This patch enables readers to optimistically spin on a rwsem when it
is owned by a writer instead of going to sleep directly. The key to
make this possible is the changes made to RWSEM_WAITING_BIAS that
enables us to check the status of the rwsem for read lock stealing
without taking the wait_lock.
The rwsem_can_spin_on_owner() function is extracted out
of rwsem_optimistic_spin() and is called directly by
rwsem_down_read_failed() and rwsem_down_write_failed().
On a 2-socket 36-core 72-thread x86-64 E5-2699 v3 system, a rwsem
microbenchmark was run with 36 locking threads (one/core) doing 250k
reader and writer lock/unlock operations each, the resulting locking
rates (avg of 3 runs) on a 4.12 based kernel were 520.1 Mop/s and
1760.2 Mop/s without and with the patch respectively. That was an
increase of about 238%.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 67 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 54 insertions(+), 13 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 4fb6cce..f82ce29 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -85,6 +85,12 @@
* (2) WAITING_BIAS - ACTIVE_WRITE_BIAS < count < 0
*/
+static inline bool count_has_writer(long count)
+{
+ return (count < RWSEM_WAITING_BIAS) || ((count < 0) &&
+ (count > RWSEM_WAITING_BIAS - RWSEM_ACTIVE_WRITE_BIAS));
+}
+
/*
* Initialize an rwsem:
*/
@@ -287,6 +293,25 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
}
}
+/*
+ * Try to acquire read lock before the reader is put on wait queue
+ */
+static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
+{
+ long count = atomic_long_read(&sem->count);
+
+ if (count_has_writer(count))
+ return false;
+ count = atomic_long_add_return_acquire(RWSEM_ACTIVE_READ_BIAS,
+ &sem->count);
+ if (!count_has_writer(count))
+ return true;
+
+ /* Back out the change */
+ atomic_long_add(-RWSEM_ACTIVE_READ_BIAS, &sem->count);
+ return false;
+}
+
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner;
@@ -356,16 +381,14 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
}
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
+ enum rwsem_waiter_type type)
{
bool taken = false;
preempt_disable();
/* sem->wait_lock should not be held when doing optimistic spinning */
- if (!rwsem_can_spin_on_owner(sem))
- goto done;
-
if (!osq_lock(&sem->osq))
goto done;
@@ -380,10 +403,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
/*
* Try to acquire the lock
*/
- if (rwsem_try_write_lock_unqueued(sem)) {
- taken = true;
+ taken = (type == RWSEM_WAITING_FOR_WRITE)
+ ? rwsem_try_write_lock_unqueued(sem)
+ : rwsem_try_read_lock_unqueued(sem);
+ if (taken)
break;
- }
/*
* When there's no owner, we might have preempted between the
@@ -417,7 +441,13 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
}
#else
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+{
+ return false;
+}
+
+static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem,
+ enum rwsem_waiter_type type)
{
return false;
}
@@ -434,7 +464,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
__visible
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
- bool first_in_queue = false;
+ bool first_in_queue = false, can_spin;
long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
@@ -444,14 +474,24 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
/*
* Undo read bias from down_read operation to stop active locking if:
- * 1) Optimistic spinners are present; or
- * 2) the wait_lock isn't free.
+ * 1) Optimistic spinners are present;
+ * 2) the wait_lock isn't free; or
+ * 3) optimistic spinning is allowed.
* Doing that after taking the wait_lock may otherwise block writer
* lock stealing for too long impacting performance.
*/
- if (rwsem_has_spinner(sem) || raw_spin_is_locked(&sem->wait_lock)) {
+ can_spin = rwsem_can_spin_on_owner(sem);
+ if (can_spin || rwsem_has_spinner(sem) ||
+ raw_spin_is_locked(&sem->wait_lock)) {
atomic_long_add(-RWSEM_ACTIVE_READ_BIAS, &sem->count);
adjustment = 0;
+
+ /*
+ * Do optimistic spinning and steal lock if possible.
+ */
+ if (can_spin &&
+ rwsem_optimistic_spin(sem, RWSEM_WAITING_FOR_READ))
+ return sem;
}
raw_spin_lock_irq(&sem->wait_lock);
@@ -509,7 +549,8 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
/* do optimistic spinning and steal lock if possible */
- if (rwsem_optimistic_spin(sem))
+ if (rwsem_can_spin_on_owner(sem) &&
+ rwsem_optimistic_spin(sem, RWSEM_WAITING_FOR_WRITE))
return sem;
/*
--
1.8.3.1
--
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