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Date: Fri, 22 Mar 2019 17:25:01 +0000 From: Russell King - ARM Linux admin <linux@...linux.org.uk> To: Waiman Long <longman@...hat.com> 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 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? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
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