Messages in this thread Patch in this message |  | | Date | Thu, 5 Apr 2018 11:37:16 +0200 | From | Peter Zijlstra <> | Subject | [RFC] locking/mutex: Optimize __mutex_trylock_fast |
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Subject: locking/mutex: Optimize __mutex_trylock_fast() From: Peter Zijlstra <peterz@infradead.org> Date: Thu Apr 5 11:05:35 CEST 2018
Use try_cmpxchg to avoid the pointless TEST instruction.. And add the (missing) atomic_long_try_cmpxchg*() wrappery.
On x86_64 this gives:
0000000000000710 <mutex_lock>: 0000000000000710 <mutex_lock>: 710: 65 48 8b 14 25 00 00 mov %gs:0x0,%rdx 710: 65 48 8b 14 25 00 00 mov %gs:0x0,%rdx 717: 00 00 717: 00 00 715: R_X86_64_32S current_task 715: R_X86_64_32S current_task 719: 31 c0 xor %eax,%eax 719: 31 c0 xor %eax,%eax 71b: f0 48 0f b1 17 lock cmpxchg %rdx,(%rdi) 71b: f0 48 0f b1 17 lock cmpxchg %rdx,(%rdi) 720: 48 85 c0 test %rax,%rax 720: 75 02 jne 724 <mutex_lock+0x14> 723: 75 02 jne 727 <mutex_lock+0x17> 722: f3 c3 repz retq 725: f3 c3 repz retq 724: eb da jmp 700 <__mutex_lock_slowpath> 727: eb d7 jmp 700 <__mutex_lock_slowpath>
On ARM64 this gives:
000000000000638 <mutex_lock>: 0000000000000638 <mutex_lock>: 638: d5384101 mrs x1, sp_el0 638: d5384101 mrs x1, sp_el0 63c: d2800002 mov x2, #0x0 63c: d2800002 mov x2, #0x0 640: f9800011 prfm pstl1strm, [x0] 640: f9800011 prfm pstl1strm, [x0] 644: c85ffc03 ldaxr x3, [x0] 644: c85ffc03 ldaxr x3, [x0] 648: ca020064 eor x4, x3, x2 648: ca020064 eor x4, x3, x2 64c: b5000064 cbnz x4, 658 <mutex_lock+0x20> 64c: b5000064 cbnz x4, 658 <mutex_lock+0x20> 650: c8047c01 stxr w4, x1, [x0] 650: c8047c01 stxr w4, x1, [x0] 654: 35ffff84 cbnz w4, 644 <mutex_lock+0xc> 654: 35ffff84 cbnz w4, 644 <mutex_lock+0xc> 658: b40000c3 cbz x3, 670 <mutex_lock+0x38> 658: b5000043 cbnz x3, 660 <mutex_lock+0x28> 65c: a9bf7bfd stp x29, x30, [sp,#-16]! 65c: d65f03c0 ret 660: 910003fd mov x29, sp 660: a9bf7bfd stp x29, x30, [sp,#-16]! 664: 97ffffef bl 620 <__mutex_lock_slowpath> 664: 910003fd mov x29, sp 668: a8c17bfd ldp x29, x30, [sp],#16 668: 97ffffee bl 620 <__mutex_lock_slowpath> 66c: d65f03c0 ret 66c: a8c17bfd ldp x29, x30, [sp],#16 670: d65f03c0 ret 670: d65f03c0 ret
Which to my untrained eye just looks different, not worse. Will?
Reported-by: Matthew Wilcox <mawilcox@microsoft.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- include/asm-generic/atomic-long.h | 17 +++++++++++++++++ kernel/locking/mutex.c | 3 ++- 2 files changed, 19 insertions(+), 1 deletion(-)
--- a/include/asm-generic/atomic-long.h +++ b/include/asm-generic/atomic-long.h @@ -25,6 +25,7 @@ typedef atomic64_t atomic_long_t; #define ATOMIC_LONG_INIT(i) ATOMIC64_INIT(i) #define ATOMIC_LONG_PFX(x) atomic64 ## x +#define ATOMIC_LONG_TYPE s64 #else @@ -32,6 +33,7 @@ typedef atomic_t atomic_long_t; #define ATOMIC_LONG_INIT(i) ATOMIC_INIT(i) #define ATOMIC_LONG_PFX(x) atomic ## x +#define ATOMIC_LONG_TYPE int #endif @@ -90,6 +92,21 @@ ATOMIC_LONG_ADD_SUB_OP(sub, _release) #define atomic_long_cmpxchg(l, old, new) \ (ATOMIC_LONG_PFX(_cmpxchg)((ATOMIC_LONG_PFX(_t) *)(l), (old), (new))) + +#define atomic_long_try_cmpxchg_relaxed(l, old, new) \ + (ATOMIC_LONG_PFX(_try_cmpxchg_relaxed)((ATOMIC_LONG_PFX(_t) *)(l), \ + (ATOMIC_LONG_TYPE *)(old), (ATOMIC_LONG_TYPE)(new))) +#define atomic_long_try_cmpxchg_acquire(l, old, new) \ + (ATOMIC_LONG_PFX(_try_cmpxchg_acquire)((ATOMIC_LONG_PFX(_t) *)(l), \ + (ATOMIC_LONG_TYPE *)(old), (ATOMIC_LONG_TYPE)(new))) +#define atomic_long_try_cmpxchg_release(l, old, new) \ + (ATOMIC_LONG_PFX(_try_cmpxchg_release)((ATOMIC_LONG_PFX(_t) *)(l), \ + (ATOMIC_LONG_TYPE *)(old), (ATOMIC_LONG_TYPE)(new))) +#define atomic_long_try_cmpxchg(l, old, new) \ + (ATOMIC_LONG_PFX(_try_cmpxchg)((ATOMIC_LONG_PFX(_t) *)(l), \ + (ATOMIC_LONG_TYPE *)(old), (ATOMIC_LONG_TYPE)(new))) + + #define atomic_long_xchg_relaxed(v, new) \ (ATOMIC_LONG_PFX(_xchg_relaxed)((ATOMIC_LONG_PFX(_t) *)(v), (new))) #define atomic_long_xchg_acquire(v, new) \ --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -139,8 +139,9 @@ static inline bool __mutex_trylock(struc static __always_inline bool __mutex_trylock_fast(struct mutex *lock) { unsigned long curr = (unsigned long)current; + unsigned long zero = 0UL; - if (!atomic_long_cmpxchg_acquire(&lock->owner, 0UL, curr)) + if (atomic_long_try_cmpxchg_acquire(&lock->owner, &zero, curr)) return true; return false;
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