From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock() Date: Thu, 21 Feb 2019 14:14:20 +0000 Message-ID: <20190221141420.GC12696@fuggles.cambridge.arm.com> References: <1550095217-12047-1-git-send-email-longman@redhat.com> <1550095217-12047-4-git-send-email-longman@redhat.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <1550095217-12047-4-git-send-email-longman@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-c6x-dev@linux-c6x.org, uclinux-h8-devel@lists.sourceforge.jp, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, nios2-dev@lists.rocketboards.org, openrisc@lists.librecores.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, x86@kernel.org, Arnd Bergmann , Borislav On Wed, Feb 13, 2019 at 05:00:17PM -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 > > 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. > > Suggested-by: Linus Torvalds > Signed-off-by: Waiman Long > --- > kernel/locking/rwsem.h | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h > index 45ee002..1f5775a 100644 > --- a/kernel/locking/rwsem.h > +++ b/kernel/locking/rwsem.h > @@ -174,14 +174,17 @@ static inline int __down_read_killable(struct rw_semaphore *sem) > > static inline int __down_read_trylock(struct rw_semaphore *sem) > { > - long tmp; > + /* > + * Optimize for the case when the rwsem is not locked at all. > + */ > + long tmp = RWSEM_UNLOCKED_VALUE; > > - while ((tmp = atomic_long_read(&sem->count)) >= 0) { > - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp, > - tmp + RWSEM_ACTIVE_READ_BIAS)) { > + do { > + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, > + tmp + RWSEM_ACTIVE_READ_BIAS)) { > return 1; > } > - } > + } while (tmp >= 0); Nit: but I guess that should be RWSEM_UNLOCKED_VALUE instead of 0. Will