blk-mq: abstract tag allocation out into sbitmap library

This is a generally useful data structure, so make it available to
anyone else who might want to use it. It's also a nice cleanup
separating the allocation logic from the rest of the tag handling logic.

The code is behind a new Kconfig option, CONFIG_SBITMAP, which is only
selected by CONFIG_BLOCK for now.

This should be a complete noop functionality-wise.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 327 insertions, 0 deletions

diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
new file mode 100644
index 000000000000..1a3b836042e1
--- /dev/null
+++ b/include/linux/sbitmap.h
@@ -0,0 +1,327 @@
+/*
+ * Fast and scalable bitmaps.
+ *
+ * Copyright (C) 2016 Facebook
+ * Copyright (C) 2013-2014 Jens Axboe
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * 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.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef __LINUX_SCALE_BITMAP_H
+#define __LINUX_SCALE_BITMAP_H
+
+#include <linux/kernel.h>
+#include <linux/slab.h>
+
+/**
+ * struct sbitmap_word - Word in a &struct sbitmap.
+ */
+struct sbitmap_word {
+	/**
+	 * @word: The bitmap word itself.
+	 */
+	unsigned long word;
+
+	/**
+	 * @depth: Number of bits being used in @word.
+	 */
+	unsigned long depth;
+} ____cacheline_aligned_in_smp;
+
+/**
+ * struct sbitmap - Scalable bitmap.
+ *
+ * A &struct sbitmap is spread over multiple cachelines to avoid ping-pong. This
+ * trades off higher memory usage for better scalability.
+ */
+struct sbitmap {
+	/**
+	 * @depth: Number of bits used in the whole bitmap.
+	 */
+	unsigned int depth;
+
+	/**
+	 * @shift: log2(number of bits used per word)
+	 */
+	unsigned int shift;
+
+	/**
+	 * @map_nr: Number of words (cachelines) being used for the bitmap.
+	 */
+	unsigned int map_nr;
+
+	/**
+	 * @map: Allocated bitmap.
+	 */
+	struct sbitmap_word *map;
+};
+
+#define SBQ_WAIT_QUEUES 8
+#define SBQ_WAKE_BATCH 8
+
+/**
+ * struct sbq_wait_state - Wait queue in a &struct sbitmap_queue.
+ */
+struct sbq_wait_state {
+	/**
+	 * @wait_cnt: Number of frees remaining before we wake up.
+	 */
+	atomic_t wait_cnt;
+
+	/**
+	 * @wait: Wait queue.
+	 */
+	wait_queue_head_t wait;
+} ____cacheline_aligned_in_smp;
+
+/**
+ * struct sbitmap_queue - Scalable bitmap with the added ability to wait on free
+ * bits.
+ *
+ * A &struct sbitmap_queue uses multiple wait queues and rolling wakeups to
+ * avoid contention on the wait queue spinlock. This ensures that we don't hit a
+ * scalability wall when we run out of free bits and have to start putting tasks
+ * to sleep.
+ */
+struct sbitmap_queue {
+	/**
+	 * @sb: Scalable bitmap.
+	 */
+	struct sbitmap sb;
+
+	/**
+	 * @wake_batch: Number of bits which must be freed before we wake up any
+	 * waiters.
+	 */
+	unsigned int wake_batch;
+
+	/**
+	 * @wake_index: Next wait queue in @ws to wake up.
+	 */
+	atomic_t wake_index;
+
+	/**
+	 * @ws: Wait queues.
+	 */
+	struct sbq_wait_state *ws;
+};
+
+/**
+ * sbitmap_init_node() - Initialize a &struct sbitmap on a specific memory node.
+ * @sb: Bitmap to initialize.
+ * @depth: Number of bits to allocate.
+ * @shift: Use 2^@shift bits per word in the bitmap; if a negative number if
+ *         given, a good default is chosen.
+ * @flags: Allocation flags.
+ * @node: Memory node to allocate on.
+ *
+ * Return: Zero on success or negative errno on failure.
+ */
+int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
+		      gfp_t flags, int node);
+
+/**
+ * sbitmap_free() - Free memory used by a &struct sbitmap.
+ * @sb: Bitmap to free.
+ */
+static inline void sbitmap_free(struct sbitmap *sb)
+{
+	kfree(sb->map);
+	sb->map = NULL;
+}
+
+/**
+ * sbitmap_resize() - Resize a &struct sbitmap.
+ * @sb: Bitmap to resize.
+ * @depth: New number of bits to resize to.
+ *
+ * Doesn't reallocate anything. It's up to the caller to ensure that the new
+ * depth doesn't exceed the depth that the sb was initialized with.
+ */
+void sbitmap_resize(struct sbitmap *sb, unsigned int depth);
+
+/**
+ * sbitmap_get() - Try to allocate a free bit from a &struct sbitmap.
+ * @sb: Bitmap to allocate from.
+ * @alloc_hint: Hint for where to start searching for a free bit.
+ * @round_robin: If true, be stricter about allocation order; always allocate
+ *               starting from the last allocated bit. This is less efficient
+ *               than the default behavior (false).
+ *
+ * Return: Non-negative allocated bit number if successful, -1 otherwise.
+ */
+int sbitmap_get(struct sbitmap *sb, unsigned int alloc_hint, bool round_robin);
