This Array-based Lock-Free (ALF) queue, is a very fast bounded
Producer-Consumer queue, supporting bulking. The MPMC
(Multi-Producer/Multi-Consumer) variant uses a locked cmpxchg, but the
cost can be amorized by utilizing bulk enqueue/dequeue.
Results on x86_64 CPU E5-2695, for variants:
MPMC = Multi-Producer-Multi-Consumer
SPSC = Single-Producer-Single-Consumer
(none-bulking): per element cost MPMC and SPSC
MPMC -- SPSC
simple : 9.519 ns -- 1.282 ns
multi(step:128): 12.905 ns -- 2.240 ns
The majority of the cost is associated with the locked cmpxchg in the
MPMC variant. Bulking helps amortize this cost:
(bulking) cost per element comparing MPMC -> SPSC:
MPMC -- SPSC
bulk2 : 5.849 ns -- 1.748 ns
bulk3 : 4.102 ns -- 1.531 ns
bulk4 : 3.281 ns -- 1.383 ns
bulk6 : 2.530 ns -- 1.238 ns
bulk8 : 2.125 ns -- 1.196 ns
bulk16: 1.552 ns -- 1.109 ns
Joint work with Hannes Frederic Sowa.
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Signed-off-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
---
Correctness of memory barries on different arch's need to be evaluated.
The API might need some adjustments/discussions regarding:
1) the semantics of enq/deq when doing bulking, choosing what
to do when e.g. a bulk enqueue cannot fit fully.
2) some better way to detect miss-use of API, e.g. using
single-enqueue variant function call on a multi-enqueue variant data
structure. Now no detection happens.
include/linux/alf_queue.h | 303 +++++++++++++++++++++++++++++++++++++++++++++
lib/Kconfig | 13 ++
lib/Makefile | 2
lib/alf_queue.c | 47 +++++++
4 files changed, 365 insertions(+), 0 deletions(-)
create mode 100644 include/linux/alf_queue.h
create mode 100644 lib/alf_queue.c
diff --git a/include/linux/alf_queue.h b/include/linux/alf_queue.h
new file mode 100644
index 0000000..fb1a774
--- /dev/null
+++ b/include/linux/alf_queue.h
@@ -0,0 +1,303 @@
+#ifndef _LINUX_ALF_QUEUE_H
+#define _LINUX_ALF_QUEUE_H
+/* linux/alf_queue.h
+ *
+ * ALF: Array-based Lock-Free queue
+ *
+ * Queue properties
+ * - Array based for cache-line optimization
+ * - Bounded by the array size
+ * - FIFO Producer/Consumer queue, no queue traversal supported
+ * - Very fast
+ * - Designed as a queue for pointers to objects
+ * - Bulk enqueue and dequeue support
+ * - Supports combinations of Multi and Single Producer/Consumer
+ *
+ * Copyright (C) 2014, Red Hat, Inc.,
+ * by Jesper Dangaard Brouer and Hannes Frederic Sowa
+ * for licensing details see kernel-base/COPYING
+ */
+#include <linux/compiler.h>
+#include <linux/kernel.h>
+
+struct alf_actor {
+ u32 head;
+ u32 tail;
+};
+
+struct alf_queue {
+ u32 size;
+ u32 mask;
+ u32 flags;
+ struct alf_actor producer ____cacheline_aligned_in_smp;
+ struct alf_actor consumer ____cacheline_aligned_in_smp;
+ void *ring[0] ____cacheline_aligned_in_smp;
+};
+
+struct alf_queue *alf_queue_alloc(u32 size, gfp_t gfp);
+void alf_queue_free(struct alf_queue *q);
+
+/* Helpers for LOAD and STORE of elements, have been split-out because:
+ * 1. They can be reused for both "Single" and "Multi" variants
+ * 2. Allow us to experiment with (pipeline) optimizations in this area.
