[PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

[Alex Kogan]

In CNA, spinning threads are organized in two queues, a main queue for
threads running on the same node as the current lock holder, and a
secondary queue for threads running on other nodes. After acquiring the
MCS lock and before acquiring the spinlock, the lock holder scans the
main queue looking for a thread running on the same node (pre-scan). If
found (call it thread T), all threads in the main queue between the
current lock holder and T are moved to the end of the secondary queue.
If such T is not found, we make another scan of the main queue when
unlocking the MCS lock (post-scan), starting at the position where
pre-scan stopped. If both scans fail to find such T, the MCS lock is
passed to the first thread in the secondary queue. If the secondary queue
is empty, the lock is passed to the next thread in the main queue.
For more details, see https://arxiv.org/abs/1810.05600.

Note that this variant of CNA may introduce starvation by continuously
passing the lock to threads running on the same node. This issue
will be addressed later in the series.

Enabling CNA is controlled via a new configuration option
(NUMA_AWARE_SPINLOCKS). By default, the CNA variant is patched in at the
boot time only if we run on a multi-node machine in native environment and
the new config is enabled. (For the time being, the patching requires
CONFIG_PARAVIRT_SPINLOCKS to be enabled as well. However, this should be
resolved once static_call() is available.) This default behavior can be
overridden with the new kernel boot command-line option
"numa_spinlock=on/off" (default is "auto").

Signed-off-by: Alex Kogan <alex.kogan@xxxxxxxxxx>
Reviewed-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
---
 arch/x86/Kconfig                 |  19 +++
 arch/x86/include/asm/qspinlock.h |   4 +
 arch/x86/kernel/alternative.c    |  41 +++++++
 kernel/locking/mcs_spinlock.h    |   2 +-
 kernel/locking/qspinlock.c       |  34 +++++-
 kernel/locking/qspinlock_cna.h   | 258 +++++++++++++++++++++++++++++++++++++++
 6 files changed, 353 insertions(+), 5 deletions(-)
 create mode 100644 kernel/locking/qspinlock_cna.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index d6e1faa28c58..1d480f190def 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1573,6 +1573,25 @@ config NUMA
 
 	  Otherwise, you should say N.
 
+config NUMA_AWARE_SPINLOCKS
+	bool "Numa-aware spinlocks"
+	depends on NUMA
+	depends on QUEUED_SPINLOCKS
+	# For now, we depend on PARAVIRT_SPINLOCKS to make the patching work.
+	# This is awkward, but hopefully would be resolved once static_call()
+	# is available.
+	depends on PARAVIRT_SPINLOCKS
+	default y
+	help
+	  Introduce NUMA (Non Uniform Memory Access) awareness into
+	  the slow path of spinlocks.
+
+	  In this variant of qspinlock, the kernel will try to keep the lock
+	  on the same node, thus reducing the number of remote cache misses,
+	  while trading some of the short term fairness for better performance.
+
+	  Say N if you want absolute first come first serve fairness.
+
 config AMD_NUMA
 	def_bool y
 	prompt "Old style AMD Opteron NUMA detection"
diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 444d6fd9a6d8..6fa8fcc5c7af 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -27,6 +27,10 @@ static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lo
 	return val;
 }
 
+#ifdef CONFIG_NUMA_AWARE_SPINLOCKS
+extern void __cna_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+#endif
+
 #ifdef CONFIG_PARAVIRT_SPINLOCKS
 extern void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
 extern void __pv_init_lock_hash(void);
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 9d3a971ea364..e0e66bd8b251 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -698,6 +698,33 @@ static void __init int3_selftest(void)
 	unregister_die_notifier(&int3_exception_nb);
 }
 
+#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
+/*
+ * Constant (boot-param configurable) flag selecting the NUMA-aware variant
+ * of spinlock.  Possible values: -1 (off) / 0 (auto, default) / 1 (on).
+ */
+static int numa_spinlock_flag;
+
+static int __init numa_spinlock_setup(char *str)
+{
+	if (!strcmp(str, "auto")) {
+		numa_spinlock_flag = 0;
+		return 1;
+	} else if (!strcmp(str, "on")) {
+		numa_spinlock_flag = 1;
+		return 1;
+	} else if (!strcmp(str, "off")) {
+		numa_spinlock_flag = -1;
+		return 1;
+	}
+
+	return 0;
+}
+
+__setup("numa_spinlock=", numa_spinlock_setup);
+
+#endif
+
 void __init alternative_instructions(void)
 {
 	int3_selftest();
@@ -738,6 +765,20 @@ void __init alternative_instructions(void)
 	}
 #endif
 
