Re: [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock()

From: Will Deacon <will.deacon@arm.com>
To: Waiman Long <longman@redhat.com>
Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	linux-mips@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	sparclinux@vger.kernel.org, linux-riscv@lists.infradead.org,
	linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-c6x-dev@linux-c6x.org, linux-hexagon@vger.kernel.org,
	x86@kernel.org, Ingo Molnar <mingo@redhat.com>,
	uclinux-h8-devel@lists.sourceforge.jp,
	linux-xtensa@linux-xtensa.org, Arnd Bergmann <arnd@arndb.de>,
	linux-um@lists.infradead.org, linux-m68k@lists.linux-m68k.org,
	openrisc@lists.librecores.org, Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	Tim Chen <tim.c.chen@linux.intel.com>,
	linux-parisc@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org,
	nios2-dev@lists.rocketboards.org,
	Andrew Morton <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock()
Date: Thu, 21 Feb 2019 14:14:20 +0000	[thread overview]
Message-ID: <20190221141420.GC12696@fuggles.cambridge.arm.com> (raw)
In-Reply-To: <1550095217-12047-4-git-send-email-longman@redhat.com>

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 <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@linux-foundation.org>
> Signed-off-by: Waiman Long <longman@redhat.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 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