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Date:   Fri, 22 Mar 2019 13:41:05 -0400
From:   Waiman Long <longman@...hat.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-ia64@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-mips@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        sparclinux@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-c6x-dev@...ux-c6x.org, linux-hexagon@...r.kernel.org,
        x86@...nel.org, uclinux-h8-devel@...ts.sourceforge.jp,
        linux-xtensa@...ux-xtensa.org, Arnd Bergmann <arnd@...db.de>,
        linux-um@...ts.infradead.org, linux-m68k@...ts.linux-m68k.org,
        openrisc@...ts.librecores.org, Borislav Petkov <bp@...en8.de>,
        linux-arm-kernel@...ts.infradead.org,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-parisc@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        nios2-dev@...ts.rocketboards.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock()

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

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