Re: [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock()

[Waiman Long]

On 02/21/2019 09:14 AM, Will Deacon wrote:
> 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

Using RWSEM_UNLOCKED_VALUE may be better. Anyway, it is not a big deal
as I am going to change this again in a later patch.

Thanks,
Longman

RWSEM_UNLOCKED_VALUE