Commit-ID: ddb20d1d3aed8f130519c0a29cd5392efcc067b8
Gitweb: https://git.kernel.org/tip/ddb20d1d3aed8f130519c0a29cd5392efcc067b8
Author: Waiman Long <longman@xxxxxxxxxx>
AuthorDate: Fri, 22 Mar 2019 10:30:08 -0400
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 3 Apr 2019 14:50:52 +0200
locking/rwsem: Optimize down_read_trylock()
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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: linux-c6x-dev@xxxxxxxxxxxxx
Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
Cc: linux-um@xxxxxxxxxxxxxxxxxxx
Cc: linux-xtensa@xxxxxxxxxxxxxxxx
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: nios2-dev@xxxxxxxxxxxxxxxxxxxxxx
Cc: openrisc@xxxxxxxxxxxxxxxxxxxx
Cc: uclinux-h8-devel@xxxxxxxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20190322143008.21313-4-longman@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@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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