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Message-Id: <20250723024923.3211744-2-yukuai1@huaweicloud.com>
Date: Wed, 23 Jul 2025 10:49:22 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: dlemoal@...nel.org,
axboe@...nel.dk,
akpm@...ux-foundation.org,
ming.lei@...hat.com,
yang.yang@...o.com
Cc: linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org,
yukuai3@...wei.com,
yukuai1@...weicloud.com,
yi.zhang@...wei.com,
yangerkun@...wei.com,
johnny.chenyi@...wei.com
Subject: [PATCH 1/2] lib/sbitmap: convert shallow_depth from one word to the whole sbitmap
From: Yu Kuai <yukuai3@...wei.com>
Currently elevators will record internal 'async_depth' to throttle
asynchronous requests, and they both calculate shallow_dpeth based on
sb->shift, with the respect that sb->shift is the available tags in one
word.
However, sb->shift is not the availbale tags in the last word, see
__map_depth:
if (index == sb->map_nr - 1)
return sb->depth - (index << sb->shift);
For consequence, if the last word is used, more tags can be get than
expected, for example, assume nr_requests=256 and there are four words,
in the worst case if user set nr_requests=32, then the first word is
the last word, and still use bits per word, which is 64, to calculate
async_depth is wrong.
One the ohter hand, due to cgroup qos, bfq can allow only one request
to be allocated, and set shallow_dpeth=1 will still allow the number
of words request to be allocated.
Fix this problems by using shallow_depth to the whole sbitmap instead
of per word, also change kyber, mq-deadline and bfq to follow this.
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
block/bfq-iosched.c | 35 ++++++++++++--------------
block/bfq-iosched.h | 3 +--
block/kyber-iosched.c | 9 ++-----
block/mq-deadline.c | 16 +-----------
include/linux/sbitmap.h | 6 ++---
lib/sbitmap.c | 55 +++++++++++++++++++++--------------------
6 files changed, 51 insertions(+), 73 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0cb1e9873aab..d68da9e92e1e 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -701,17 +701,13 @@ static void bfq_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
{
struct bfq_data *bfqd = data->q->elevator->elevator_data;
struct bfq_io_cq *bic = bfq_bic_lookup(data->q);
- int depth;
- unsigned limit = data->q->nr_requests;
- unsigned int act_idx;
+ unsigned int limit, act_idx;
/* Sync reads have full depth available */
- if (op_is_sync(opf) && !op_is_write(opf)) {
- depth = 0;
- } else {
- depth = bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(opf)];
- limit = (limit * depth) >> bfqd->full_depth_shift;
- }
+ if (op_is_sync(opf) && !op_is_write(opf))
+ limit = data->q->nr_requests;
+ else
+ limit = bfqd->async_depths[!!bfqd->wr_busy_queues][op_is_sync(opf)];
for (act_idx = 0; bic && act_idx < bfqd->num_actuators; act_idx++) {
/* Fast path to check if bfqq is already allocated. */
@@ -725,14 +721,16 @@ static void bfq_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
* available requests and thus starve other entities.
*/
if (bfqq_request_over_limit(bfqd, bic, opf, act_idx, limit)) {
- depth = 1;
+ limit = 1;
break;
}
}
+
bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
- __func__, bfqd->wr_busy_queues, op_is_sync(opf), depth);
- if (depth)
- data->shallow_depth = depth;
+ __func__, bfqd->wr_busy_queues, op_is_sync(opf), limit);
+
+ if (limit < data->q->nr_requests)
+ data->shallow_depth = limit;
}
static struct bfq_queue *
@@ -7128,9 +7126,8 @@ void bfq_put_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg)
*/
static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
{
- unsigned int depth = 1U << bt->sb.shift;
+ unsigned int nr_requests = bfqd->queue->nr_requests;
- bfqd->full_depth_shift = bt->sb.shift;
/*
* In-word depths if no bfq_queue is being weight-raised:
* leaving 25% of tags only for sync reads.
@@ -7142,13 +7139,13 @@ static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
* limit 'something'.
