Re: [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock()

Waiman Long
2019-03-22T17:41:30Z

On 03/22/2019 01:25 PM, Russell King - ARM Linux admin wrote:
> On Fri, Mar 22, 2019 at 10:30:08AM -0400, 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.
> So, 70% for 4 threads, 61% for 4 threads - does this trend
> continue tailing off as the number of threads (and cores)
> increase?
>
I didn't try higher number of contending threads. I won't worry too much
about contention as trylock is a one-off event. The chance of having
more than one trylock happening simultaneously is very small.

Cheers,
Longman