+ */
+static inline void
+__helper_alf_enqueue_store(u32 p_head, struct alf_queue *q,
+ void **ptr, const u32 n)
+{
+ int i, index = p_head;
+
+ for (i = 0; i < n; i++, index++)
+ q->ring[index & q->mask] = ptr[i];
+}
+
+static inline void
+__helper_alf_dequeue_load(u32 c_head, struct alf_queue *q,
+ void **ptr, const u32 elems)
+{
+ int i, index = c_head;
+
+ for (i = 0; i < elems; i++, index++)
+ ptr[i] = q->ring[index & q->mask];
+}
+
+/* Main Multi-Producer ENQUEUE
+ *
+ * Even-though current API have a "fixed" semantics of aborting if it
+ * cannot enqueue the full bulk size. Users of this API should check
+ * on the returned number of enqueue elements match, to verify enqueue
+ * was successful. This allow us to introduce a "variable" enqueue
+ * scheme later.
+ *
+ * Not preemption safe. Multiple CPUs can enqueue elements, but the
+ * same CPU is not allowed to be preempted and access the same
+ * queue. Due to how the tail is updated, this can result in a soft
+ * lock-up. (Same goes for alf_mc_dequeue).
+ */
+static inline int
+alf_mp_enqueue(const u32 n;
+ struct alf_queue *q, void *ptr[n], const u32 n)
+{
+ u32 p_head, p_next, c_tail, space;
+
+ /* Reserve part of the array for enqueue STORE/WRITE */
+ do {
+ p_head = ACCESS_ONCE(q->producer.head);
+ c_tail = ACCESS_ONCE(q->consumer.tail);
+
+ space = q->size + c_tail - p_head;
+ if (n > space)
+ return 0;
+
+ p_next = p_head + n;
+ }
+ while (unlikely(cmpxchg(&q->producer.head, p_head, p_next) != p_head));
+
+ /* STORE the elems into the queue array */
+ __helper_alf_enqueue_store(p_head, q, ptr, n);
+ smp_wmb(); /* Write-Memory-Barrier matching dequeue LOADs */
+
+ /* Wait for other concurrent preceding enqueues not yet done,
+ * this part make us none-wait-free and could be problematic
+ * in case of congestion with many CPUs
+ */
+ while (unlikely(ACCESS_ONCE(q->producer.tail) != p_head))
+ cpu_relax();
+ /* Mark this enq done and avail for consumption */
+ ACCESS_ONCE(q->producer.tail) = p_next;
+
+ return n;
+}
+
+/* Main Multi-Consumer DEQUEUE */
+static inline int
+alf_mc_dequeue(const u32 n;
+ struct alf_queue *q, void *ptr[n], const u32 n)
+{
+ u32 c_head, c_next, p_tail, elems;
+
+ /* Reserve part of the array for dequeue LOAD/READ */
+ do {
+ c_head = ACCESS_ONCE(q->consumer.head);
+ p_tail = ACCESS_ONCE(q->producer.tail);
+
+ elems = p_tail - c_head;
+
+ if (elems == 0)
+ return 0;
+ else
+ elems = min(elems, n);
+
+ c_next = c_head + elems;
+ }
+ while (unlikely(cmpxchg(&q->consumer.head, c_head, c_next) != c_head));
+
+ /* LOAD the elems from the queue array.
+ * We don't need a smb_rmb() Read-Memory-Barrier here because
+ * the above cmpxchg is an implied full Memory-Barrier.
+ */
+ __helper_alf_dequeue_load(c_head, q, ptr, elems);
+
+ /* Archs with weak Memory Ordering need a memory barrier here.
+ * As the STORE to q->consumer.tail, must happen after the
+ * dequeue LOADs. Dequeue LOADs have a dependent STORE into
+ * ptr, thus a smp_wmb() is enough. Paired with enqueue
+ * implicit full-MB in cmpxchg.