*/
/* no more than 50% of tags for async I/O */
- bfqd->word_depths[0][0] = max(depth >> 1, 1U);
+ bfqd->async_depths[0][0] = max(nr_requests >> 1, 1U);
/*
* no more than 75% of tags for sync writes (25% extra tags
* w.r.t. async I/O, to prevent async I/O from starving sync
* writes)
*/
- bfqd->word_depths[0][1] = max((depth * 3) >> 2, 1U);
+ bfqd->async_depths[0][1] = max((nr_requests * 3) >> 2, 1U);
/*
* In-word depths in case some bfq_queue is being weight-
@@ -7158,9 +7155,9 @@ static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
* shortage.
*/
/* no more than ~18% of tags for async I/O */
- bfqd->word_depths[1][0] = max((depth * 3) >> 4, 1U);
+ bfqd->async_depths[1][0] = max((nr_requests * 3) >> 4, 1U);
/* no more than ~37% of tags for sync writes (~20% extra tags) */
- bfqd->word_depths[1][1] = max((depth * 6) >> 4, 1U);
+ bfqd->async_depths[1][1] = max((nr_requests * 6) >> 4, 1U);
}
static void bfq_depth_updated(struct blk_mq_hw_ctx *hctx)
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 687a3a7ba784..31217f196f4f 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -813,8 +813,7 @@ struct bfq_data {
* Depth limits used in bfq_limit_depth (see comments on the
* function)
*/
- unsigned int word_depths[2][2];
- unsigned int full_depth_shift;
+ unsigned int async_depths[2][2];
/*
* Number of independent actuators. This is equal to 1 in
diff --git a/block/kyber-iosched.c b/block/kyber-iosched.c
index 4dba8405bd01..bfd9a40bb33d 100644
--- a/block/kyber-iosched.c
+++ b/block/kyber-iosched.c
@@ -157,10 +157,7 @@ struct kyber_queue_data {
*/
struct sbitmap_queue domain_tokens[KYBER_NUM_DOMAINS];
- /*
- * Async request percentage, converted to per-word depth for
- * sbitmap_get_shallow().
- */
+ /* Number of allowed async requests. */
unsigned int async_depth;
struct kyber_cpu_latency __percpu *cpu_latency;
@@ -454,10 +451,8 @@ static void kyber_depth_updated(struct blk_mq_hw_ctx *hctx)
{
struct kyber_queue_data *kqd = hctx->queue->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;
- unsigned int shift = tags->bitmap_tags.sb.shift;
-
- kqd->async_depth = (1U << shift) * KYBER_ASYNC_PERCENT / 100U;
+ kqd->async_depth = hctx->queue->nr_requests * KYBER_ASYNC_PERCENT / 100U;
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, kqd->async_depth);
}
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 2edf1cac06d5..9ab6c6256695 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -487,20 +487,6 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
return rq;
}
-/*
- * 'depth' is a number in the range 1..INT_MAX representing a number of
- * requests. Scale it with a factor (1 << bt->sb.shift) / q->nr_requests since
- * 1..(1 << bt->sb.shift) is the range expected by sbitmap_get_shallow().
- * Values larger than q->nr_requests have the same effect as q->nr_requests.
- */
-static int dd_to_word_depth(struct blk_mq_hw_ctx *hctx, unsigned int qdepth)
-{
- struct sbitmap_queue *bt = &hctx->sched_tags->bitmap_tags;
- const unsigned int nrr = hctx->queue->nr_requests;
-
- return ((qdepth << bt->sb.shift) + nrr - 1) / nrr;
-}
-
/*
* Called by __blk_mq_alloc_request(). The shallow_depth value set by this
* function is used by __blk_mq_get_tag().
@@ -517,7 +503,7 @@ static void dd_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
* Throttle asynchronous requests and writes such that these requests
* do not block the allocation of synchronous requests.
*/
- data->shallow_depth = dd_to_word_depth(data->hctx, dd->async_depth);
+ data->shallow_depth = dd->async_depth;
}
/* Called by blk_mq_update_nr_requests(). */
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 189140bf11fc..4adf4b364fcd 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -213,12 +213,12 @@ int sbitmap_get(struct sbitmap *sb);
* sbitmap_get_shallow() - Try to allocate a free bit from a &struct sbitmap,
* limiting the depth used from each word.
