[PATCH v10 11/19] qspinlock: Split the MCS queuing code into a separate slowerpath

[Waiman Long]

With the pending addition of more codes to support unfair lock and
PV spinlock, the complexity of the slowpath function increases to
the point that the number of scratch-pad registers in the x86-64
architecture is not enough and so those additional non-scratch-pad
registers will need to be used. This has the downside of requiring
saving and restoring of those registers in the prolog and epilog of
the slowpath function slowing down the nominally faster pending bit
and trylock code path at the beginning of the slowpath function.

This patch separates out the actual MCS queuing code into a slowerpath
function. This avoids the slow down of the pending bit and trylock
code path at the expense of a little bit of additional overhead to
the MCS queuing code path.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 kernel/locking/qspinlock.c |  120 +++++++++++++++++++++++++------------------
 1 files changed, 70 insertions(+), 50 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 10e87e1..a14241e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -335,57 +335,23 @@ static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
 }
 
 /**
- * queue_spin_lock_slowpath - acquire the queue spinlock
+ * queue_spin_lock_slowerpath - a slower path for acquiring queue spinlock
  * @lock: Pointer to queue spinlock structure
- * @val: Current value of the queue spinlock 32-bit word
- *
- * (queue tail, pending bit, lock bit)
- *
- *              fast     :    slow                                  :    unlock
- *                       :                                          :
- * uncontended  (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0)
- *                       :       | ^--------.------.             /  :
- *                       :       v           \      \            |  :
- * pending               :    (0,1,1) +--> (0,1,0)   \           |  :
- *                       :       | ^--'              |           |  :
- *                       :       v                   |           |  :
- * uncontended           :    (n,x,y) +--> (n,0,0) --'           |  :
- *   queue               :       | ^--'                          |  :
- *                       :       v                               |  :
- * contended             :    (*,x,y) +--> (*,0,0) ---> (*,0,1) -'  :
- *   queue               :         ^--'                             :
- *
- * The pending bit processing is in the trylock_pending() function
- * whereas the uncontended and contended queue processing is in the
- * queue_spin_lock_slowpath() function.
+ * @val : Current value of the queue spinlock 32-bit word
+ * @node: Pointer to the queue node
+ * @tail: The tail code
  *
+ * The reason for splitting a slowerpath from slowpath is to avoid the
+ * unnecessary overhead of non-scratch pad register pushing and popping
+ * due to increased complexity with unfair and PV spinlock from slowing
+ * down the nominally faster pending bit and trylock code path. So this
+ * function is not inlined.
  */
-void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
+static noinline void queue_spin_lock_slowerpath(struct qspinlock *lock,
+			u32 val, struct qnode *node, u32 tail)
 {
-	struct qnode *prev, *next, *node;
-	u32 old, tail;
-	int idx;
-
-	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
-
-	if (trylock_pending(lock, &val))
-		return;	/* Lock acquired */
-
-	node = this_cpu_ptr(&qnodes[0]);
-	idx = node->mcs.count++;
-	tail = encode_tail(smp_processor_id(), idx);
-
-	node += idx;
-	node->qhead = 0;
-	node->mcs.next = NULL;
-
-	/*
-	 * We touched a (possibly) cold cacheline in the per-cpu queue node;
-	 * attempt the trylock once more in the hope someone let go while we
-	 * weren't watching.
-	 */
-	if (queue_spin_trylock(lock))
-		goto release;
+	struct qnode *prev, *next;
+	u32 old;
 
 	/*
 	 * we already touched the queueing cacheline; don't bother with pending
@@ -442,7 +408,7 @@ retry_queue_wait:
 		}
 		old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
 		if (old == val)
-			goto release;	/* No contention */
+			return;	/* No contention */
 		else if (old & _Q_LOCKED_MASK)
 			goto retry_queue_wait;
 
@@ -450,14 +416,68 @@ retry_queue_wait:
 	}
 
 	/*
-	 * contended path; wait for next, release.
+	 * contended path; wait for next, return.
 	 */
 	while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
 		arch_mutex_cpu_relax();
 
 	arch_mcs_spin_unlock_contended(&next->qhead);
+}
+
+/**
+ * queue_spin_lock_slowpath - acquire the queue spinlock
+ * @lock: Pointer to queue spinlock structure
+ * @val: Current value of the queue spinlock 32-bit word
+ *
+ * (queue tail, pending bit, lock bit)
+ *
+ *              fast     :    slow                                  :    unlock
+ *                       :                                          :
+ * uncontended  (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0)
+ *                       :       | ^--------.------.             /  :
+ *                       :       v           \      \            |  :
+ * pending               :    (0,1,1) +--> (0,1,0)   \           |  :
+ *                       :       | ^--'              |           |  :
+ *                       :       v                   |           |  :
+ * uncontended           :    (n,x,y) +--> (n,0,0) --'           |  :
+ *   queue               :       | ^--'                          |  :
+ *                       :       v                               |  :
+ * contended             :    (*,x,y) +--> (*,0,0) ---> (*,0,1) -'  :
+ *   queue               :         ^--'                             :
+ *
+ * This slowpath only contains the faster pending bit and trylock codes.
+ * The slower queuing code is in the slowerpath function.
+ *
+ * The pending bit processing is in the trylock_pending() function
+ * whereas the uncontended and contended queue processing is in the
+ * queue_spin_lock_slowerpath() function.
+ */
+void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
+{
+	struct qnode *node;
+	u32 tail, idx;
+
+	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
+
+	if (trylock_pending(lock, &val))
+		return;	/* Lock acquired */
+
+	node = this_cpu_ptr(&qnodes[0]);
+	idx = node->mcs.count++;
+	tail = encode_tail(smp_processor_id(), idx);
+
+	node += idx;
+	node->qhead = 0;
+	node->mcs.next = NULL;
+
+	/*
+	 * We touched a (possibly) cold cacheline in the per-cpu queue node;
+	 * attempt the trylock once more in the hope someone let go while we
+	 * weren't watching.
+	 */
+	if (!queue_spin_trylock(lock))
+		queue_spin_lock_slowerpath(lock, val, node, tail);
 
-release:
 	/*
 	 * release the node
 	 */
-- 
1.7.1

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