+ */
+ smp_wmb();
+
+ /* Wait for other concurrent preceding dequeues not yet done */
+ while (unlikely(ACCESS_ONCE(q->consumer.tail) != c_head))
+ cpu_relax();
+ /* Mark this deq done and avail for producers */
+ ACCESS_ONCE(q->consumer.tail) = c_next;
+
+ return elems;
+}
+
+/* #define ASSERT_DEBUG_SPSC 1 */
+#ifndef ASSERT_DEBUG_SPSC
+#define ASSERT(x) do { } while (0)
+#else
+#define ASSERT(x) \
+ do { \
+ if (unlikely(!(x))) { \
+ pr_crit("Assertion failed %s:%d: \"%s\"\n", \
+ __FILE__, __LINE__, #x); \
+ BUG(); \
+ } \
+ } while (0)
+#endif
+
+/* Main SINGLE Producer ENQUEUE
+ * caller MUST make sure preemption is disabled
+ */
+static inline int
+alf_sp_enqueue(const u32 n;
+ struct alf_queue *q, void *ptr[n], const u32 n)
+{
+ u32 p_head, p_next, c_tail, space;
+
+ /* Reserve part of the array for enqueue STORE/WRITE */
+ p_head = q->producer.head;
+ smp_rmb(); /* for consumer.tail write, making sure deq loads are done */
+ c_tail = ACCESS_ONCE(q->consumer.tail);
+
+ space = q->size + c_tail - p_head;
+ if (n > space)
+ return 0;
+
+ p_next = p_head + n;
+ ASSERT(ACCESS_ONCE(q->producer.head) == p_head);
+ q->producer.head = p_next;
+
+ /* STORE the elems into the queue array */
+ __helper_alf_enqueue_store(p_head, q, ptr, n);
+ smp_wmb(); /* Write-Memory-Barrier matching dequeue LOADs */
+
+ /* Assert no other CPU (or same CPU via preemption) changed queue */
+ ASSERT(ACCESS_ONCE(q->producer.tail) == p_head);
+
+ /* Mark this enq done and avail for consumption */
+ ACCESS_ONCE(q->producer.tail) = p_next;
+
+ return n;
+}
+
+/* Main SINGLE Consumer DEQUEUE
+ * caller MUST make sure preemption is disabled
+ */
+static inline int
+alf_sc_dequeue(const u32 n;
+ struct alf_queue *q, void *ptr[n], const u32 n)
+{
+ u32 c_head, c_next, p_tail, elems;
+
+ /* Reserve part of the array for dequeue LOAD/READ */
+ c_head = q->consumer.head;
+ p_tail = ACCESS_ONCE(q->producer.tail);
+
+ elems = p_tail - c_head;
+
+ if (elems == 0)
+ return 0;
+ else
+ elems = min(elems, n);
+
+ c_next = c_head + elems;
+ ASSERT(ACCESS_ONCE(q->consumer.head) == c_head);
+ q->consumer.head = c_next;
+
+ smp_rmb(); /* Read-Memory-Barrier matching enq STOREs */
+ __helper_alf_dequeue_load(c_head, q, ptr, elems);
+
+ /* Archs with weak Memory Ordering need a memory barrier here.
+ * As the STORE to q->consumer.tail, must happen after the
+ * dequeue LOADs. Dequeue LOADs have a dependent STORE into
+ * ptr, thus a smp_wmb() is enough.
+ */
+ smp_wmb();
+
+ /* Assert no other CPU (or same CPU via preemption) changed queue */
+ ASSERT(ACCESS_ONCE(q->consumer.tail) == c_head);
+
+ /* Mark this deq done and avail for producers */
+ ACCESS_ONCE(q->consumer.tail) = c_next;
+
+ return elems;
+}
+
+static inline bool
+alf_queue_empty(struct alf_queue *q)
+{
+ u32 c_tail = ACCESS_ONCE(q->consumer.tail);
+ u32 p_tail = ACCESS_ONCE(q->producer.tail);
+
+ /* The empty (and initial state) is when consumer have reached
+ * up with producer.
+ *
+ * DOUBLE-CHECK: Should we use producer.head, as this indicate
+ * a producer is in-progress(?)