* @sb: Bitmap to allocate from.
- * @shallow_depth: The maximum number of bits to allocate from a single word.
+ * @shallow_depth: The maximum number of bits to allocate from the bitmap.
*
* This rather specific operation allows for having multiple users with
* different allocation limits. E.g., there can be a high-priority class that
* uses sbitmap_get() and a low-priority class that uses sbitmap_get_shallow()
- * with a @shallow_depth of (1 << (@sb->shift - 1)). Then, the low-priority
+ * with a @shallow_depth of (sb->depth >> 1). Then, the low-priority
* class can only allocate half of the total bits in the bitmap, preventing it
* from starving out the high-priority class.
*
@@ -478,7 +478,7 @@ unsigned long __sbitmap_queue_get_batch(struct sbitmap_queue *sbq, int nr_tags,
* sbitmap_queue, limiting the depth used from each word, with preemption
* already disabled.
* @sbq: Bitmap queue to allocate from.
- * @shallow_depth: The maximum number of bits to allocate from a single word.
+ * @shallow_depth: The maximum number of bits to allocate from the queue.
* See sbitmap_get_shallow().
*
* If you call this, make sure to call sbitmap_queue_min_shallow_depth() after
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index d3412984170c..f2e90ac6b56e 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -208,8 +208,27 @@ static int sbitmap_find_bit_in_word(struct sbitmap_word *map,
return nr;
}
+static unsigned int __map_depth_with_shallow(const struct sbitmap *sb,
+ int index,
+ unsigned int shallow_depth)
+{
+ unsigned int lower_bound = 0;
+
+ if (shallow_depth >= sb->depth)
+ return __map_depth(sb, index);
+
+ if (index > 0)
+ lower_bound += (index - 1) << sb->shift;
+
+ if (shallow_depth <= lower_bound)
+ return 0;
+
+ return min_t(unsigned int, __map_depth(sb, index),
+ shallow_depth - lower_bound);
+}
+
static int sbitmap_find_bit(struct sbitmap *sb,
- unsigned int depth,
+ unsigned int shallow_depth,
unsigned int index,
unsigned int alloc_hint,
bool wrap)
@@ -218,12 +237,12 @@ static int sbitmap_find_bit(struct sbitmap *sb,
int nr = -1;
for (i = 0; i < sb->map_nr; i++) {
- nr = sbitmap_find_bit_in_word(&sb->map[index],
- min_t(unsigned int,
- __map_depth(sb, index),
- depth),
- alloc_hint, wrap);
+ unsigned int depth = __map_depth_with_shallow(sb, index,
+ shallow_depth);
+ if (depth)
+ nr = sbitmap_find_bit_in_word(&sb->map[index], depth,
+ alloc_hint, wrap);
if (nr != -1) {
nr += index << sb->shift;
break;
@@ -406,27 +425,9 @@ EXPORT_SYMBOL_GPL(sbitmap_bitmap_show);
static unsigned int sbq_calc_wake_batch(struct sbitmap_queue *sbq,
unsigned int depth)
{
- unsigned int wake_batch;
- unsigned int shallow_depth;
-
- /*
- * Each full word of the bitmap has bits_per_word bits, and there might
- * be a partial word. There are depth / bits_per_word full words and
- * depth % bits_per_word bits left over. In bitwise arithmetic:
- *
- * bits_per_word = 1 << shift
- * depth / bits_per_word = depth >> shift
- * depth % bits_per_word = depth & ((1 << shift) - 1)
- *
- * Each word can be limited to sbq->min_shallow_depth bits.
- */
- shallow_depth = min(1U << sbq->sb.shift, sbq->min_shallow_depth);
- depth = ((depth >> sbq->sb.shift) * shallow_depth +
- min(depth & ((1U << sbq->sb.shift) - 1), shallow_depth));
- wake_batch = clamp_t(unsigned int, depth / SBQ_WAIT_QUEUES, 1,
- SBQ_WAKE_BATCH);
-
- return wake_batch;
+ return clamp_t(unsigned int,
+ min(depth, sbq->min_shallow_depth) / SBQ_WAIT_QUEUES,
+ 1, SBQ_WAKE_BATCH);
}
int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
--
2.39.2
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