kernel/locking/osq_lock.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
Replace this pattern in osq_unlock():
atomic_cmpxchg(*ptr, old, new) == old
... with the simpler and faster:
atomic_try_cmpxchg(*ptr, &old, new)
The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG. The code
in the fast path of osq_unlock() improves from:
11b: 31 c9 xor %ecx,%ecx
11d: 8d 50 01 lea 0x1(%rax),%edx
120: 89 d0 mov %edx,%eax
122: f0 0f b1 0f lock cmpxchg %ecx,(%rdi)
126: 39 c2 cmp %eax,%edx
128: 75 05 jne 12f <...>
to:
12b: 31 d2 xor %edx,%edx
12d: 83 c0 01 add $0x1,%eax
130: f0 0f b1 17 lock cmpxchg %edx,(%rdi)
134: 75 05 jne 13b <...>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
kernel/locking/osq_lock.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f6133866..b4233dc2c2b0 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
/*
* Fast path for the uncontended case.
*/
- if (likely(atomic_cmpxchg_release(&lock->tail, curr,
- OSQ_UNLOCKED_VAL) == curr))
+ if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
return;
/*
--
2.46.2On Tue, Oct 01, 2024 at 01:45:57PM +0200, Uros Bizjak wrote: > Replace this pattern in osq_unlock(): > > atomic_cmpxchg(*ptr, old, new) == old > > ... with the simpler and faster: > > atomic_try_cmpxchg(*ptr, &old, new) > > The x86 CMPXCHG instruction returns success in the ZF flag, > so this change saves a compare after the CMPXCHG. The code > in the fast path of osq_unlock() improves from: > > 11b: 31 c9 xor %ecx,%ecx > 11d: 8d 50 01 lea 0x1(%rax),%edx > 120: 89 d0 mov %edx,%eax > 122: f0 0f b1 0f lock cmpxchg %ecx,(%rdi) > 126: 39 c2 cmp %eax,%edx > 128: 75 05 jne 12f <...> > > to: > > 12b: 31 d2 xor %edx,%edx > 12d: 83 c0 01 add $0x1,%eax > 130: f0 0f b1 17 lock cmpxchg %edx,(%rdi) > 134: 75 05 jne 13b <...> > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> Thanks!
On 10/1/24 07:45, Uros Bizjak wrote: > Replace this pattern in osq_unlock(): > > atomic_cmpxchg(*ptr, old, new) == old > > ... with the simpler and faster: > > atomic_try_cmpxchg(*ptr, &old, new) > > The x86 CMPXCHG instruction returns success in the ZF flag, > so this change saves a compare after the CMPXCHG. The code > in the fast path of osq_unlock() improves from: > > 11b: 31 c9 xor %ecx,%ecx > 11d: 8d 50 01 lea 0x1(%rax),%edx > 120: 89 d0 mov %edx,%eax > 122: f0 0f b1 0f lock cmpxchg %ecx,(%rdi) > 126: 39 c2 cmp %eax,%edx > 128: 75 05 jne 12f <...> > > to: > > 12b: 31 d2 xor %edx,%edx > 12d: 83 c0 01 add $0x1,%eax > 130: f0 0f b1 17 lock cmpxchg %edx,(%rdi) > 134: 75 05 jne 13b <...> > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Will Deacon <will@kernel.org> > Cc: Waiman Long <longman@redhat.com> > Cc: Boqun Feng <boqun.feng@gmail.com> > --- > kernel/locking/osq_lock.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > index 75a6f6133866..b4233dc2c2b0 100644 > --- a/kernel/locking/osq_lock.c > +++ b/kernel/locking/osq_lock.c > @@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock) > /* > * Fast path for the uncontended case. > */ > - if (likely(atomic_cmpxchg_release(&lock->tail, curr, > - OSQ_UNLOCKED_VAL) == curr)) > + if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL)) > return; > > /* LGTM Acked-by: Waiman Long <longman@redhat.com>
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 0d75e0c420e52b4057a2de274054a5274209a2ae
Gitweb: https://git.kernel.org/tip/0d75e0c420e52b4057a2de274054a5274209a2ae
Author: Uros Bizjak <ubizjak@gmail.com>
AuthorDate: Tue, 01 Oct 2024 13:45:57 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 25 Oct 2024 10:01:50 +02:00
locking/osq_lock: Use atomic_try_cmpxchg_release() in osq_unlock()
Replace this pattern in osq_unlock():
atomic_cmpxchg(*ptr, old, new) == old
... with the simpler and faster:
atomic_try_cmpxchg(*ptr, &old, new)
The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG. The code
in the fast path of osq_unlock() improves from:
11b: 31 c9 xor %ecx,%ecx
11d: 8d 50 01 lea 0x1(%rax),%edx
120: 89 d0 mov %edx,%eax
122: f0 0f b1 0f lock cmpxchg %ecx,(%rdi)
126: 39 c2 cmp %eax,%edx
128: 75 05 jne 12f <...>
to:
12b: 31 d2 xor %edx,%edx
12d: 83 c0 01 add $0x1,%eax
130: f0 0f b1 17 lock cmpxchg %edx,(%rdi)
134: 75 05 jne 13b <...>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20241001114606.820277-1-ubizjak@gmail.com
---
kernel/locking/osq_lock.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f61..b4233dc 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
/*
* Fast path for the uncontended case.
*/
- if (likely(atomic_cmpxchg_release(&lock->tail, curr,
- OSQ_UNLOCKED_VAL) == curr))
+ if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
return;
/*© 2016 - 2024 Red Hat, Inc.