[PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()

[Will Deacon]

On Thu, Feb 14, 2019 at 11:33:33AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 03:32:12PM -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
> 
> Urgh, yes LL/SC is the obvious exception that can actually do better
> here :/
> 
> Will, what say you?

What machine were these numbers generated on and is it using LL/SC or LSE
atomics for arm64? If you stick the microbenchmark somewhere, I can go play
with a broader variety of h/w.

Will