[PATCH RFC 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

[Waiman Long]

This patch introduces a new queue spinlock implementation that can
serve as an alternative to the default ticket spinlock. Compared
with the ticket spinlock, this queue spinlock should be as fair as
the ticket spinlock. It is faster both in single-thread and in high
contention situations. Only in light to moderate contention where
the average queue depth is around 1-2 will this queue spinlock be
potentially a bit slower due to the higher slowpath overhead.

This queue spinlock is especially suit to NUMA machines with a large
number of cores as the chance of spinlock contention is much higher
in those machines.

The idea behind this spinlock implementation is the fact that spinlocks
are acquired with preemption disabled. In other words, the process
will not be migrated to another CPU while it is trying to get a
spinlock. Ignoring interrupt handling, a CPU can only be contending
in one spinlock at any one time. Of course, interrupt handler can try
to acquire one spinlock while the interrupted user process is in the
process of getting another spinlock. By allocating a set of per-cpu
queue nodes and used them to form a waiting queue, we can encode the
queue node address into a much smaller 16-bit size. Together with
the 1-byte lock bit, this queue spinlock implementation will only
need 4 bytes to hold all the information that it needs.

The current queue node address encoding is as follows:
Bits 0-1 : queue node index in the per-cpu array (4 entries)
Bits 2-16: cpu number + 1 (max cpus = 16383)

By using also the unused byte, we could support more cpus and queue
node entries in the array at the expense of more coding and probably
performance overheads.

In the extremely unlikely case that all the queue node entries are
used up, the current code will fall back to busy spinning without
waiting in a queue with warning message.

With minor modification to the lock fastpath, we could also make a
less fair version of queue spinlock that allows lock stealing. This
can potentially provide better performance for some workloads.

This patch allows the optional replacement of architecture specific
implementation of ticket spinlock by this generic version of queue
spinlock. Two new config parameters are introduced:

1. QUEUE_SPINLOCK
   A select-able option that enables the building and replacement of
   architecture specific spinlock by queue spinlock.
2. ARCH_QUEUE_SPINLOCK
   Have to be defined in arch/$(ARCH)/Kconfig to enable QUEUE_SPINLOCK
   option. This option, by itself, will not enable the queue spinlock
   feature.

For single-thread performance (no contention), a 256K lock/unlock
loop was run on a 2.93Ghz Westmere x86-64 CPU.  The following table
shows the average time (in ns) for a single lock/unlock sequence
(including the looping and timing overhead):

Lock Type		Time (ns)
---------		---------
Ticket spinlock		  12.1
Queue spinlock		  10.9

So the queue spinlock is slightly faster.

The AIM7 benchmark was run on a 8-socket 80-core DL980 with Westmere
x86-64 CPUs to evaluate the performance impact of this patch on the
3.10.1 kernel. The jobs per minute (JPM) results for the 1100-2000
user ranges are shown below:

+------------+--------+--------+--------+--------+--------+--------+
| Kernel     | 3.10.1 | 3.10.1 |%Change | 3.10.1 | 3.10.1 |%Change |
|            |        | qlock  |        |        | qlock  |        |
| HT         |  off   |  off   |        |   on   |   on   |        |
+------------+--------+--------+--------+--------+--------+--------+
|alltests    | 403590 | 397622 |  -1.5% | 411479 | 411428 |   0.0% |
|custom      | 304581 | 307165 |  +0.9% | 230335 | 254798 | +10.6% |
|dbase       | 896252 | 900597 |  +0.5% |1134659 |1137730 |  +0.3% |
|disk        | 213679 | 270597 | +26.6% | 203974 | 249998 | +22.6% |
|five_sec    | 144012 | 157286 |  +9.2% | 132599 | 150812 | +13.7% |
|fserver     | 472741 | 467213 |  -1.2% | 270601 | 285737 |  +5.6% |
|high_systime| 131106 | 139015 |  +6.0% | 120294 | 125802 |  +4.6% |
|new_dbase   | 934871 | 936860 |  +0.2  |1177640 |1181679 |  +0.3% |
|new_fserver | 452544 | 444967 |  -1.7% | 262368 | 275827 |  +5.1% |
|shared      | 366750 | 369901 |  +0.9% | 403645 | 415840 |  +3.0% |
|short       |1061663 |3003942 |+183.0% |1040392 |2834437 |+172.4% |
+------------+--------+--------+--------+--------+--------+--------+

There are cases where the performance drops 1-2%, but the majority
of them are performance gains and some of them are pretty significant.

For the disk workload, the spinlock bottleneck is the standalone
mb_cache_spinlock in fs/mbcache.c. For the short workload, the spinlock
bottleneck is the d_lock in the dentry structure.

