On 03/22/2019 01:25 PM, Russell King - ARM Linux admin wrote: > On Fri, Mar 22, 2019 at 10:30:08AM -0400, 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. > So, 70% for 4 threads, 61% for 4 threads - does this trend > continue tailing off as the number of threads (and cores) > increase? > I didn't try higher number of contending threads. I won't worry too much about contention as trylock is a one-off event. The chance of having more than one trylock happening simultaneously is very small. Cheers, Longman
