[PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()

[Waiman Long]

On 02/14/2019 01:02 PM, Will Deacon wrote:
> On Thu, Feb 14, 2019 at 11:33:33AM +0100, Peter Zijlstra wrote:
>> On Wed, Feb 13, 2019 at 03:32:12PM -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
>> Urgh, yes LL/SC is the obvious exception that can actually do better
>> here :/
>>
>> Will, what say you?
> What machine were these numbers generated on and is it using LL/SC or LSE
> atomics for arm64? If you stick the microbenchmark somewhere, I can go play
> with a broader variety of h/w.
>
> Will

The machine is a 2-socket Cavium ThunderX2 99xx system with 64 cores and
256 threads. I was just using threads from the first socket for this
test. The microbenchmark that I used is attached. I used the command
"./run-locktest -ltryrwsem -r100 -i-10 -c10 -n<threads>" to generate the
locking rates.

The lscpu flags were:

fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics cpuid asimdrdm

Cheers,
Longman

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