On Wed, Feb 13, 2019 at 05:00:17PM -0500, Waiman Long wrote:
> Modify __down_read_trylock() to optimize for an unlocked rwsem and make
> it generate slightly better code.
>
> 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>: xor %eax,%eax
> 0x0000000000000007 <+7>: lea 0x1(%rax),%rdx
> 0x000000000000000b <+11>: lock cmpxchg %rdx,(%rdi)
> 0x0000000000000010 <+16>: jne 0x29 <down_read_trylock+41>
> 0x0000000000000012 <+18>: mov %gs:0x0,%rax
> 0x000000000000001b <+27>: or $0x3,%rax
> 0x000000000000001f <+31>: mov %rax,0x20(%rdi)
> 0x0000000000000023 <+35>: mov $0x1,%eax
> 0x0000000000000028 <+40>: retq
> 0x0000000000000029 <+41>: test %rax,%rax
> 0x000000000000002c <+44>: jns 0x7 <down_read_trylock+7>
> 0x000000000000002e <+46>: xor %eax,%eax
> 0x0000000000000030 <+48>: retq
>
> By using a rwsem microbenchmark, the down_read_trylock() rate (with a
> load of 10 to lengthen the lock critical section) on a x86-64 system
> before and after the patch were:
>
> Before Patch After Patch
> # of Threads rlock rlock
> ------------ ----- -----
> 1 14,496 14,716
> 2 8,644 8,453
> 4 6,799 6,983
> 8 5,664 7,190
>
> On a ARM64 system, the performance results were:
>
> Before Patch After Patch
> # of Threads rlock rlock
> ------------ ----- -----
> 1 23,676 24,488
> 2 7,697 9,502
> 4 4,945 3,440
> 8 2,641 1,603
>
> For the uncontended case (1 thread), the new down_read_trylock() is a
> little bit faster. For the contended cases, the new down_read_trylock()
> perform pretty well in x86-64, but performance degrades at high
> contention level on ARM64.
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> kernel/locking/rwsem.h | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index 45ee002..1f5775a 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -174,14 +174,17 @@ static inline int __down_read_killable(struct rw_semaphore *sem)
>
> static inline int __down_read_trylock(struct rw_semaphore *sem)
> {
> - long tmp;
> + /*
> + * Optimize for the case when the rwsem is not locked at all.
> + */
> + long tmp = RWSEM_UNLOCKED_VALUE;
>
> - while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> - tmp + RWSEM_ACTIVE_READ_BIAS)) {
> + do {
> + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
> + tmp + RWSEM_ACTIVE_READ_BIAS)) {
> return 1;
> }
> - }
> + } while (tmp >= 0);
Nit: but I guess that should be RWSEM_UNLOCKED_VALUE instead of 0.
Will
