[PATCH v2 2/2] mm/vmalloc: Use __this_cpu_try_cmpxchg() in preload_this_cpu_lock()

Uros Bizjak
2024-05-28T14:44:05Z

Use __this_cpu_try_cmpxchg() instead of
__this_cpu_cmpxchg (*ptr, old, new) == old in
preload_this_cpu_lock(). x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg.

The generated code improves from:

4bb6: 48 85 f6 test %rsi,%rsi
4bb9: 0f 84 10 fa ff ff je 45cf <...>
4bbf: 4c 89 e8 mov %r13,%rax
4bc2: 65 48 0f b1 35 00 00 cmpxchg %rsi,%gs:0x0(%rip)
4bc9: 00 00
4bcb: 48 85 c0 test %rax,%rax
4bce: 0f 84 fb f9 ff ff je 45cf <...>

to:

4bb6: 48 85 f6 test %rsi,%rsi
4bb9: 0f 84 10 fa ff ff je 45cf <...>
4bbf: 4c 89 e8 mov %r13,%rax
4bc2: 65 48 0f b1 35 00 00 cmpxchg %rsi,%gs:0x0(%rip)
4bc9: 00 00
4bcb: 0f 84 fe f9 ff ff je 45cf <...>

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
---
v2: Show generated code improvement in the commit message.
---
mm/vmalloc.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5d3aa2dc88a8..4f34d935d648 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1816,7 +1816,7 @@ static void free_vmap_area(struct vmap_area *va)
static inline void
preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)
{
- struct vmap_area *va = NULL;
+ struct vmap_area *va = NULL, *tmp;

/*
* Preload this CPU with one extra vmap_area object. It is used
@@ -1832,7 +1832,8 @@ preload_this_cpu_lock(spinlock_t *lock, gfp_t gfp_mask, int node)

spin_lock(lock);

- if (va && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, va))
+ tmp = NULL;
+ if (va && !__this_cpu_try_cmpxchg(ne_fit_preload_node, &tmp, va))
kmem_cache_free(vmap_area_cachep, va);
}

--
2.42.0