linux/block/blk-mq-tag.c
Jens Axboe 0d2602ca30 blk-mq: improve support for shared tags maps
This adds support for active queue tracking, meaning that the
blk-mq tagging maintains a count of active users of a tag set.
This allows us to maintain a notion of fairness between users,
so that we can distribute the tag depth evenly without starving
some users while allowing others to try unfair deep queues.

If sharing of a tag set is detected, each hardware queue will
track the depth of its own queue. And if this exceeds the total
depth divided by the number of active queues, the user is actively
throttled down.

The active queue count is done lazily to avoid bouncing that data
between submitter and completer. Each hardware queue gets marked
active when it allocates its first tag, and gets marked inactive
when 1) the last tag is cleared, and 2) the queue timeout grace
period has passed.

Signed-off-by: Jens Axboe <axboe@fb.com>
2014-05-13 15:10:52 -06:00

549 lines
12 KiB
C

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/random.h>
#include <linux/blk-mq.h>
#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-tag.h"
void blk_mq_wait_for_tags(struct blk_mq_hw_ctx *hctx, bool reserved)
{
int tag, zero = 0;
tag = blk_mq_get_tag(hctx, &zero, __GFP_WAIT, reserved);
blk_mq_put_tag(hctx, tag, &zero);
}
static bool bt_has_free_tags(struct blk_mq_bitmap_tags *bt)
{
int i;
for (i = 0; i < bt->map_nr; i++) {
struct blk_mq_bitmap *bm = &bt->map[i];
int ret;
ret = find_first_zero_bit(&bm->word, bm->depth);
if (ret < bm->depth)
return true;
}
return false;
}
bool blk_mq_has_free_tags(struct blk_mq_tags *tags)
{
if (!tags)
return true;
return bt_has_free_tags(&tags->bitmap_tags);
}
static inline void bt_index_inc(unsigned int *index)
{
*index = (*index + 1) & (BT_WAIT_QUEUES - 1);
}
/*
* If a previously inactive queue goes active, bump the active user count.
*/
bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
{
if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
!test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
atomic_inc(&hctx->tags->active_queues);
return true;
}
/*
* If a previously busy queue goes inactive, potential waiters could now
* be allowed to queue. Wake them up and check.
*/
void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
{
struct blk_mq_tags *tags = hctx->tags;
struct blk_mq_bitmap_tags *bt;
int i, wake_index;
if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
return;
atomic_dec(&tags->active_queues);
/*
* Will only throttle depth on non-reserved tags
*/
bt = &tags->bitmap_tags;
wake_index = bt->wake_index;
for (i = 0; i < BT_WAIT_QUEUES; i++) {
struct bt_wait_state *bs = &bt->bs[wake_index];
if (waitqueue_active(&bs->wait))
wake_up(&bs->wait);
bt_index_inc(&wake_index);
}
}
/*
* For shared tag users, we track the number of currently active users
* and attempt to provide a fair share of the tag depth for each of them.
*/
static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
struct blk_mq_bitmap_tags *bt)
{
unsigned int depth, users;
if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_SHARED))
return true;
if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
return true;
/*
* Don't try dividing an ant
*/
if (bt->depth == 1)
return true;
users = atomic_read(&hctx->tags->active_queues);
if (!users)
return true;
/*
* Allow at least some tags
*/
depth = max((bt->depth + users - 1) / users, 4U);
return atomic_read(&hctx->nr_active) < depth;
}
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;
end = bm->depth;
do {
restart:
tag = find_next_zero_bit(&bm->word, end, last_tag);
if (unlikely(tag >= end)) {
/*
* We started with an offset, start from 0 to
* exhaust the map.
*/
if (org_last_tag && last_tag) {
end = last_tag;
last_tag = 0;
goto restart;
}
return -1;
}
last_tag = tag + 1;
} while (test_and_set_bit_lock(tag, &bm->word));
return tag;
}
/*
* Straight forward bitmap tag implementation, where each bit is a tag