+#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
+	/*
+	 * By default, switch to the NUMA-friendly slow path for
+	 * spinlocks when we have multiple NUMA nodes in native environment.
+	 */
+	if ((numa_spinlock_flag == 1) ||
+	    (numa_spinlock_flag == 0 && nr_node_ids > 1 &&
+		    pv_ops.lock.queued_spin_lock_slowpath ==
+			native_queued_spin_lock_slowpath)) {
+		pv_ops.lock.queued_spin_lock_slowpath =
+		    __cna_queued_spin_lock_slowpath;
+	}
+#endif
+
 	apply_paravirt(__parainstructions, __parainstructions_end);
 
 	restart_nmi();
diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index 52d06ec6f525..e40b9538b79f 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -17,7 +17,7 @@
 
 struct mcs_spinlock {
 	struct mcs_spinlock *next;
-	int locked; /* 1 if lock acquired */
+	unsigned int locked; /* 1 if lock acquired */
 	int count;  /* nesting count, see qspinlock.c */
 };
 
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index c06d1e8075d9..6d8c4a52e44e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -11,7 +11,7 @@
  *          Peter Zijlstra <peterz@xxxxxxxxxxxxx>
  */
 
-#ifndef _GEN_PV_LOCK_SLOWPATH
+#if !defined(_GEN_PV_LOCK_SLOWPATH) && !defined(_GEN_CNA_LOCK_SLOWPATH)
 
 #include <linux/smp.h>
 #include <linux/bug.h>
@@ -70,7 +70,8 @@
 /*
  * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
  * size and four of them will fit nicely in one 64-byte cacheline. For
- * pvqspinlock, however, we need more space for extra data. To accommodate
+ * pvqspinlock, however, we need more space for extra data. The same also
+ * applies for the NUMA-aware variant of spinlocks (CNA). To accommodate
  * that, we insert two more long words to pad it up to 32 bytes. IOW, only
  * two of them can fit in a cacheline in this case. That is OK as it is rare
  * to have more than 2 levels of slowpath nesting in actual use. We don't
@@ -79,7 +80,7 @@
  */
 struct qnode {
 	struct mcs_spinlock mcs;
-#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS)
 	long reserved[2];
 #endif
 };
@@ -103,6 +104,8 @@ struct qnode {
  * Exactly fits one 64-byte cacheline on a 64-bit architecture.
  *
  * PV doubles the storage and uses the second cacheline for PV state.
+ * CNA also doubles the storage and uses the second cacheline for
+ * CNA-specific state.
  */
 static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
 
@@ -316,7 +319,7 @@ static __always_inline void __mcs_pass_lock(struct mcs_spinlock *node,
 #define try_clear_tail	__try_clear_tail
 #define mcs_pass_lock		__mcs_pass_lock
 
-#endif /* _GEN_PV_LOCK_SLOWPATH */
+#endif /* _GEN_PV_LOCK_SLOWPATH && _GEN_CNA_LOCK_SLOWPATH */
 
 /**
  * queued_spin_lock_slowpath - acquire the queued spinlock
@@ -589,6 +592,29 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 EXPORT_SYMBOL(queued_spin_lock_slowpath);
 
