Re: [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock()

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Hi,

Could you share the microbenchmark you are using ?

I'd like to test the series on powerpc.

Thanks
Christophe

Le 22/03/2019 à 15:30, Waiman Long a écrit :
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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
  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 45ee00236e03..1f5775aa6a1d 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);
  	return 0;
  }



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