The following table shows the %time used up by the locks as reported
by the perf command at 1000 users for disk workload & 1500 users for
short workload with HT off.

-----------------+----------+----------+----------+
    Lock 	 | ticket   |  qlock   |  qlock   |
		 | spinlock | fastpath | slowpath |
-----------------+----------+----------+----------+
Disk w/o patch	 |  69.0%   |    -     |    -     |
Disk with patch	 |    -     |  0.31%   |  73.6%   |
Short w/o patch	 |  74.3%   |    -     |    -     |
Short with patch |    -     |  0.72%   |  72.3%   |
-----------------+----------+----------+----------+

There are 2 observations here:
1. The %CPU time actually increases in the disk workload with the
   patch. The fact that most of them are spinning on their own
   cacheline with less congestion can probably skew the number up.

2. Both the short and disk workloads spend about 70% of their time
   in the lock. However, performance improvement is 2.8X vs 1.3X.
   This is probably due to the fact that d_lock is embedded next
   to the d_count to be updated whereas mb_cache_spinlock is in a
   standalone cacheline not shared by the data to be updated.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 include/asm-generic/qspinlock.h |  120 ++++++++++++++++++++
 lib/Kconfig                     |   14 +++
 lib/Makefile                    |    1 +
 lib/qspinlock.c                 |  237 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 372 insertions(+), 0 deletions(-)
 create mode 100644 include/asm-generic/qspinlock.h
 create mode 100644 lib/qspinlock.c

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
new file mode 100644
index 0000000..4cfb369
--- /dev/null
+++ b/include/asm-generic/qspinlock.h
@@ -0,0 +1,120 @@
+/*
+ * Queue spinlock
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * (C) Copyright 2013 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.long@xxxxxx>
+ */
+#ifndef __ASM_GENERIC_QSPINLOCK_H
+#define __ASM_GENERIC_QSPINLOCK_H
+
+#include <linux/types.h>
+#include <asm/cmpxchg.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#define	qspinlock arch_spinlock
+#endif
+
+/*
+ * The queue spinlock data structure
+ */
+typedef struct qspinlock {
+	union {
+		struct {
+			u8	locked;		/* Bit lock */
+			u8	reserved;
+			u16	qcode;		/* Wait queue code */
+		};
+		u32		qlock;
+	};
+} arch_spinlock_t;
+
+/*
+ * External function declarations
+ */
+extern void queue_spin_lock_slowpath(struct qspinlock *lock);
+
+/**
+ * queue_spin_is_locked - is the spinlock locked?
+ * @lock: Pointer to queue spinlock structure
+ * Return: 1 if it is locked, 0 otherwise
+ */
+static __always_inline int queue_spin_is_locked(struct qspinlock *lock)
+{
+	return ACCESS_ONCE(lock->locked);
+}
+
+/**
+ * queue_spin_is_contended - check if the lock is contended
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock contended, 0 otherwise
+ */
+static __always_inline int queue_spin_is_contended(struct qspinlock *lock)
+{
+	return ACCESS_ONCE(lock->qcode);
+}
+/**
+ * queue_spin_trylock - try to acquire the queue spinlock
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static __always_inline int queue_spin_trylock(struct qspinlock *lock)
+{
+	if (!queue_spin_is_contended(lock) && (xchg(&lock->locked, 1) == 0))
+		return 1;
+	return 0;
+}
+
+/**
+ * queue_spin_lock - acquire a queue spinlock
+ * @lock: Pointer to queue spinlock structure
+ */
+static __always_inline void queue_spin_lock(struct qspinlock *lock)
+{
+	if (likely(queue_spin_trylock(lock)))
+		return;
+	queue_spin_lock_slowpath(lock);
+}
+
+/**
+ * queue_spin_unlock - release a queue spinlock
+ * @lock : Pointer to queue spinlock structure
+ */
+static __always_inline void queue_spin_unlock(struct qspinlock *lock)
+{
+	barrier();
+	ACCESS_ONCE(lock->locked) = 0;
+	smp_wmb();
+}
+
+/*
+ * Initializier
+ */
+#define	__ARCH_SPIN_LOCK_UNLOCKED	{ { .qlock = 0 } }
+
+#ifndef CONFIG_PARAVIRT_SPINLOCKS
+/*
+ * Remapping spinlock architecture specific functions to the corresponding
+ * queue spinlock functions.
+ */
+#define arch_spin_is_locked(l)		queue_spin_is_locked(l)
+#define arch_spin_is_contended(l)	queue_spin_is_contended(l)
+#define arch_spin_lock(l)		queue_spin_lock(l)
+#define arch_spin_trylock(l)		queue_spin_trylock(l)
+#define arch_spin_unlock(l)		queue_spin_unlock(l)
+#define arch_spin_lock_flags(l, f)	queue_spin_lock(l)
+#endif
+
+#endif /* __ASM_GENERIC_QSPINLOCK_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index 35da513..15e0e02 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -412,6 +412,20 @@ config SIGNATURE
 	  Implementation is done using GnuPG MPI library
 