 /*
+ * Generate the code for NUMA-aware spinlocks
+ */
+#if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
+#define _GEN_CNA_LOCK_SLOWPATH
+
+#undef pv_wait_head_or_lock
+#define pv_wait_head_or_lock		cna_pre_scan
+
+#undef try_clear_tail
+#define try_clear_tail			cna_try_change_tail
+
+#undef mcs_pass_lock
+#define mcs_pass_lock			cna_pass_lock
+
+#undef  queued_spin_lock_slowpath
+#define queued_spin_lock_slowpath	__cna_queued_spin_lock_slowpath
+
+#include "qspinlock_cna.h"
+#include "qspinlock.c"
+
+#endif
+
+/*
  * Generate the paravirt code for queued_spin_unlock_slowpath().
  */
 #if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS)
diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
new file mode 100644
index 000000000000..4d095f742d31
--- /dev/null
+++ b/kernel/locking/qspinlock_cna.h
@@ -0,0 +1,258 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _GEN_CNA_LOCK_SLOWPATH
+#error "do not include this file"
+#endif
+
+#include <linux/topology.h>
+
+/*
+ * Implement a NUMA-aware version of MCS (aka CNA, or compact NUMA-aware lock).
+ *
+ * In CNA, spinning threads are organized in two queues, a main queue for
+ * threads running on the same NUMA node as the current lock holder, and a
+ * secondary queue for threads running on other nodes. Schematically, it
+ * looks like this:
+ *
+ *    cna_node
+ *   +----------+    +--------+        +--------+
+ *   |mcs:next  | -> |mcs:next| -> ... |mcs:next| -> NULL      [Main queue]
+ *   |mcs:locked| -+ +--------+        +--------+
+ *   +----------+  |
+ *                 +----------------------+
+ *                                        \/
+ *                 +--------+         +--------+
+ *                 |mcs:next| -> ...  |mcs:next|          [Secondary queue]
+ *                 +--------+         +--------+
+ *                     ^                    |
+ *                     +--------------------+
+ *
+ * N.B. locked = 1 if secondary queue is absent. Othewrise, it contains the
+ * encoded pointer to the tail of the secondary queue, which is organized as a
+ * circular list.
+ *
+ * After acquiring the MCS lock and before acquiring the spinlock, the lock
+ * holder scans the main queue looking for a thread running on the same node
+ * (pre-scan). If found (call it thread T), all threads in the main queue
+ * between the current lock holder and T are moved to the end of the secondary
+ * queue.  If such T is not found, we make another scan of the main queue when
+ * unlocking the MCS lock (post-scan), starting at the node where pre-scan
+ * stopped. If both scans fail to find such T, the MCS lock is passed to the
+ * first thread in the secondary queue. If the secondary queue is empty, the
+ * lock is passed to the next thread in the main queue.
+ *
+ * For more details, see https://arxiv.org/abs/1810.05600.
+ *
+ * Authors: Alex Kogan <alex.kogan@xxxxxxxxxx>
+ *          Dave Dice <dave.dice@xxxxxxxxxx>
+ */
+
+struct cna_node {
+	struct mcs_spinlock	mcs;
+	int			numa_node;
+	u32			encoded_tail;
+	u32			pre_scan_result; /* 0 or an encoded tail */
+};
+
+static void __init cna_init_nodes_per_cpu(unsigned int cpu)
+{
+	struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
+	int numa_node = cpu_to_node(cpu);
+	int i;
+
+	for (i = 0; i < MAX_NODES; i++) {
+		struct cna_node *cn = (struct cna_node *)grab_mcs_node(base, i);
+
+		cn->numa_node = numa_node;
+		cn->encoded_tail = encode_tail(cpu, i);
+		/*
+		 * @encoded_tail has to be larger than 1, so we do not confuse
+		 * it with other valid values for @locked or @pre_scan_result
+		 * (0 or 1)
+		 */
+		WARN_ON(cn->encoded_tail <= 1);
+	}
+}
+
+static void __init cna_init_nodes(void)
+{
+	unsigned int cpu;
+
+	BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
+	/* we store an ecoded tail word in the node's @locked field */
+	BUILD_BUG_ON(sizeof(u32) > sizeof(unsigned int));
+
+	for_each_possible_cpu(cpu)
+		cna_init_nodes_per_cpu(cpu);
+}
+early_initcall(cna_init_nodes);
+
+static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val,
+				       struct mcs_spinlock *node)
+{
+	struct mcs_spinlock *head_2nd, *tail_2nd;
+	u32 new;
+
+	/* If the secondary queue is empty, do what MCS does. */
+	if (node->locked <= 1)
+		return __try_clear_tail(lock, val, node);
+
+	/*
+	 * Try to update the tail value to the last node in the secondary queue.
+	 * If successful, pass the lock to the first thread in the secondary
+	 * queue. Doing those two actions effectively moves all nodes from the
+	 * secondary queue into the main one.
+	 */
+	tail_2nd = decode_tail(node->locked);
+	head_2nd = tail_2nd->next;
+	new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL;
+
+	if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) {
+		/*
+		 * Try to reset @next in tail_2nd to NULL, but no need to check
+		 * the result - if failed, a new successor has updated it.
+		 */
+		cmpxchg64_relaxed(&tail_2nd->next, head_2nd, NULL);
