From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Date: Tue, 12 Feb 2019 18:36:32 +0000 Subject: Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock() Message-Id: List-Id: References: <1549913486-16799-1-git-send-email-longman@redhat.com> <1549913486-16799-3-git-send-email-longman@redhat.com> <20190212132404.GI32494@hirez.programming.kicks-ass.net> <20190212132537.GL32534@hirez.programming.kicks-ass.net> In-Reply-To: <20190212132537.GL32534@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 02/12/2019 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 >>> 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>: mov (%rdi),%rax >>> 0x0000000000000008 <+8>: test %rax,%rax >>> 0x000000000000000b <+11>: js 0x2f >>> 0x000000000000000d <+13>: lea 0x1(%rax),%rdx >>> 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) >>> 0x0000000000000016 <+22>: jne 0x8 >>> 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