[PATCH v4 10/16] locking/rwsem: Wake up almost all readers in wait queue

[Waiman Long]

When the front of the wait queue is a reader, other readers
immediately following the first reader will also be woken up at the
same time. However, if there is a writer in between. Those readers
behind the writer will not be woken up.

Because of optimistic spinning, the lock acquisition order is not FIFO
anyway. The lock handoff mechanism will ensure that lock starvation
will not happen.

Assuming that the lock hold times of the other readers still in the
queue will be about the same as the readers that are being woken up,
there is really not much additional cost other than the additional
latency due to the wakeup of additional tasks by the waker. Therefore
all the readers up to a maximum of 256 in the queue are woken up when
the first waiter is a reader to improve reader throughput.

With a locking microbenchmark running on 5.1 based kernel, the total
locking rates (in kops/s) on a 8-socket IvyBridge-EX system with
equal numbers of readers and writers before and after this patch were
as follows:

   # of Threads  Pre-Patch   Post-patch
   ------------  ---------   ----------
        4          1,641        1,674
        8            731        1,062
       16            564          924
       32             78          300
       64             38          195
      240             50          149

There is no performance gain at low contention level. At high contention
level, however, this patch gives a pretty decent performance boost.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/locking/rwsem.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 8e19b5141595..cf0a90d251aa 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -260,6 +260,13 @@ enum writer_wait_state {
  */
 #define RWSEM_WAIT_TIMEOUT	(HZ/250)
 
+/*
+ * We limit the maximum number of readers that can be woken up for a
+ * wake-up call to not penalizing the waking thread for spending too
+ * much time doing it.
+ */
+#define MAX_READERS_WAKEUP	0x100
+
 /*
  * handle the lock release when processes blocked on it that can now run
  * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
@@ -332,16 +339,16 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 	}
 
 	/*
-	 * Grant an infinite number of read locks to the readers at the front
-	 * of the queue. We know that woken will be at least 1 as we accounted
-	 * for above. Note we increment the 'active part' of the count by the
+	 * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
+	 * queue. We know that woken will be at least 1 as we accounted for
+	 * above. Note we increment the 'active part' of the count by the
 	 * number of readers before waking any processes up.
 	 */
 	list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
 		struct task_struct *tsk;
 
 		if (waiter->type == RWSEM_WAITING_FOR_WRITE)
-			break;
+			continue;
 
 		woken++;
 		tsk = waiter->task;
@@ -360,6 +367,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 		 * after setting the reader waiter to nil.
 		 */
 		wake_q_add_safe(wake_q, tsk);
+
+		/*
+		 * Limit # of readers that can be woken up per wakeup call.
+		 */
+		if (woken >= MAX_READERS_WAKEUP)
+			break;
 	}
 
 	adjustment = woken * RWSEM_READER_BIAS - adjustment;
-- 
2.18.1