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

[Waiman Long]

On 02/12/2019 01:36 PM, Waiman Long wrote:
> On 02/12/2019 08:25 AM, Peter Zijlstra wrote:
>> On Tue, Feb 12, 2019 at 02:24:04PM +0100, Peter Zijlstra wrote:
>>> 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
>> Argh, fat fingered and send before I was done typing.
>>
>> What I wanted to say was; those rlock numbers don't match up. What
>> gives?
>>
>> The before _this_ patch number of 27k787 should be the same as the after
>> first patch number of 30k172.
> The rlock number in patch 1 refers to down_read() which uses xadd. The
> number here in patch 2 refers specifically to down_read_trylock() which
> uses cmpxchg() as this patch changes only __down_read_tryulock(). So the
> performance data differ.

You can see that the performance is worse if we use cmpxchg for
down_read instead of using xadd.

Cheers,
Longman