 #
+# Generic queue spinlock
+#
+config QUEUE_SPINLOCK
+	bool "Generic queue spinlock"
+	depends on ARCH_QUEUE_SPINLOCK
+	default n
+	help
+	  Use an alternative spinlock implementation that has the same
+	  spinlock structure size but is more optimized for larger
+	  NUMA systems with a lot of CPU cores. Specifically, waiting
+	  lock spinners are put to wait in a queue on a local cacheline
+	  rather than all spinning on the same lock cacheline.
+
+#
 # libfdt files, only selected if needed.
 #
 config LIBFDT
diff --git a/lib/Makefile b/lib/Makefile
index 7baccfd..b53710a 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -187,3 +187,4 @@ quiet_cmd_build_OID_registry = GEN     $@
 clean-files	+= oid_registry_data.c
 
 obj-$(CONFIG_UCS2_STRING) += ucs2_string.o
+obj-$(CONFIG_QUEUE_SPINLOCK) += qspinlock.o
diff --git a/lib/qspinlock.c b/lib/qspinlock.c
new file mode 100644
index 0000000..21d5f6e
--- /dev/null
+++ b/lib/qspinlock.c
@@ -0,0 +1,237 @@
+/*
+ * Queue spinlock
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * (C) Copyright 2013 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.long@xxxxxx>
+ */
+#include <linux/smp.h>
+#include <linux/bug.h>
+#include <linux/cpumask.h>
+#include <linux/percpu.h>
+#include <linux/hardirq.h>
+#include <asm-generic/qspinlock.h>
+
+/*
+ * The queue spinlock fastpath is as simple as it can get, all the heavy
+ * lifting is done in the lock slowpath. The main idea behind this queue
+ * spinlock implementation is to keep the spinlock size at 4 bytes while
+ * at the same time implement a queue structure to queue up the waiting
+ * lock spinners.
+ *
+ * Since preemption is disabled before getting the lock, a given CPU will
+ * only need to use one queue node structure in a non-interrupt context.
+ * A percpu queue node structure will be allocated for this purpose and the
+ * cpu number will be put into the queue spinlock structure to indicate the
+ * tail of the queue.
+ *
+ * To handle spinlock acquisition at interrupt context (softirq or hardirq),
+ * the queue node structure is actually an array for supporting nested spin
+ * locking operations in interrupt handlers. If all the entries in the
+ * array are used up, a warning message will be printed (as that shouldn't
+ * happen in normal circumstances) and the lock spinner will fall back to
+ * busy spinning instead of waiting in a queue.
+ */
+
+/*
+#ifndef CONFIG_DEBUG_SPINLOCK
+#define CONFIG_DEBUG_SPINLOCK 1
+#endif
+ */
+
+/*
+ * The queue node structure
+ */
+struct qnode {
+	struct qnode	*next;
+	u8		 wait;		/* Waiting flag	*/
+	u8		 used;		/* Used flag	*/
+#ifdef	CONFIG_DEBUG_SPINLOCK
+	u16		 cpu_nr;	/* CPU number	*/
+	void		*lock;		/* Lock address */
+#endif
+};
+
+/*
+ * The 16-bit wait queue code is divided into the following 2 fields:
+ * Bits 0-1 : queue node index
+ * Bits 2-15: cpu number + 1
+ *
+ * The current implementation will allow a maximum of (1<<14)-1 = 16383 CPUs.
+ */
+#define	GET_QN_IDX(code)	((code) & 0x3)
+#define GET_CPU_NR(code)	(((code) >> 2) - 1)
+#define	SET_CPU_CODE(cpu, idx)	((((cpu) + 1) << 2) | idx)
+#define	MAX_QNODES		4
+#define	MAX_CPUS		((1<<14)-1)
+
+/*
+ * Per-CPU queue node structures
+ */
+static DEFINE_PER_CPU(struct qnode [MAX_QNODES], qnodes) = { { 0 } };
+
+/**
+ * queue_trylock - try to acquire the lock bit ignoring the qcode in lock
+ * @lock: Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static __always_inline int queue_trylock(struct qspinlock *lock)
+{