* (cleared == free, and set == busy). The small twist is using per-cpu
* last_tag caches, which blk-mq stores in the blk_mq_ctx software queue
* contexts. This enables us to drastically limit the space searched,
* without dirtying an extra shared cacheline like we would if we stored
* the cache value inside the shared blk_mq_bitmap_tags structure. On top
* of that, each word of tags is in a separate cacheline. This means that
* multiple users will tend to stick to different cachelines, at least
* until the map is exhausted.
*/
static int __bt_get(struct blk_mq_hw_ctx *hctx, struct blk_mq_bitmap_tags *bt,
unsigned int *tag_cache)
{
unsigned int last_tag, org_last_tag;
int index, i, tag;
if (!hctx_may_queue(hctx, bt))
return -1;
last_tag = org_last_tag = *tag_cache;
index = TAG_TO_INDEX(bt, last_tag);
for (i = 0; i < bt->map_nr; i++) {
tag = __bt_get_word(&bt->map[index], TAG_TO_BIT(bt, last_tag));
if (tag != -1) {
tag += (index << bt->bits_per_word);
goto done;
}
last_tag = 0;
if (++index >= bt->map_nr)
index = 0;
}
*tag_cache = 0;
return -1;
/*
* Only update the cache from the allocation path, if we ended
* up using the specific cached tag.
*/
done:
if (tag == org_last_tag) {
last_tag = tag + 1;
if (last_tag >= bt->depth - 1)
last_tag = 0;
*tag_cache = last_tag;
}
return tag;
}
static struct bt_wait_state *bt_wait_ptr(struct blk_mq_bitmap_tags *bt,
struct blk_mq_hw_ctx *hctx)
{
struct bt_wait_state *bs;
if (!hctx)
return &bt->bs[0];
bs = &bt->bs[hctx->wait_index];
bt_index_inc(&hctx->wait_index);
return bs;
}
static int bt_get(struct blk_mq_bitmap_tags *bt, struct blk_mq_hw_ctx *hctx,
unsigned int *last_tag, gfp_t gfp)
{
struct bt_wait_state *bs;
DEFINE_WAIT(wait);
int tag;
tag = __bt_get(hctx, bt, last_tag);
if (tag != -1)
return tag;
if (!(gfp & __GFP_WAIT))
return -1;
bs = bt_wait_ptr(bt, hctx);
do {
bool was_empty;
was_empty = list_empty(&wait.task_list);
prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE);
tag = __bt_get(hctx, bt, last_tag);
if (tag != -1)
break;
if (was_empty)
atomic_set(&bs->wait_cnt, bt->wake_cnt);
io_schedule();
} while (1);
finish_wait(&bs->wait, &wait);
return tag;
}
static unsigned int __blk_mq_get_tag(struct blk_mq_tags *tags,
struct blk_mq_hw_ctx *hctx,
unsigned int *last_tag, gfp_t gfp)
{
int tag;
tag = bt_get(&tags->bitmap_tags, hctx, last_tag, gfp);
if (tag >= 0)
return tag + tags->nr_reserved_tags;
return BLK_MQ_TAG_FAIL;
}
static unsigned int __blk_mq_get_reserved_tag(struct blk_mq_tags *tags,
gfp_t gfp)
{
int tag, zero = 0;
if (unlikely(!tags->nr_reserved_tags)) {
WARN_ON_ONCE(1);
return BLK_MQ_TAG_FAIL;
}
tag = bt_get(&tags->breserved_tags, NULL, &zero, gfp);
if (tag < 0)
return BLK_MQ_TAG_FAIL;
return tag;
}
unsigned int blk_mq_get_tag(struct blk_mq_hw_ctx *hctx, unsigned int *last_tag,
gfp_t gfp, bool reserved)
{
if (!reserved)
return __blk_mq_get_tag(hctx->tags, hctx, last_tag, gfp);
return __blk_mq_get_reserved_tag(hctx->tags, gfp);
}
static struct bt_wait_state *bt_wake_ptr(struct blk_mq_bitmap_tags *bt)
{
int i, wake_index;
wake_index = bt->wake_index;
for (i = 0; i < BT_WAIT_QUEUES; i++) {
struct bt_wait_state *bs = &bt->bs[wake_index];
if (waitqueue_active(&bs->wait)) {
if (wake_index != bt->wake_index)
bt->wake_index = wake_index;
return bs;
}
bt_index_inc(&wake_index);
}
return NULL;
}
static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag)
{
const int index = TAG_TO_INDEX(bt, tag);
struct bt_wait_state *bs;
/*
* The unlock memory barrier need to order access to req in free
* path and clearing tag bit
*/
clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word);
bs = bt_wake_ptr(bt);
if (bs && atomic_dec_and_test(&bs->wait_cnt)) {
atomic_set(&bs->wait_cnt, bt->wake_cnt);
bt_index_inc(&bt->wake_index);
wake_up(&bs->wait);
}
}
static void __blk_mq_put_tag(struct blk_mq_tags *tags, unsigned int tag)
{
BUG_ON(tag >= tags->nr_tags);
bt_clear_tag(&tags->bitmap_tags, tag);
}
