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

Cheers,
Longman

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