+	if (!ACCESS_ONCE(lock->locked) && (xchg(&lock->locked, 1) == 0))
+		return 1;
+	return 0;
+}
+
+/**
+ * queue_spin_lock_slowpath - acquire the queue spinlock
+ * @lock: Pointer to queue spinlock structure
+ */
+void queue_spin_lock_slowpath(struct qspinlock *lock)
+{
+	unsigned int cpu_nr, qn_idx;
+	struct qnode *node, *prev, *next;
+	u16 oldcode, newcode;
+
+	/*
+	 * The current code does not work with more than 64K-1 cpus.
+	 */
+	BUILD_BUG_ON(CONFIG_NR_CPUS >= MAX_CPUS);
+
+	/*
+	 * Get the queue node
+	 */
+	cpu_nr = smp_processor_id();
+	node   = per_cpu_ptr(&qnodes[0], cpu_nr);
+	qn_idx = 0;
+
+	if (unlikely(node->used)) {
+		/*
+		 * The node has bee used, try to find an empty queue node entry
+		 */
+		for (qn_idx = 1; qn_idx < MAX_QNODES; qn_idx++)
+			if (!node[qn_idx].used)
+				break;
+		if (unlikely(qn_idx == MAX_QNODES)) {
+			/*
+			 * This shouldn't happen, print a warning message
+			 * & busy spinning on the lock.
+			 */
+			pr_warn("qspinlock: queue node table exhausted at "
+				"cpu %d!\n", cpu_nr);
+			while (!queue_trylock(lock))
+				cpu_relax();
+			return;
+		}
+		/* Adjust node pointer */
+		node += qn_idx;
+	}
+
+	/*
+	 * Set up the new cpu code to be exchanged
+	 */
+	newcode = SET_CPU_CODE(cpu_nr, qn_idx);
+
+	/*
+	 * The lock may be available at this point, try again before waiting
+	 * in a queue.
+	 */
+	if (queue_spin_trylock(lock))
+		return;
+
+	/*
+	 * Set up the queue node
+	 */
+#ifdef	CONFIG_DEBUG_SPINLOCK
+	BUG_ON(node->used);
+#endif
+	node->used = true;
+	barrier();
+#ifdef	CONFIG_DEBUG_SPINLOCK
+	BUG_ON(node->lock || node->next || node->wait);
+	node->cpu_nr = cpu_nr;
+	node->lock   = (void *)lock;
+#endif
+	node->wait = true;
+	node->next = NULL;
+
+	/*
+	 * Exchange current copy of the cpu code
+	 */
+	oldcode = xchg(&lock->qcode, newcode);
+	if (oldcode) {
+		/*
+		 * Not at the queue head, get the address of the previous node
+		 * and setup the "next" pointer of that node.
+		 */
+		prev = per_cpu_ptr(&qnodes[GET_QN_IDX(oldcode)],
+				   GET_CPU_NR(oldcode));
+#ifdef	CONFIG_DEBUG_SPINLOCK
+		BUG_ON(prev->cpu_nr == cpu_nr);
+		BUG_ON(prev->lock != (void *)lock);
+		BUG_ON(!prev->used || (prev->next != NULL));
+#endif
+		ACCESS_ONCE(prev->next) = node;
+		smp_wmb();
+		/*
+		 * Wait until the waiting flag is off
+		 */
+		while (ACCESS_ONCE(node->wait))
+			cpu_relax();
+	}
+
+	/*
+	 * At the head of the wait queue now, try to get the lock in a
+	 * spin loop.
+	 */
+	while (!queue_trylock(lock))
+		cpu_relax();
+
+	next = ACCESS_ONCE(node->next);
+	if (next)
+		goto notify_next;
+	/*
+	 * Clear the wait queue if it is the last node
+	 */
+	if ((ACCESS_ONCE(lock->qcode) == newcode) &&
+	    (cmpxchg(&lock->qcode, newcode, 0) == newcode))
+			goto release_qnode;
+	/*
+	 * Wait until the next one in queue set up the next field
+	 */
+	while (!(next = ACCESS_ONCE(node->next)))
+		cpu_relax();
+notify_next:
+#ifdef	CONFIG_DEBUG_SPINLOCK
+	BUG_ON(next->cpu_nr == cpu_nr);
+	BUG_ON(next->lock != (void *)lock);
+	BUG_ON(!next->wait || !next->used);
+#endif
+	/*
+	 * The next one in queue is now at the head
+	 */
+	ACCESS_ONCE(next->wait) = false;
+
+release_qnode:
+	/*
+	 * Release the queue node
+	 */
+	node->next = NULL;
+	node->used = false;
+	node->wait = false;
+#ifdef	CONFIG_DEBUG_SPINLOCK
+	node->lock = NULL;
+	BUG_ON(cpu_nr != smp_processor_id());
+#endif
+	smp_wmb();
+}
+EXPORT_SYMBOL(queue_spin_lock_slowpath);
-- 
1.7.1

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