+ */
+ return c_tail == p_tail;
+}
+
+static inline int
+alf_queue_count(struct alf_queue *q)
+{
+ u32 c_head = ACCESS_ONCE(q->consumer.head);
+ u32 p_tail = ACCESS_ONCE(q->producer.tail);
+ u32 elems;
+
+ /* Due to u32 arithmetic the values are implicitly
+ * masked/modulo 32-bit, thus saving one mask operation
+ */
+ elems = p_tail - c_head;
+ /* Thus, same as:
+ * elems = (p_tail - c_head) & q->mask;
+ */
+ return elems;
+}
+
+static inline int
+alf_queue_avail_space(struct alf_queue *q)
+{
+ u32 p_head = ACCESS_ONCE(q->producer.head);
+ u32 c_tail = ACCESS_ONCE(q->consumer.tail);
+ u32 space;
+
+ /* The max avail space is q->size and
+ * the empty state is when (consumer == producer)
+ */
+
+ /* Due to u32 arithmetic the values are implicitly
+ * masked/modulo 32-bit, thus saving one mask operation
+ */
+ space = q->size + c_tail - p_head;
+ /* Thus, same as:
+ * space = (q->size + c_tail - p_head) & q->mask;
+ */
+ return space;
+}
+
+#endif /* _LINUX_ALF_QUEUE_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index 54cf309..3c0cd58 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -439,6 +439,19 @@ config NLATTR
bool
#
+# ALF queue
+#
+config ALF_QUEUE
+ bool "ALF: Array-based Lock-Free (Producer-Consumer) queue"
+ default y
+ help
+ This Array-based Lock-Free (ALF) queue, is a very fast
+ bounded Producer-Consumer queue, supporting bulking. The
+ MPMC (Multi-Producer/Multi-Consumer) variant uses a locked
+ cmpxchg, but the cost can be amorized by utilizing bulk
+ enqueue/dequeue.
+
+#
# Generic 64-bit atomic support is selected if needed
#
config GENERIC_ATOMIC64
diff --git a/lib/Makefile b/lib/Makefile
index 0211d2b..cd3a2d0 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -119,6 +119,8 @@ obj-$(CONFIG_DYNAMIC_DEBUG) += dynamic_debug.o
obj-$(CONFIG_NLATTR) += nlattr.o
+obj-$(CONFIG_ALF_QUEUE) += alf_queue.o
+
obj-$(CONFIG_LRU_CACHE) += lru_cache.o
obj-$(CONFIG_DMA_API_DEBUG) += dma-debug.o
diff --git a/lib/alf_queue.c b/lib/alf_queue.c
new file mode 100644
index 0000000..d6c9b69
--- /dev/null
+++ b/lib/alf_queue.c
@@ -0,0 +1,47 @@
+/*
+ * lib/alf_queue.c
+ *
+ * ALF: Array-based Lock-Free queue
+ * - Main implementation in: include/linux/alf_queue.h
+ *
+ * Copyright (C) 2014, Red Hat, Inc.,
+ * by Jesper Dangaard Brouer and Hannes Frederic Sowa
+ * for licensing details see kernel-base/COPYING
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/slab.h> /* kzalloc */
+#include <linux/alf_queue.h>
+#include <linux/log2.h>
+
+struct alf_queue *alf_queue_alloc(u32 size, gfp_t gfp)
+{
+ struct alf_queue *q;
+ size_t mem_size;
+
+ if (!(is_power_of_2(size)) || size > 65536)
+ return ERR_PTR(-EINVAL);
+
+ /* The ring array is allocated together with the queue struct */
+ mem_size = size * sizeof(void *) + sizeof(struct alf_queue);
+ q = kzalloc(mem_size, gfp);
+ if (!q)
+ return ERR_PTR(-ENOMEM);
+
+ q->size = size;
+ q->mask = size - 1;
+
+ return q;
+}
+EXPORT_SYMBOL_GPL(alf_queue_alloc);
+
+void alf_queue_free(struct alf_queue *q)
+{
+ kfree(q);
+}
+EXPORT_SYMBOL_GPL(alf_queue_free);
+
+MODULE_DESCRIPTION("ALF: Array-based Lock-Free queue");
+MODULE_AUTHOR("Jesper Dangaard Brouer <netoptimizer@xxxxxxxxxx>");
+MODULE_LICENSE("GPL");
--
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