Commit 4acb8341 authored by Keith Busch's avatar Keith Busch Committed by Jens Axboe
Browse files

sbitmap: fix batched wait_cnt accounting 

Batched completions can clear multiple bits, but we're only decrementing
the wait_cnt by one each time. This can cause waiters to never be woken,
stalling IO. Use the batched count instead.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215679


Signed-off-by: default avatarKeith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20220909184022.1709476-1-kbusch@fb.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent c35227d4

block/blk-mq-tag.c

+1 −1
Original line number Diff line number Diff line
@@ -196,7 +196,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data) 
		 * other allocations on previous queue won't be starved.

		 */

		if (bt != bt_prev)

			sbitmap_queue_wake_up(bt_prev);

			sbitmap_queue_wake_up(bt_prev, 1);



		ws = bt_wait_ptr(bt, data->hctx);

	} while (1);

include/linux/sbitmap.h

+2 −1
Original line number Diff line number Diff line
@@ -575,8 +575,9 @@ void sbitmap_queue_wake_all(struct sbitmap_queue *sbq); 
 * sbitmap_queue_wake_up() - Wake up some of waiters in one waitqueue

 * on a &struct sbitmap_queue.

 * @sbq: Bitmap queue to wake up.

 * @nr: Number of bits cleared.

 */

void sbitmap_queue_wake_up(struct sbitmap_queue *sbq);

void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr);



/**

 * sbitmap_queue_show() - Dump &struct sbitmap_queue information to a &struct

lib/sbitmap.c

+23 −14
Original line number Diff line number Diff line
@@ -599,24 +599,31 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq) 
	return NULL;

}



static bool __sbq_wake_up(struct sbitmap_queue *sbq)

static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)

{

	struct sbq_wait_state *ws;

	unsigned int wake_batch;

	int wait_cnt;

	int wait_cnt, cur, sub;

	bool ret;



	if (*nr <= 0)

		return false;



	ws = sbq_wake_ptr(sbq);

	if (!ws)

		return false;



	wait_cnt = atomic_dec_return(&ws->wait_cnt);

	cur = atomic_read(&ws->wait_cnt);

	do {

		/*

	 * For concurrent callers of this, callers should call this function

	 * again to wakeup a new batch on a different 'ws'.

		 * For concurrent callers of this, callers should call this

		 * function again to wakeup a new batch on a different 'ws'.

		 */

	if (wait_cnt < 0)

		if (cur == 0)

			return true;

		sub = min(*nr, cur);

		wait_cnt = cur - sub;

	} while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));



	/*

	 * If we decremented queue without waiters, retry to avoid lost
@@ -625,6 +632,8 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq) 
	if (wait_cnt > 0)

		return !waitqueue_active(&ws->wait);



	*nr -= sub;



	/*

	 * When wait_cnt == 0, we have to be particularly careful as we are

	 * responsible to reset wait_cnt regardless whether we've actually
@@ -660,12 +669,12 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq) 
	sbq_index_atomic_inc(&sbq->wake_index);

	atomic_set(&ws->wait_cnt, wake_batch);



	return ret;

	return ret || *nr;

}



void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)

void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)

{

	while (__sbq_wake_up(sbq))

	while (__sbq_wake_up(sbq, &nr))

		;

}

EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
@@ -705,7 +714,7 @@ void sbitmap_queue_clear_batch(struct sbitmap_queue *sbq, int offset, 
		atomic_long_andnot(mask, (atomic_long_t *) addr);



	smp_mb__after_atomic();

	sbitmap_queue_wake_up(sbq);

	sbitmap_queue_wake_up(sbq, nr_tags);

	sbitmap_update_cpu_hint(&sbq->sb, raw_smp_processor_id(),

					tags[nr_tags - 1] - offset);

}
@@ -733,7 +742,7 @@ void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr, 
	 * waiter. See the comment on waitqueue_active().

	 */

	smp_mb__after_atomic();

	sbitmap_queue_wake_up(sbq);

	sbitmap_queue_wake_up(sbq, 1);

	sbitmap_update_cpu_hint(&sbq->sb, cpu, nr);

}

EXPORT_SYMBOL_GPL(sbitmap_queue_clear);