[PATCH] mm/page_alloc: use try_cmpxchg in set_pfnblock_flags_mask

Uros Bizjak
2022-07-08T14:07:52Z

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 <...>

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
mm/page_alloc.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3df0485..d415d5b539b7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -524,7 +524,7 @@ 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));
@@ -540,12 +540,8 @@ 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)
--
2.35.3