+
+/**
+ * sbitmap_any_bit_set() - Check for a set bit in a &struct sbitmap.
+ * @sb: Bitmap to check.
+ *
+ * Return: true if any bit in the bitmap is set, false otherwise.
+ */
+bool sbitmap_any_bit_set(const struct sbitmap *sb);
+
+/**
+ * sbitmap_any_bit_clear() - Check for an unset bit in a &struct
+ * sbitmap.
+ * @sb: Bitmap to check.
+ *
+ * Return: true if any bit in the bitmap is clear, false otherwise.
+ */
+bool sbitmap_any_bit_clear(const struct sbitmap *sb);
+
+typedef bool (*sb_for_each_fn)(struct sbitmap *, unsigned int, void *);
+
+/**
+ * sbitmap_for_each_set() - Iterate over each set bit in a &struct sbitmap.
+ * @sb: Bitmap to iterate over.
+ * @fn: Callback. Should return true to continue or false to break early.
+ * @data: Pointer to pass to callback.
+ *
+ * This is inline even though it's non-trivial so that the function calls to the
+ * callback will hopefully get optimized away.
+ */
+static inline void sbitmap_for_each_set(struct sbitmap *sb, sb_for_each_fn fn,
+					void *data)
+{
+	unsigned int i;
+
+	for (i = 0; i < sb->map_nr; i++) {
+		struct sbitmap_word *word = &sb->map[i];
+		unsigned int off, nr;
+
+		if (!word->word)
+			continue;
+
+		nr = 0;
+		off = i << sb->shift;
+		while (1) {
+			nr = find_next_bit(&word->word, word->depth, nr);
+			if (nr >= word->depth)
+				break;
+
+			if (!fn(sb, off + nr, data))
+				return;
+
+			nr++;
+		}
+	}
+}
+
+#define SB_NR_TO_INDEX(sb, bitnr) ((bitnr) >> (sb)->shift)
+#define SB_NR_TO_BIT(sb, bitnr) ((bitnr) & ((1U << (sb)->shift) - 1U))
+
+static inline unsigned long *__sbitmap_word(struct sbitmap *sb,
+					    unsigned int bitnr)
+{
+	return &sb->map[SB_NR_TO_INDEX(sb, bitnr)].word;
+}
+
+/* Helpers equivalent to the operations in asm/bitops.h and linux/bitmap.h */
+
+static inline void sbitmap_set_bit(struct sbitmap *sb, unsigned int bitnr)
+{
+	set_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr));
+}
+
+static inline void sbitmap_clear_bit(struct sbitmap *sb, unsigned int bitnr)
+{
+	clear_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr));
+}
+
+static inline int sbitmap_test_bit(struct sbitmap *sb, unsigned int bitnr)
+{
+	return test_bit(SB_NR_TO_BIT(sb, bitnr), __sbitmap_word(sb, bitnr));
+}
+
+unsigned int sbitmap_weight(const struct sbitmap *sb);
+
+/**
+ * sbitmap_queue_init_node() - Initialize a &struct sbitmap_queue on a specific
+ * memory node.
+ * @sbq: Bitmap queue to initialize.
+ * @depth: See sbitmap_init_node().
+ * @shift: See sbitmap_init_node().
+ * @flags: Allocation flags.
+ * @node: Memory node to allocate on.
+ *
+ * Return: Zero on success or negative errno on failure.
+ */
+int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
+			    int shift, gfp_t flags, int node);
+
+/**
+ * sbitmap_queue_free() - Free memory used by a &struct sbitmap_queue.
+ *
+ * @sbq: Bitmap queue to free.
+ */
+static inline void sbitmap_queue_free(struct sbitmap_queue *sbq)
+{
+	kfree(sbq->ws);
+	sbitmap_free(&sbq->sb);
+}
+
+/**
+ * sbitmap_queue_resize() - Resize a &struct sbitmap_queue.
+ * @sbq: Bitmap queue to resize.
+ * @depth: New number of bits to resize to.
+ *
+ * Like sbitmap_resize(), this doesn't reallocate anything. It has to do
+ * some extra work on the &struct sbitmap_queue, so it's not safe to just
+ * resize the underlying &struct sbitmap.
+ */
+void sbitmap_queue_resize(struct sbitmap_queue *sbq, unsigned int depth);
+
+/**
+ * sbitmap_queue_clear() - Free an allocated bit and wake up waiters on a
+ * &struct sbitmap_queue.
+ * @sbq: Bitmap to free from.
+ * @nr: Bit number to free.
+ */
+void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr);
+
+static inline int sbq_index_inc(int index)
+{
+	return (index + 1) & (SBQ_WAIT_QUEUES - 1);
+}
+
+static inline void sbq_index_atomic_inc(atomic_t *index)
+{
+	int old = atomic_read(index);
+	int new = sbq_index_inc(old);
+	atomic_cmpxchg(index, old, new);
+}
+
+/**
+ * sbq_wait_ptr() - Get the next wait queue to use for a &struct
+ * sbitmap_queue.
+ * @sbq: Bitmap queue to wait on.
+ * @wait_index: A counter per "user" of @sbq.
+ */
+static inline struct sbq_wait_state *sbq_wait_ptr(struct sbitmap_queue *sbq,
+						  atomic_t *wait_index)
+{
+	struct sbq_wait_state *ws;
+
+	ws = &sbq->ws[atomic_read(wait_index)];
+	sbq_index_atomic_inc(wait_index);
+	return ws;
+}
+
+/**
+ * sbitmap_queue_wake_all() - Wake up everything waiting on a &struct
+ * sbitmap_queue.
+ * @sbq: Bitmap queue to wake up.
+ */
+void sbitmap_queue_wake_all(struct sbitmap_queue *sbq);
+
+#endif /* __LINUX_SCALE_BITMAP_H */