From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
Davidlohr Bueso <dave@stgolabs.net>,
linux-ia64@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
Arnd Bergmann <arnd@arndb.de>,
linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org,
x86@kernel.org, Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
linuxppc-dev@lists.ozlabs.org,
Andrew Morton <akpm@linux-foundation.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock()
Date: Tue, 12 Feb 2019 14:24:04 +0100 [thread overview]
Message-ID: <20190212132404.GI32494@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1549913486-16799-3-git-send-email-longman@redhat.com>
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 <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>: mov (%rdi),%rax
> 0x0000000000000008 <+8>: test %rax,%rax
> 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47>
> 0x000000000000000d <+13>: lea 0x1(%rax),%rdx
> 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi)
> 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8>
> 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
>
> On a ARM64 system, the performance results were:
>
> Before Patch After Patch
> # of Threads rlock rlock
> ------------ ----- -----
> 1 24,155 25,000
> 2 6,820 8,699
>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/locking/rwsem.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index 067e265..028bc33 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -175,11 +175,11 @@ static inline int __down_read_killable(struct rw_semaphore *sem)
>
> static inline int __down_read_trylock(struct rw_semaphore *sem)
> {
> - long tmp;
> + long tmp = atomic_long_read(&sem->count);
>
> - while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> - tmp + RWSEM_ACTIVE_READ_BIAS)) {
> + while (tmp >= 0) {
> + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
> + tmp + RWSEM_ACTIVE_READ_BIAS)) {
> return 1;
> }
> }
> --
> 1.8.3.1
>
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next prev parent reply other threads:[~2019-02-12 13:24 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-11 19:31 [PATCH v2 0/2] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-11 19:31 ` [PATCH v2 1/2] " Waiman Long
2019-02-11 19:31 ` [PATCH v2 2/2] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-02-12 13:24 ` Peter Zijlstra [this message]
2019-02-12 13:25 ` Peter Zijlstra
2019-02-12 18:36 ` Waiman Long
2019-02-12 18:38 ` Waiman Long
2019-02-12 19:58 ` Linus Torvalds
2019-02-12 21:21 ` Waiman Long
2019-02-13 7:45 ` Ingo Molnar
2019-02-13 15:33 ` Waiman Long
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