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?
