[PATCH v3 1/4] locking/rwsem: Add reader-owned state to the owner field

[Waiman Long]

Currently, it is not possible to determine for sure if a reader
owns a rwsem by looking at the content of the rwsem data structure.
This patch adds a new state RWSEM_READER_OWNED to the owner field
to indicate that readers currently own the lock. This enables us to
address the following 2 issues in the rwsem optimistic spinning code:

 1) rwsem_can_spin_on_owner() will disallow optimistic spinning if
    the owner field is NULL which can mean either the readers own
    the lock or the owning writer hasn't set the owner field yet.
    In the latter case, we miss the chance to do optimistic spinning.

 2) While a writer is waiting in the OSQ and a reader takes the lock,
    the writer will continue to spin when out of the OSQ in the main
    rwsem_optimistic_spin() loop as the owner field is NULL wasting
    CPU cycles if some of readers are sleeping.

Adding the new state will allow optimistic spinning to go forward as
long as the owner field is not RWSEM_READER_OWNED and the owner is
running, if set, but stop immediately when that state has been reached.

On a 4-socket Haswell machine running on a 4.6-rc1 based kernel, the
fio test with multithreaded randrw and randwrite tests on the same
file on a XFS partition on top of a NVDIMM were run, the aggregated
bandwidths before and after the patch were as follows:

  Test      BW before patch     BW after patch  % change
  ----      ---------------     --------------  --------
  randrw         988 MB/s          1192 MB/s      +21%
  randwrite     1513 MB/s          1623 MB/s      +7.3%

The perf profile of the rwsem_down_write_failed() function in randrw
before and after the patch were:

   19.95%  5.88%  fio  [kernel.vmlinux]  [k] rwsem_down_write_failed
   14.20%  1.52%  fio  [kernel.vmlinux]  [k] rwsem_down_write_failed

The actual CPU cycles spend in rwsem_down_write_failed() dropped from
5.88% to 1.52% after the patch.

The xfstests was also run and no regression was observed.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
Acked-by: Jason Low <jason.low2@xxxxxx>
Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
---
 kernel/locking/rwsem-xadd.c |   41 ++++++++++++++++++++++-------------------
 kernel/locking/rwsem.c      |    8 ++++++--
 kernel/locking/rwsem.h      |   41 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+), 21 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 09e30c6..7ccab5c 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -155,6 +155,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 			/* Last active locker left. Retry waking readers. */
 			goto try_reader_grant;
 		}
+		/*
+		 * It is not really necessary to set it to reader-owned here,
+		 * but it gives the spinners an early indication that the
+		 * readers now have the lock.
+		 */
+		rwsem_set_reader_owned(sem);
 	}
 
 	/* Grant an infinite number of read locks to the readers at the front
@@ -306,16 +312,11 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 
 	rcu_read_lock();
 	owner = READ_ONCE(sem->owner);
-	if (!owner) {
-		long count = READ_ONCE(sem->count);
+	if (!rwsem_owner_is_writer(owner)) {
 		/*
-		 * If sem->owner is not set, yet we have just recently entered the
-		 * slowpath with the lock being active, then there is a possibility
-		 * reader(s) may have the lock. To be safe, bail spinning in these
-		 * situations.
+		 * Don't spin if the rwsem is readers owned.
 		 */
-		if (count & RWSEM_ACTIVE_MASK)
-			ret = false;
+		ret = !rwsem_owner_is_reader(owner);
 		goto done;
 	}
 
@@ -328,8 +329,6 @@ done:
 static noinline
 bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 {
-	long count;
-
 	rcu_read_lock();
 	while (sem->owner == owner) {
 		/*
@@ -350,16 +349,11 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 	}
 	rcu_read_unlock();
 
-	if (READ_ONCE(sem->owner))
-		return true; /* new owner, continue spinning */
-
 	/*
-	 * When the owner is not set, the lock could be free or
-	 * held by readers. Check the counter to verify the
-	 * state.
+	 * If there is a new owner or the owner is not set, we continue
+	 * spinning.
 	 */
-	count = READ_ONCE(sem->count);
-	return (count == 0 || count == RWSEM_WAITING_BIAS);
+	return !rwsem_owner_is_reader(sem->owner);
 }
 
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
@@ -378,7 +372,16 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 
 	while (true) {
 		owner = READ_ONCE(sem->owner);
-		if (owner && !rwsem_spin_on_owner(sem, owner))
+		/*
+		 * Don't spin if
+		 * 1) the owner is a reader as we we can't determine if the
+		 *    reader is actively running or not.
+		 * 2) The rwsem_spin_on_owner() returns false which means
+		 *    the owner isn't running.
+		 */
+		if (rwsem_owner_is_reader(owner) ||
+		   (rwsem_owner_is_writer(owner) &&
+		   !rwsem_spin_on_owner(sem, owner)))
 			break;
 
 		/* wait_lock will be acquired if write_lock is obtained */
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index c817216..5838f56 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -22,6 +22,7 @@ void __sched down_read(struct rw_semaphore *sem)
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
+	rwsem_set_reader_owned(sem);
 }
 
 EXPORT_SYMBOL(down_read);
@@ -33,8 +34,10 @@ int down_read_trylock(struct rw_semaphore *sem)
 {
 	int ret = __down_read_trylock(sem);
 
-	if (ret == 1)
+	if (ret == 1) {
 		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
+		rwsem_set_reader_owned(sem);
+	}
 	return ret;
 }
 
@@ -124,7 +127,7 @@ void downgrade_write(struct rw_semaphore *sem)
 	 * lockdep: a downgraded write will live on as a write
 	 * dependency.
 	 */
-	rwsem_clear_owner(sem);
+	rwsem_set_reader_owned(sem);
 	__downgrade_write(sem);
 }
 
@@ -138,6 +141,7 @@ void down_read_nested(struct rw_semaphore *sem, int subclass)
 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
+	rwsem_set_reader_owned(sem);
 }
 
 EXPORT_SYMBOL(down_read_nested);
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 870ed9a..8f43ba2 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -1,3 +1,20 @@
+/*
+ * The owner field of the rw_semaphore structure will be set to
+ * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
+ * the owner field when it unlocks. A reader, on the other hand, will
+ * not touch the owner field when it unlocks.
+ *
+ * In essence, the owner field now has the following 3 states:
+ *  1) 0
+ *     - lock is free or the owner hasn't set the field yet
+ *  2) RWSEM_READER_OWNED
+ *     - lock is currently or previously owned by readers (lock is free
+ *       or not set by owner yet)
+ *  3) Other non-zero value
+ *     - a writer owns the lock
+ */
+#define RWSEM_READER_OWNED	((struct task_struct *)1UL)
+
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
@@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 	sem->owner = NULL;
 }
 
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+	/*
+	 * We check the owner value first to make sure that we will only
+	 * do a write to the rwsem cacheline when it is really necessary
+	 * to minimize cacheline contention.
+	 */
+	if (sem->owner != RWSEM_READER_OWNED)
+		sem->owner = RWSEM_READER_OWNED;
+}
+
+static inline bool rwsem_owner_is_writer(struct task_struct *owner)
+{
+	return owner && owner != RWSEM_READER_OWNED;
+}
+
+static inline bool rwsem_owner_is_reader(struct task_struct *owner)
+{
+	return owner == RWSEM_READER_OWNED;
+}
 #else
 static inline void rwsem_set_owner(struct rw_semaphore *sem)
 {
@@ -17,4 +54,8 @@ static inline void rwsem_set_owner(struct rw_semaphore *sem)
 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 {
 }
+
+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
+{
+}
 #endif
-- 
1.7.1