[PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()

[Peter Zijlstra]

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 at infradead.org>
> Signed-off-by: Waiman Long <longman at 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
>