    %rcx,%rdx
    1c63:	48 21 c2             	and    %rax,%rdx
    1c66:	4c 09 c2             	or     %r8,%rdx
    1c69:	f0 48 0f b1 16       	lock cmpxchg %rdx,(%rsi)
    1c6e:	75 f0                	jne    1c60 <...>

Link: https://lkml.kernel.org/r/20220708140736.8737-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Uros Bizjak 2022-07-08 16:07:36 +02:00 committed by akpm
parent dcadcf1c30
commit 04ec006171
1 changed files with 3 additions and 7 deletions
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10
mm/page_alloc.c
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@ -602,7 +602,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
{
unsigned long *bitmap;
unsigned long bitidx, word_bitidx;
unsigned long old_word, word;
unsigned long word;
BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
@ -618,12 +618,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
flags <<= bitidx;
word = READ_ONCE(bitmap[word_bitidx]);
for (;;) {
old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
if (word == old_word)
break;
word = old_word;
}
do {
} while (!try_cmpxchg(&bitmap[word_bitidx], &word, (word & ~mask) | flags));
}
void set_pageblock_migratetype(struct page *page, int migratetype)