All the locking related cmpxchg's in the following functions are
replaced with the _acquire variants:
- pv_queued_spin_steal_lock()
- trylock_clear_pending()
This change should help performance on architectures that use LL/SC.
On a 2-core 16-thread Power8 system with pvqspinlock explicitly
enabled, the performance of a locking microbenchmark with and without
this patch on a 4.10-rc8 kernel with Xinhui's PPC qspinlock patch
were as follows:
# of thread w/o patch with patch % Change
----------- --------- ---------- --------
4 4053.3 Mop/s 4223.7 Mop/s +4.2%
8 3310.4 Mop/s 3406.0 Mop/s +2.9%
12 2576.4 Mop/s 2674.6 Mop/s +3.8%
Signed-off-by: Waiman Long <long...@redhat.com>
---
v4->v5:
- Correct some grammatical issues in comment.
v3->v4:
- Update the comment in pv_kick_node() to mention that the code
may not work in some archs.
v2->v3:
- Reduce scope by relaxing cmpxchg's in fast path only.
v1->v2:
- Add comments in changelog and code for the rationale of the change.
kernel/locking/qspinlock_paravirt.h | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/kernel/locking/qspinlock_paravirt.h
b/kernel/locking/qspinlock_paravirt.h
index e6b2f7a..4614e39 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock
*lock)
struct __qspinlock *l = (void *)lock;
if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
- (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
+ (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
qstat_inc(qstat_pv_lock_stealing, true);
return true;
}
@@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct
qspinlock *lock)
/*
* The pending bit check in pv_queued_spin_steal_lock() isn't a memory
- * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock
- * just to be sure that it will get it.
+ * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
+ * lock just to be sure that it will get it.
*/
static __always_inline int trylock_clear_pending(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
return !READ_ONCE(l->locked) &&
- (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)
- == _Q_PENDING_VAL);
+ (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
+ _Q_LOCKED_VAL) == _Q_PENDING_VAL);
}
#else /* _Q_PENDING_BITS == 8 */
static __always_inline void set_pending(struct qspinlock *lock)
@@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct
qspinlock *lock)
*/
old = val;
new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
- val = atomic_cmpxchg(&lock->val, old, new);
+ val = atomic_cmpxchg_acquire(&lock->val, old, new);
if (val == old)
return 1;
@@ -361,6 +361,13 @@ static void pv_kick_node(struct qspinlock *lock, struct
mcs_spinlock *node)
* observe its next->locked value and advance itself.
*
* Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
+ *
+ * The write to next->locked in arch_mcs_spin_unlock_contended()
+ * must be ordered before the read of pn->state in the cmpxchg()
+ * below for the code to work correctly. However, this is not
+ * guaranteed on all architectures when the cmpxchg() call fails.
+ * Both x86 and PPC can provide that guarantee, but other
+ * architectures not necessarily.
*/
if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
return;
--
1.8.3.1
