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Subject[PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()
Date
Modify __down_read_trylock() to make it generate slightly better code
(smaller and maybe a tiny bit faster).

Before this patch, down_read_trylock:

0x0000000000000000 <+0>: callq 0x5 <down_read_trylock+5>
0x0000000000000005 <+5>: jmp 0x18 <down_read_trylock+24>
0x0000000000000007 <+7>: lea 0x1(%rdx),%rcx
0x000000000000000b <+11>: mov %rdx,%rax
0x000000000000000e <+14>: lock cmpxchg %rcx,(%rdi)
0x0000000000000013 <+19>: cmp %rax,%rdx
0x0000000000000016 <+22>: je 0x23 <down_read_trylock+35>
0x0000000000000018 <+24>: mov (%rdi),%rdx
0x000000000000001b <+27>: test %rdx,%rdx
0x000000000000001e <+30>: jns 0x7 <down_read_trylock+7>
0x0000000000000020 <+32>: xor %eax,%eax
0x0000000000000022 <+34>: retq
0x0000000000000023 <+35>: mov %gs:0x0,%rax
0x000000000000002c <+44>: or $0x3,%rax
0x0000000000000030 <+48>: mov %rax,0x20(%rdi)
0x0000000000000034 <+52>: mov $0x1,%eax
0x0000000000000039 <+57>: retq

After patch, down_read_trylock:

0x0000000000000000 <+0>: callq 0x5 <down_read_trylock+5>
0x0000000000000005 <+5>: mov (%rdi),%rax
0x0000000000000008 <+8>: test %rax,%rax
0x000000000000000b <+11>: js 0x2f <down_read_trylock+47>
0x000000000000000d <+13>: lea 0x1(%rax),%rdx
0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi)
0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8>
0x0000000000000018 <+24>: mov %gs:0x0,%rax
0x0000000000000021 <+33>: or $0x3,%rax
0x0000000000000025 <+37>: mov %rax,0x20(%rdi)
0x0000000000000029 <+41>: mov $0x1,%eax
0x000000000000002e <+46>: retq
0x000000000000002f <+47>: xor %eax,%eax
0x0000000000000031 <+49>: retq

By using a rwsem microbenchmark, the down_read_trylock() rate on a
x86-64 system before and after the patch were:

Before Patch After Patch
# of Threads rlock rlock
------------ ----- -----
1 27,787 28,259
2 8,359 9,234

On a ARM64 system, the performance results were:

Before Patch After Patch
# of Threads rlock rlock
------------ ----- -----
1 24,155 25,000
2 6,820 8,699

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/rwsem.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 067e265..028bc33 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -175,11 +175,11 @@ static inline int __down_read_killable(struct rw_semaphore *sem)

static inline int __down_read_trylock(struct rw_semaphore *sem)
{
- long tmp;
+ long tmp = atomic_long_read(&sem->count);

- while ((tmp = atomic_long_read(&sem->count)) >= 0) {
- if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
- tmp + RWSEM_ACTIVE_READ_BIAS)) {
+ while (tmp >= 0) {
+ if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
+ tmp + RWSEM_ACTIVE_READ_BIAS)) {
return 1;
}
}
--
1.8.3.1
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