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Commit 59d13bf5 authored by Jens Axboe's avatar Jens Axboe
Browse files

blk-mq: use sparser tag layout for lower queue depth 



For best performance, spreading tags over multiple cachelines
makes the tagging more efficient on multicore systems. But since
we have 8 * sizeof(unsigned long) tags per cacheline, we don't
always get a nice spread.

Attempt to spread the tags over at least 4 cachelines, using fewer
number of bits per unsigned long if we have to. This improves
tagging performance in setups with 32-128 tags. For higher depths,
the spread is the same as before (BITS_PER_LONG tags per cacheline).

Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent 4bb659b1

block/blk-mq-tag.c

+32 −13
Original line number Diff line number Diff line
@@ -44,7 +44,7 @@ static int __bt_get_word(struct blk_mq_bitmap *bm, unsigned int last_tag) 
{

	int tag, org_last_tag, end;



	org_last_tag = last_tag = TAG_TO_BIT(last_tag);

	org_last_tag = last_tag;

	end = bm->depth;

	do {

restart:
@@ -84,12 +84,12 @@ static int __bt_get(struct blk_mq_bitmap_tags *bt, unsigned int *tag_cache) 
	int index, i, tag;



	last_tag = org_last_tag = *tag_cache;

	index = TAG_TO_INDEX(last_tag);

	index = TAG_TO_INDEX(bt, last_tag);



	for (i = 0; i < bt->map_nr; i++) {

		tag = __bt_get_word(&bt->map[index], last_tag);

		tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag));

		if (tag != -1) {

			tag += index * BITS_PER_LONG;

			tag += (index << bt->bits_per_word);

			goto done;

		}
@@ -233,10 +233,10 @@ static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt) 


static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)

{

	const int index = TAG_TO_INDEX(tag);

	const int index = TAG_TO_INDEX(bt, tag);

	struct bt_wait_state *bs;



	clear_bit(TAG_TO_BIT(tag), &bt->map[index].word);

	clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word);



	bs = bt_wake_ptr(bt);

	if (bs && atomic_dec_and_test(&bs->wait_cnt)) {
@@ -292,7 +292,7 @@ static void bt_for_each_free(struct blk_mq_bitmap_tags *bt, 
			bit++;

		} while (1);



		off += BITS_PER_LONG;

		off += (1 << bt->bits_per_word);

	}

}
@@ -333,14 +333,31 @@ static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth, 
{

	int i;



	bt->bits_per_word = ilog2(BITS_PER_LONG);



	/*

	 * Depth can be zero for reserved tags, that's not a failure

	 * condition.

	 */

	if (depth) {

		int nr, i, map_depth;

		unsigned int nr, i, map_depth, tags_per_word;



		tags_per_word = (1 << bt->bits_per_word);



		/*

		 * If the tag space is small, shrink the number of tags

		 * per word so we spread over a few cachelines, at least.

		 * If less than 4 tags, just forget about it, it's not

		 * going to work optimally anyway.

		 */

		if (depth >= 4) {

			while (tags_per_word * 4 > depth) {

				bt->bits_per_word--;

				tags_per_word = (1 << bt->bits_per_word);

			}

		}



		nr = ALIGN(depth, BITS_PER_LONG) / BITS_PER_LONG;

		nr = ALIGN(depth, tags_per_word) / tags_per_word;

		bt->map = kzalloc_node(nr * sizeof(struct blk_mq_bitmap),

						GFP_KERNEL, node);

		if (!bt->map)
@@ -349,8 +366,8 @@ static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth, 
		bt->map_nr = nr;

		map_depth = depth;

		for (i = 0; i < nr; i++) {

			bt->map[i].depth = min(map_depth, BITS_PER_LONG);

			map_depth -= BITS_PER_LONG;

			bt->map[i].depth = min(map_depth, tags_per_word);

			map_depth -= tags_per_word;

		}

	}
@@ -443,8 +460,10 @@ ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page) 
	if (!tags)

		return 0;



	page += sprintf(page, "nr_tags=%u, reserved_tags=%u\n",

			tags->nr_tags, tags->nr_reserved_tags);

	page += sprintf(page, "nr_tags=%u, reserved_tags=%u, "

			"bits_per_word=%u\n",

			tags->nr_tags, tags->nr_reserved_tags,

			tags->bitmap_tags.bits_per_word);



	free = bt_unused_tags(&tags->bitmap_tags);

	res = bt_unused_tags(&tags->breserved_tags);

block/blk-mq-tag.h

+4 −3
Original line number Diff line number Diff line
@@ -11,8 +11,8 @@ struct bt_wait_state { 
	wait_queue_head_t wait;

} ____cacheline_aligned_in_smp;



#define TAG_TO_INDEX(tag)	((tag) / BITS_PER_LONG)

#define TAG_TO_BIT(tag)		((tag) & (BITS_PER_LONG - 1))

#define TAG_TO_INDEX(bt, tag)	((tag) >> (bt)->bits_per_word)

#define TAG_TO_BIT(bt, tag)	((tag) & ((1 << (bt)->bits_per_word) - 1))



struct blk_mq_bitmap {

	unsigned long word;
@@ -22,9 +22,10 @@ struct blk_mq_bitmap { 
struct blk_mq_bitmap_tags {

	unsigned int depth;

	unsigned int wake_cnt;

	unsigned int bits_per_word;



	struct blk_mq_bitmap *map;

	unsigned int map_nr;

	struct blk_mq_bitmap *map;



	unsigned int wake_index;

	struct bt_wait_state *bs;