In most cases, a writer acquires the lock in two steps - first setting
the writer mode byte to _QW_WAITING and then to _QW_LOCKED. So two
atomic operations are required. This 2-step dance is only needed if
readers are present. This patch modifies the logic so that a writer
will try to acquire the lock in a single step as long as possible
until it see some readers.
Using a locking microbenchmark, a 10-threads 5M locking loop of only
writers has the following performance numbers in a Haswell-EX box:
Kernel Locking Rate (Kops/s)
------ ---------------------
4.1.1 11,939,648
Patched 4.1.1 12,906,593
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
kernel/locking/qrwlock.c | 20 +++++++++++++-------
1 files changed, 13 insertions(+), 7 deletions(-)
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 6a7a3b8..9f64493 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -109,15 +109,22 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
*/
void queued_write_lock_slowpath(struct qrwlock *lock)
{
- u32 cnts;
-
/* Put the writer into the wait queue */
arch_spin_lock(&lock->lock);
/* Try to acquire the lock directly if no reader is present */
- if (!atomic_read(&lock->cnts) &&
- (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
- goto unlock;
+ for (;;) {
+ u32 cnts = atomic_read(&lock->cnts);
+
+ if (!cnts) {
+ cnts = atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED);
+ if (cnts == 0)
+ goto unlock;
+ }
+ if (cnts & ~_QW_WMASK)
+ break; /* Reader is present */
+ cpu_relax_lowlatency();
+ }
/*
* Set the waiting flag to notify readers that a writer is pending,
@@ -135,8 +142,7 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
/* When no more readers, set the locked flag */
for (;;) {
- cnts = atomic_read(&lock->cnts);
- if ((cnts == _QW_WAITING) &&
+ if ((atomic_read(&lock->cnts) == _QW_WAITING) &&
(atomic_cmpxchg(&lock->cnts, _QW_WAITING,
_QW_LOCKED) == _QW_WAITING))
break;
--
1.7.1
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