[PATCH v10 13/19] unfair qspinlock: Enable lock stealing in lock waiters
From: Waiman Long
Date: Wed May 07 2014 - 11:06:30 EST
The simple unfair queue lock cannot completely solve the lock waiter
preemption problem as a preempted CPU at the front of the queue will
block forward progress in all the other CPUs behind it in the queue.
To allow those CPUs to move forward, it is necessary to enable lock
stealing for those lock waiters as well.
Enabling those lock waiters to try to steal the lock increases the
cacheline pressure on the lock word. As a result, performance can
suffer on a workload with heavy spinlock contention.
The tables below shows the the performance (jobs/minutes) of other
kernel flavors of a 3.14-based kernel at 3000 users of the AIM7 disk
workload on a 4-socket Westmere-EX bare-metal system. The ebizzy test
was run.
AIM7 XFS Disk Test
kernel JPM Real Time Sys Time Usr Time
----- --- --------- -------- --------
ticketlock 5678233 3.17 96.61 5.81
qspinlock 5750799 3.13 94.83 5.97
simple test-and-set 5625000 3.20 98.29 5.93
simple unfair 5750799 3.13 95.91 5.98
qspinlock
complex unfair 5678233 3.17 97.40 5.93
qspinlock
AIM7 EXT4 Disk Test
kernel JPM Real Time Sys Time Usr Time
----- --- --------- -------- --------
ticketlock 1114551 16.15 509.72 7.11
qspinlock 2184466 8.24 232.99 6.01
simple test-and-set 593081 30.35 967.55 9.00
simple unfair 2292994 7.85 222.84 5.89
qspinlock
complex unfair 972447 18.51 589.88 6.65
qspinlock
Ebizzy -m test
kernel records/s Real Time Sys Time Usr Time
----- --------- --------- -------- --------
ticketlock 2075 10.00 216.35 3.49
qspinlock 3023 10.00 198.20 4.80
simple test-and-set 1667 10.00 198.93 2.89
simple unfair 2915 10.00 165.68 4.31
qspinlock
complex unfair 1965 10.00 191.96 3.17
qspinlock
With heavy spinlock contention, the complex unfair lock is faster
than the simple test-and-set lock, but it is still slower than the
baseline ticketlock.
The table below shows the execution times (in ms) of a spinlock
micro-benchmark on the same 4-socket Westmere-EX system.
# of Ticket Fair Unfair simple Unfair complex
tasks lock queue lock queue lock queue lock
------ ------- ---------- ---------- ---------
1 135 135 137 137
2 890 1082 421 663
3 1932 2248 708 1263
4 2829 2819 1030 1806
5 3834 3522 1323 2315
6 4963 4173 1723 2831
7 6299 4875 2067 2878
8 7691 5563 2360 3256
Executing one task per node, the performance data were:
# of Ticket Fair Unfair simple Unfair complex
nodes lock queue lock queue lock queue lock
------ ------- ---------- ---------- ---------
1 135 135 137 137
2 4603 1034 670 888
3 10940 12087 1389 2041
4 21555 10507 1869 4307
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
kernel/locking/qspinlock.c | 160 ++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 154 insertions(+), 6 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 06dd486..0c86a6f 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -166,6 +166,23 @@ xchg_tail(struct qspinlock *lock, u32 tail, u32 *pval)
return (u32)xchg(&l->tail, tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
}
+/*
+ * cmpxchg_tail - Put in the new tail code if it matches the old one
+ * @lock : Pointer to queue spinlock structure
+ * @old : The old tail code value
+ * @new : The new tail code value
+ * Return: true if operation succeeds, fail otherwise
+ */
+static __always_inline int
+cmpxchg_tail(struct qspinlock *lock, u32 old, u32 new)
+{
+ struct __qspinlock *l = (void *)lock;
+
+ old >>= _Q_TAIL_OFFSET;
+ new >>= _Q_TAIL_OFFSET;
+ return cmpxchg(&l->tail, old, new) == old;
+}
+
#else /* _Q_PENDING_BITS == 8 */
/**
@@ -218,6 +235,35 @@ xchg_tail(struct qspinlock *lock, u32 tail, u32 *pval)
*pval = new;
return old;
}
+
+/*
+ * cmpxchg_tail - Put in the new tail code if it matches the old one
+ * @lock : Pointer to queue spinlock structure
+ * @old : The old tail code value
+ * @new : The new tail code value
+ * Return: true if operation succeeds, fail otherwise
+ *
+ * It is assumed that the caller has grabbed the lock before calling it.
+ */
+static __always_inline int
+cmpxchg_tail(struct qspinlock *lock, u32 old, u32 new)
+{
+ u32 val;
+ u32 lp = _Q_LOCKED_VAL; /* Lock & pending bits value */
+
+ for (;;) {
+ u32 val = atomic_cmpxchg(&lock->val, old | lp, new | lp);
+
+ /*
+ * If the lock or pending bits somehow changes, redo it again
+ */
+ if ((val & _Q_LOCKED_PENDING_MASK) != lp) {
+ lp = val & _Q_LOCKED_PENDING_MASK;
+ continue;
+ }
+ return val == (old | lp);
+ }
+}
#endif /* _Q_PENDING_BITS == 8 */
/*
@@ -302,6 +348,25 @@ unfair_set_vars(struct qnode *node, struct qnode *prev, u32 prev_tail)
}
/**
+ * unfair_check_and_clear_tail - check the tail value in lock & clear it if
+ * it matches the given tail
+ * @lock : Pointer to queue spinlock structure
+ * @tail : The tail code to be checked against
+ * Return: true if the tail code matches and is cleared, false otherwise
+ */
+static inline int unfair_check_and_clear_tail(struct qspinlock *lock, u32 tail)
+{
+ if (!static_key_false(¶virt_unfairlocks_enabled))
+ return false;
+
+ /*
+ * Try to clear the current tail code if it matches the given tail
+ */
+ return ((atomic_read(&lock->val) & _Q_TAIL_MASK) == tail) &&
+ cmpxchg_tail(lock, tail, 0);
+}
+
+/**
* unfair_get_lock - try to steal the lock periodically
* @lock : Pointer to queue spinlock structure
* @node : Current queue node address
@@ -313,7 +378,7 @@ unfair_set_vars(struct qnode *node, struct qnode *prev, u32 prev_tail)
* node only if the qhead flag is set and the next pointer in the queue
* node is not NULL.
