From: Waiman Long <longman@redhat.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
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Davidlohr Bueso <dave@stgolabs.net>,
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Linus Torvalds <torvalds@linux-foundation.org>,
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Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock()
Date: Fri, 22 Mar 2019 13:41:05 -0400 [thread overview]
Message-ID: <27c2fb96-daa4-ba2c-da06-e559dc5b693e@redhat.com> (raw)
In-Reply-To: <20190322172501.3nbjw6e2wqsaisgw@shell.armlinux.org.uk>
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
next prev parent reply other threads:[~2019-03-22 17:41 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 14:30 [PATCH v5 0/3] locking/rwsem: Rwsem rearchitecture part 0 Waiman Long
2019-03-22 14:30 ` [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-03-22 17:01 ` Linus Torvalds
2019-03-22 17:23 ` Waiman Long
2019-03-22 19:30 ` Davidlohr Bueso
2019-03-22 20:27 ` Waiman Long
2019-03-22 14:30 ` [PATCH v5 2/3] locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all archs Waiman Long
2019-03-22 17:07 ` Linus Torvalds
2019-03-22 14:30 ` [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-03-22 17:08 ` Linus Torvalds
2019-03-22 17:25 ` Russell King - ARM Linux admin
2019-03-22 17:41 ` Waiman Long [this message]
2019-03-25 15:25 ` Christophe Leroy
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