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SubjectRe: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()
On Mon, Feb 11, 2019 at 02:31:26PM -0500, Waiman Long wrote:
> 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

From 1/2:

1 29,201 30,143 29,458 28,615 30,172 29,201
2 6,807 13,299 1,171 7,725 15,025 1,804

>
> 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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