From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock() Date: Thu, 14 Feb 2019 09:53:12 -0500 Message-ID: <9e01d4ef-56df-7af8-a0f5-b49644e298bf@redhat.com> References: <1550089932-6888-1-git-send-email-longman@redhat.com> <1550089932-6888-3-git-send-email-longman@redhat.com> <20190214103333.GH32494@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190214103333.GH32494@hirez.programming.kicks-ass.net> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, x86@kernel.org, Arnd Bergmann , Borislav Petkov , "H. Peter Anvin" , Davidlohr Bueso , Linus Torvalds , Andrew Morton , Tim Chen On 02/14/2019 05:33 AM, Peter Zijlstra wrote: > 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 >> 0x0000000000000005 <+5>: jmp 0x18 >> 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 >> 0x0000000000000018 <+24>: mov (%rdi),%rdx >> 0x000000000000001b <+27>: test %rdx,%rdx >> 0x000000000000001e <+30>: jns 0x7 >> 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 >> 0x0000000000000005 <+5>: xor %eax,%eax >> 0x0000000000000007 <+7>: lea 0x1(%rax),%rdx >> 0x000000000000000b <+11>: lock cmpxchg %rdx,(%rdi) >> 0x0000000000000010 <+16>: jne 0x29 >> 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 >> 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? The ARM64 result is what I would have expected given that the change was to optimize for the uncontended case. The x86-64 result is kind of an anomaly to me, but I haven't bothered to dig into that. Cheers, Longman