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Commit 07b05bcc authored by Dennis Zhou (Facebook)'s avatar Dennis Zhou (Facebook) Committed by Jens Axboe
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blkcg: convert blkg_lookup_create to find closest blkg 



There are several scenarios where blkg_lookup_create can fail. Examples
include the blkcg dying, request_queue is dying, or simply being OOM. At
the end of the day, most handle this by simply falling back to the
q->root_blkg and calling it a day.

This patch implements the notion of closest blkg. During
blkg_lookup_create, if it fails to create, return the closest blkg
found or the q->root_blkg. blkg_try_get_closest is introduced and used
during association so a bio is always attached to a blkg.

Acked-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarDennis Zhou <dennisszhou@gmail.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 49f4c2dc

block/bio.c

+10 −7
Original line number Diff line number Diff line
@@ -2007,21 +2007,24 @@ int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css) 
EXPORT_SYMBOL_GPL(bio_associate_blkcg);



/**

 * bio_associate_blkg - associate a bio with the specified blkg

 * bio_associate_blkg - associate a bio with the a blkg

 * @bio: target bio

 * @blkg: the blkg to associate

 *

 * Associate @bio with the blkg specified by @blkg.  This is the queue specific

 * blkcg information associated with the @bio, a reference will be taken on the

 * @blkg and will be freed when the bio is freed.

 * This tries to associate @bio with the specified blkg.  Association failure

 * is handled by walking up the blkg tree.  Therefore, the blkg associated can

 * be anything between @blkg and the root_blkg.  This situation only happens

 * when a cgroup is dying and then the remaining bios will spill to the closest

 * alive blkg.

 *

 * A reference will be taken on the @blkg and will be released when @bio is

 * freed.

 */

int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)

{

	if (unlikely(bio->bi_blkg))

		return -EBUSY;

	if (!blkg_try_get(blkg))

		return -ENODEV;

	bio->bi_blkg = blkg;

	bio->bi_blkg = blkg_try_get_closest(blkg);

	return 0;

}

block/blk-cgroup.c

+17 −8
Original line number Diff line number Diff line
@@ -268,9 +268,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, 
 * that all non-root blkg's have access to the parent blkg.  This function

 * should be called under RCU read lock and @q->queue_lock.

 *

 * Returns pointer to the looked up or created blkg on success, ERR_PTR()

 * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not

 * dead and bypassing, returns ERR_PTR(-EBUSY).

 * Returns the blkg or the closest blkg if blkg_create fails as it walks

 * down from root.

 */

struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,

				      struct request_queue *q)
@@ -285,7 +284,7 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg, 
	 * we shouldn't allow anything to go through for a bypassing queue.

	 */

	if (unlikely(blk_queue_bypass(q)))

		return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);

		return q->root_blkg;



	blkg = __blkg_lookup(blkcg, q, true);

	if (blkg)
@@ -293,19 +292,29 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg, 


	/*

	 * Create blkgs walking down from blkcg_root to @blkcg, so that all

	 * non-root blkgs have access to their parents.

	 * non-root blkgs have access to their parents.  Returns the closest

	 * blkg to the intended blkg should blkg_create() fail.

	 */

	while (true) {

		struct blkcg *pos = blkcg;

		struct blkcg *parent = blkcg_parent(blkcg);

		struct blkcg_gq *ret_blkg = q->root_blkg;



		while (parent && !__blkg_lookup(parent, q, false)) {

		while (parent) {

			blkg = __blkg_lookup(parent, q, false);

			if (blkg) {

				/* remember closest blkg */

				ret_blkg = blkg;

				break;

			}

			pos = parent;

			parent = blkcg_parent(parent);

		}



		blkg = blkg_create(pos, q, NULL);

		if (pos == blkcg || IS_ERR(blkg))

		if (IS_ERR(blkg))

			return ret_blkg;

		if (pos == blkcg)

			return blkg;

	}

}

include/linux/blk-cgroup.h

+14 −0
Original line number Diff line number Diff line
@@ -549,6 +549,20 @@ static inline struct blkcg_gq *blkg_try_get(struct blkcg_gq *blkg) 
	return NULL;

}



/**

 * blkg_try_get_closest - try and get a blkg ref on the closet blkg

 * @blkg: blkg to get

 *

 * This walks up the blkg tree to find the closest non-dying blkg and returns

 * the blkg that it did association with as it may not be the passed in blkg.

 */

static inline struct blkcg_gq *blkg_try_get_closest(struct blkcg_gq *blkg)

{

	while (!atomic_inc_not_zero(&blkg->refcnt))

		blkg = blkg->parent;



	return blkg;

}



void __blkg_release_rcu(struct rcu_head *rcu);