+		arch_mcs_pass_lock(&head_2nd->locked, 1);
+		return true;
+	}
+
+	return false;
+}
+
+/*
+ * cna_splice_tail -- splice nodes in the main queue between [first, last]
+ * onto the secondary queue.
+ */
+static void cna_splice_tail(struct mcs_spinlock *node,
+			    struct mcs_spinlock *first,
+			    struct mcs_spinlock *last)
+{
+	/* remove [first,last] */
+	node->next = last->next;
+
+	/* stick [first,last] on the secondary queue tail */
+	if (node->locked <= 1) { /* if secondary queue is empty */
+		/* create secondary queue */
+		last->next = first;
+	} else {
+		/* add to the tail of the secondary queue */
+		struct mcs_spinlock *tail_2nd = decode_tail(node->locked);
+		struct mcs_spinlock *head_2nd = tail_2nd->next;
+
+		tail_2nd->next = first;
+		last->next = head_2nd;
+	}
+
+	node->locked = ((struct cna_node *)last)->encoded_tail;
+}
+
+/*
+ * cna_scan_main_queue - scan the main waiting queue looking for the first
+ * thread running on the same NUMA node as the lock holder. If found (call it
+ * thread T), move all threads in the main queue between the lock holder and
+ * T to the end of the secondary queue and return 0; otherwise, return the
+ * encoded pointer of the last scanned node in the primary queue (so a
+ * subsequent scan can be resumed from that node)
+ *
+ * Schematically, this may look like the following (nn stands for numa_node and
+ * et stands for encoded_tail).
+ *
+ *   when cna_scan_main_queue() is called (the secondary queue is empty):
+ *
+ *  A+------------+   B+--------+   C+--------+   T+--------+
+ *   |mcs:next    | -> |mcs:next| -> |mcs:next| -> |mcs:next| -> NULL
+ *   |mcs:locked=1|    |cna:nn=0|    |cna:nn=2|    |cna:nn=1|
+ *   |cna:nn=1    |    +--------+    +--------+    +--------+
+ *   +----------- +
+ *
+ *   when cna_scan_main_queue() returns (the secondary queue contains B and C):
+ *
+ *  A+----------------+    T+--------+
+ *   |mcs:next        | ->  |mcs:next| -> NULL
+ *   |mcs:locked=C.et | -+  |cna:nn=1|
+ *   |cna:nn=1        |  |  +--------+
+ *   +--------------- +  +-----+
+ *                             \/
+ *          B+--------+   C+--------+
+ *           |mcs:next| -> |mcs:next| -+
+ *           |cna:nn=0|    |cna:nn=2|  |
+ *           +--------+    +--------+  |
+ *               ^                     |
+ *               +---------------------+
+ *
+ * The worst case complexity of the scan is O(n), where n is the number
+ * of current waiters. However, the amortized complexity is close to O(1),
+ * as the immediate successor is likely to be running on the same node once
+ * threads from other nodes are moved to the secondary queue.
+ */
+static u32 cna_scan_main_queue(struct mcs_spinlock *node,
+			       struct mcs_spinlock *pred_start)
+{
+	struct cna_node *cn = (struct cna_node *)node;
+	struct cna_node *cni = (struct cna_node *)READ_ONCE(pred_start->next);
+	struct cna_node *last;
+	int my_numa_node = cn->numa_node;
+
+	/* find any next waiter on 'our' NUMA node */
+	for (last = cn;
+	     cni && cni->numa_node != my_numa_node;
+	     last = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next))
+		;
+
+	/* if found, splice any skipped waiters onto the secondary queue */
+	if (cni) {
+		if (last != cn)	/* did we skip any waiters? */
+			cna_splice_tail(node, node->next,
+					(struct mcs_spinlock *)last);
+		return 0;
+	}
+
+	return last->encoded_tail;
+}
+
+__always_inline u32 cna_pre_scan(struct qspinlock *lock,
+				  struct mcs_spinlock *node)
+{
+	struct cna_node *cn = (struct cna_node *)node;
+
+	cn->pre_scan_result = cna_scan_main_queue(node, node);
+
+	return 0;
+}
+
+static inline void cna_pass_lock(struct mcs_spinlock *node,
+				 struct mcs_spinlock *next)
+{
+	struct cna_node *cn = (struct cna_node *)node;
+	struct mcs_spinlock *next_holder = next, *tail_2nd;
+	u32 val = 1;
+
+	u32 scan = cn->pre_scan_result;
+
+	/*
+	 * check if a successor from the same numa node has not been found in
+	 * pre-scan, and if so, try to find it in post-scan starting from the
+	 * node where pre-scan stopped (stored in @pre_scan_result)
+	 */
+	if (scan > 0)
+		scan = cna_scan_main_queue(node, decode_tail(scan));
+
+	if (!scan) { /* if found a successor from the same numa node */
+		next_holder = node->next;
+		/*
+		 * make sure @val gets 1 if current holder's @locked is 0 as
+		 * we have to store a non-zero value in successor's @locked
+		 * to pass the lock
+		 */
+		val = node->locked + (node->locked == 0);
+	} else if (node->locked > 1) {	  /* if secondary queue is not empty */
+		/* next holder will be the first node in the secondary queue */
+		tail_2nd = decode_tail(node->locked);
+		/* @tail_2nd->next points to the head of the secondary queue */
+		next_holder = tail_2nd->next;
+		/* splice the secondary queue onto the head of the main queue */
+		tail_2nd->next = next;
+	}
+
+	arch_mcs_pass_lock(&next_holder->locked, val);
+}
-- 
2.11.0 (Apple Git-81)