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Date: Mon, 25 Mar 2019 16:25:24 +0100
From: Christophe Leroy <christophe.leroy@....fr>
To: Waiman Long <longman@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Will Deacon <will.deacon@....com>,
Thomas Gleixner <tglx@...utronix.de>
Cc: 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()
Hi,
Could you share the microbenchmark you are using ?
I'd like to test the series on powerpc.
Thanks
Christophe
Le 22/03/2019 à 15:30, Waiman Long a écrit :
> 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.
>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
> 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 45ee00236e03..1f5775aa6a1d 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);
> return 0;
> }
>
>
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