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Commit 97889f9a authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
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blk-mq: remove synchronize_rcu() from blk_mq_del_queue_tag_set() 



We have to remove synchronize_rcu() from blk_queue_cleanup(),
otherwise long delay can be caused during lun probe. For removing
it, we have to avoid to iterate the set->tag_list in IO path, eg,
blk_mq_sched_restart().

This patch reverts 5b79413946d (Revert "blk-mq: don't handle
TAG_SHARED in restart"). Given we have fixed enough IO hang issue,
and there isn't any reason to restart all queues in one tags any more,
see the following reasons:

1) blk-mq core can deal with shared-tags case well via blk_mq_get_driver_tag(),
which can wake up queues waiting for driver tag.

2) SCSI is a bit special because it may return BLK_STS_RESOURCE if queue,
target or host is ready, but SCSI built-in restart can cover all these well,
see scsi_end_request(), queue will be rerun after any request initiated from
this host/target is completed.

In my test on scsi_debug(8 luns), this patch may improve IOPS by 20% ~ 30%
when running I/O on these 8 luns concurrently.

Fixes: 705cda97 ("blk-mq: Make it safe to use RCU to iterate over blk_mq_tag_set.tag_list")
Cc: Omar Sandoval <osandov@fb.com>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: linux-scsi@vger.kernel.org
Reported-by: default avatarAndrew Jones <drjones@redhat.com>
Tested-by: default avatarAndrew Jones <drjones@redhat.com>
Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 5815839b

block/blk-mq-sched.c

+5 −80
Original line number Diff line number Diff line
@@ -59,29 +59,16 @@ static void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx) 
	if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

		return;



	if (hctx->flags & BLK_MQ_F_TAG_SHARED) {

		struct request_queue *q = hctx->queue;



		if (!test_and_set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

			atomic_inc(&q->shared_hctx_restart);

	} else

	set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);

}



static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)

void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)

{

	if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

		return false;



	if (hctx->flags & BLK_MQ_F_TAG_SHARED) {

		struct request_queue *q = hctx->queue;



		if (test_and_clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

			atomic_dec(&q->shared_hctx_restart);

	} else

		return;

	clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);



	return blk_mq_run_hw_queue(hctx, true);

	blk_mq_run_hw_queue(hctx, true);

}



/*
@@ -380,68 +367,6 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx, 
	return false;

}



/**

 * list_for_each_entry_rcu_rr - iterate in a round-robin fashion over rcu list

 * @pos:    loop cursor.

 * @skip:   the list element that will not be examined. Iteration starts at

 *          @skip->next.

 * @head:   head of the list to examine. This list must have at least one

 *          element, namely @skip.

 * @member: name of the list_head structure within typeof(*pos).

 */

#define list_for_each_entry_rcu_rr(pos, skip, head, member)		\

	for ((pos) = (skip);						\

	     (pos = (pos)->member.next != (head) ? list_entry_rcu(	\

			(pos)->member.next, typeof(*pos), member) :	\

	      list_entry_rcu((pos)->member.next->next, typeof(*pos), member)), \

	     (pos) != (skip); )



/*

 * Called after a driver tag has been freed to check whether a hctx needs to

 * be restarted. Restarts @hctx if its tag set is not shared. Restarts hardware

 * queues in a round-robin fashion if the tag set of @hctx is shared with other

 * hardware queues.

 */

void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)

{

	struct blk_mq_tags *const tags = hctx->tags;

	struct blk_mq_tag_set *const set = hctx->queue->tag_set;

	struct request_queue *const queue = hctx->queue, *q;

	struct blk_mq_hw_ctx *hctx2;

	unsigned int i, j;



	if (set->flags & BLK_MQ_F_TAG_SHARED) {

		/*

		 * If this is 0, then we know that no hardware queues

		 * have RESTART marked. We're done.

		 */

		if (!atomic_read(&queue->shared_hctx_restart))

			return;



		rcu_read_lock();

		list_for_each_entry_rcu_rr(q, queue, &set->tag_list,

					   tag_set_list) {

			queue_for_each_hw_ctx(q, hctx2, i)

				if (hctx2->tags == tags &&

				    blk_mq_sched_restart_hctx(hctx2))

					goto done;

		}

		j = hctx->queue_num + 1;

		for (i = 0; i < queue->nr_hw_queues; i++, j++) {

			if (j == queue->nr_hw_queues)

				j = 0;

			hctx2 = queue->queue_hw_ctx[j];

			if (hctx2->tags == tags &&

			    blk_mq_sched_restart_hctx(hctx2))

				break;

		}

done:

		rcu_read_unlock();

	} else {

		blk_mq_sched_restart_hctx(hctx);

	}

}



void blk_mq_sched_insert_request(struct request *rq, bool at_head,

				 bool run_queue, bool async)

{

block/blk-mq.c

+2 −8
Original line number Diff line number Diff line
@@ -2335,17 +2335,12 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared) 
	int i;



	queue_for_each_hw_ctx(q, hctx, i) {

		if (shared) {

			if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

				atomic_inc(&q->shared_hctx_restart);

		if (shared)

			hctx->flags |= BLK_MQ_F_TAG_SHARED;

		} else {

			if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))

				atomic_dec(&q->shared_hctx_restart);

		else

			hctx->flags &= ~BLK_MQ_F_TAG_SHARED;

	}

}

}



static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set,

					bool shared)
@@ -2374,7 +2369,6 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q) 
		blk_mq_update_tag_set_depth(set, false);

	}

	mutex_unlock(&set->tag_list_lock);

	synchronize_rcu();

	INIT_LIST_HEAD(&q->tag_set_list);

}

include/linux/blkdev.h

+0 −2
Original line number Diff line number Diff line
@@ -442,8 +442,6 @@ struct request_queue { 
	int			nr_rqs[2];	/* # allocated [a]sync rqs */

	int			nr_rqs_elvpriv;	/* # allocated rqs w/ elvpriv */



	atomic_t		shared_hctx_restart;



	struct blk_queue_stats	*stats;

	struct rq_wb		*rq_wb;