Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH 3/4] locking/qrwlock: Reduce writer to writer lock transfer latency | Date | Mon, 6 Jul 2015 11:43:05 -0400 |
| |
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@hp.com> --- 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 ecd2d19..87e2d6b 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -109,15 +109,22 @@ EXPORT_SYMBOL(queue_read_lock_slowpath); */ void queue_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 queue_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
| |