static void __blk_mq_put_reserved_tag(struct blk_mq_tags *tags,
unsigned int tag)
{
BUG_ON(tag >= tags->nr_reserved_tags);
bt_clear_tag(&tags->breserved_tags, tag);
}
void blk_mq_put_tag(struct blk_mq_hw_ctx *hctx, unsigned int tag,
unsigned int *last_tag)
{
struct blk_mq_tags *tags = hctx->tags;
if (tag >= tags->nr_reserved_tags) {
const int real_tag = tag - tags->nr_reserved_tags;
__blk_mq_put_tag(tags, real_tag);
*last_tag = real_tag;
} else
__blk_mq_put_reserved_tag(tags, tag);
}
static void bt_for_each_free(struct blk_mq_bitmap_tags *bt,
unsigned long *free_map, unsigned int off)
{
int i;
for (i = 0; i < bt->map_nr; i++) {
struct blk_mq_bitmap *bm = &bt->map[i];
int bit = 0;
do {
bit = find_next_zero_bit(&bm->word, bm->depth, bit);
if (bit >= bm->depth)
break;
__set_bit(bit + off, free_map);
bit++;
} while (1);
off += (1 << bt->bits_per_word);
}
}
void blk_mq_tag_busy_iter(struct blk_mq_tags *tags,
void (*fn)(void *, unsigned long *), void *data)
{
unsigned long *tag_map;
size_t map_size;
map_size = ALIGN(tags->nr_tags, BITS_PER_LONG) / BITS_PER_LONG;
tag_map = kzalloc(map_size * sizeof(unsigned long), GFP_ATOMIC);
if (!tag_map)
return;
bt_for_each_free(&tags->bitmap_tags, tag_map, tags->nr_reserved_tags);
if (tags->nr_reserved_tags)
bt_for_each_free(&tags->breserved_tags, tag_map, 0);
fn(data, tag_map);
kfree(tag_map);
}
static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
{
unsigned int i, used;
for (i = 0, used = 0; i < bt->map_nr; i++) {
struct blk_mq_bitmap *bm = &bt->map[i];
used += bitmap_weight(&bm->word, bm->depth);
}
return bt->depth - used;
}
static int bt_alloc(struct blk_mq_bitmap_tags *bt, unsigned int depth,
int node, bool reserved)
{
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) {
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, tags_per_word) / tags_per_word;
bt->map = kzalloc_node(nr * sizeof(struct blk_mq_bitmap),
GFP_KERNEL, node);
if (!bt->map)
return -ENOMEM;
bt->map_nr = nr;
map_depth = depth;
for (i = 0; i < nr; i++) {
bt->map[i].depth = min(map_depth, tags_per_word);
map_depth -= tags_per_word;
}
}
bt->bs = kzalloc(BT_WAIT_QUEUES * sizeof(*bt->bs), GFP_KERNEL);
if (!bt->bs) {
kfree(bt->map);
return -ENOMEM;
}
for (i = 0; i < BT_WAIT_QUEUES; i++)
init_waitqueue_head(&bt->bs[i].wait);
bt->wake_cnt = BT_WAIT_BATCH;
if (bt->wake_cnt > depth / 4)
bt->wake_cnt = max(1U, depth / 4);
bt->depth = depth;
return 0;
}
static void bt_free(struct blk_mq_bitmap_tags *bt)
{
kfree(bt->map);
kfree(bt->bs);
}
static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
int node)
{
unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
if (bt_alloc(&tags->bitmap_tags, depth, node, false))
goto enomem;
if (bt_alloc(&tags->breserved_tags, tags->nr_reserved_tags, node, true))
goto enomem;
return tags;
enomem:
bt_free(&tags->bitmap_tags);
kfree(tags);
return NULL;
}
struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
unsigned int reserved_tags, int node)
{
struct blk_mq_tags *tags;
if (total_tags > BLK_MQ_TAG_MAX) {
pr_err("blk-mq: tag depth too large\n");
return NULL;
}
tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
if (!tags)
return NULL;
tags->nr_tags = total_tags;
tags->nr_reserved_tags = reserved_tags;
return blk_mq_init_bitmap_tags(tags, node);
}
void blk_mq_free_tags(struct blk_mq_tags *tags)
{
bt_free(&tags->bitmap_tags);
bt_free(&tags->breserved_tags);
kfree(tags);
}
void blk_mq_tag_init_last_tag(struct blk_mq_tags *tags, unsigned int *tag)
{
unsigned int depth = tags->nr_tags - tags->nr_reserved_tags;
*tag = prandom_u32() % depth;
}
ssize_t blk_mq_tag_sysfs_show(struct blk_mq_tags *tags, char *page)
{
char *orig_page = page;
unsigned int free, res;
if (!tags)
return 0;
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);
page += sprintf(page, "nr_free=%u, nr_reserved=%u\n", free, res);
page += sprintf(page, "active_queues=%u\n", atomic_read(&tags->active_queues));
return page - orig_page;
}