From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
Davidlohr Bueso <dave@stgolabs.net>,
linux-ia64@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
Arnd Bergmann <arnd@arndb.de>,
linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org,
x86@kernel.org, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
linuxppc-dev@lists.ozlabs.org,
Andrew Morton <akpm@linux-foundation.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()
Date: Thu, 14 Feb 2019 11:33:33 +0100 [thread overview]
Message-ID: <20190214103333.GH32494@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1550089932-6888-3-git-send-email-longman@redhat.com>
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?
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next prev parent reply other threads:[~2019-02-14 10:33 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-13 20:32 [PATCH v3 0/2] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-13 20:32 ` [PATCH v3 1/2] " Waiman Long
2019-02-13 20:32 ` [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-02-14 10:33 ` Peter Zijlstra [this message]
2019-02-14 14:53 ` Waiman Long
2019-02-14 17:51 ` Linus Torvalds
2019-02-14 18:09 ` Linus Torvalds
2019-02-15 18:35 ` Will Deacon
2019-02-14 18:02 ` Will Deacon
2019-02-14 18:35 ` Waiman Long
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