*/
-static noinline int
+static inline int
unfair_get_lock(struct qspinlock *lock, struct qnode *node, u32 tail, int count)
{
u32 prev_tail;
@@ -337,8 +402,64 @@ unfair_get_lock(struct qspinlock *lock, struct qnode *node, u32 tail, int count)
node->lsteal_mask = LSTEAL_MAX_MASK;
return false;
}
- queue_spin_unlock(lock);
- return false;
+
+ /*
+ * Have stolen the lock, need to remove itself from the wait queue.
+ * There are 3 steps that need to be done:
+ * 1) If it is at the end of the queue, change the tail code in the
+ * lock to the one before it. If the current node happens to be
+ * the queue head, the previous tail code is 0.
+ * 2) Change the next pointer in the previous queue node to point
+ * to the next one in queue or NULL if it is at the end of queue.
+ * 3) If a next node is present, copy the prev_tail and qprev values
+ * to the next node.
+ */
+ isqhead = ACCESS_ONCE(node->qhead);
+ prev_tail = isqhead ? 0 : node->prev_tail;
+
+ /* Step 1 */
+ if (((atomic_read(&lock->val) & _Q_TAIL_MASK) == tail) &&
+ cmpxchg_tail(lock, tail, prev_tail)) {
+ /*
+ * Successfully change the tail code back to the previous one.
+ * Now need to clear the next pointer in the previous node
+ * only if it contains my queue node address and is not
+ * the queue head. The cmpxchg() call below may fail if
+ * a new task comes along and put its node address into the
+ * next pointer. Whether the operation succeeds or fails
+ * doesn't really matter.
+ */
+ /* Step 2 */
+ if (!isqhead)
+ (void)cmpxchg(&node->qprev->mcs.next, &node->mcs, NULL);
+ node->mcs.next = NULL;
+ return true;
+ }
+
+ /*
+ * A next node has to be present if the lock has a different tail
+ * code. So wait until the next pointer is set.
+ */
+ while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
+ arch_mutex_cpu_relax();
+
+ if (!isqhead) {
+ /*
+ * Change the node data only if it is not the queue head
+ * Steps 2 & 3
+ */
+ ACCESS_ONCE(node->qprev->mcs.next) =
+ (struct mcs_spinlock *)next;
+ ACCESS_ONCE(next->qprev) = node->qprev;
+ ACCESS_ONCE(next->prev_tail) = node->prev_tail;
+
+ /*
+ * Make sure all the new node information are visible
+ * before proceeding.
+ */
+ smp_wmb();
+ }
+ return true;
}
#else /* CONFIG_PARAVIRT_UNFAIR_LOCKS */
@@ -351,6 +472,8 @@ static void unfair_set_vars(struct qnode *node, struct qnode *prev,
u32 prev_tail) {}
static int unfair_get_lock(struct qspinlock *lock, struct qnode *node,
u32 tail, int count) { return false; }
+static int unfair_check_and_clear_tail(struct qspinlock *lock, u32 tail)
+ { return false; }
#endif /* CONFIG_PARAVIRT_UNFAIR_LOCKS */
/**
@@ -511,7 +634,16 @@ static noinline void queue_spin_lock_slowerpath(struct qspinlock *lock,
while (!smp_load_acquire(&node->qhead)) {
INC_LOOP_CNT(cnt);
- unfair_get_lock(lock, node, tail, LOOP_CNT(cnt));
+ if (unfair_get_lock(lock, node, tail, LOOP_CNT(cnt))) {
+ /*
+ * Need to notify the next node only if both
+ * the qhead flag and the next pointer in the
+ * queue node are set.
+ */
+ if (unlikely(node->qhead && node->mcs.next))
+ goto notify_next;
+ return;
+ }
arch_mutex_cpu_relax();
}
}
@@ -545,10 +677,25 @@ retry_queue_wait:
* The get_qlock function will only failed if the
* lock was stolen.
*/
- if (get_qlock(lock))
+ if (get_qlock(lock)) {
+ /*
+ * It is possible that in between the reading
+ * of the lock value and the acquisition of
+ * the lock, the next node may have stolen the
+ * lock and be removed from the queue. So
+ * the lock value may contain the tail code
+ * of the current node. We need to recheck
+ * the tail value here to be sure. And if
+ * the tail code was cleared, we don't need
+ * to notify the next node.
+ */
+ if (unlikely(unfair_check_and_clear_tail(lock,
+ tail)))
+ return;
break;
- else
+ } else {
goto retry_queue_wait;
+ }
}
old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
@@ -562,6 +709,7 @@ retry_queue_wait:
/*
* contended path; wait for next, return.
*/
+notify_next:
while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
arch_mutex_cpu_relax();
--
1.7.1
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