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Commit a637120e authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
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blkcg: use radix tree to index blkgs from blkcg 



blkg lookup is currently performed by traversing linked list anchored
at blkcg->blkg_list.  This is very unscalable and with blk-throttle
enabled and enough request queues on the system, this can get very
ugly quickly (blk-throttle performs look up on every bio submission).

This patch makes blkcg use radix tree to index blkgs combined with
simple last-looked-up hint.  This is mostly identical to how icqs are
indexed from ioc.

Note that because __blkg_lookup() may be invoked without holding queue
lock, hint is only updated from __blkg_lookup_create().  Due to cfq's
cfqq caching, this makes hint updates overly lazy.  This will be
improved with scheduled blkcg aware request allocation.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 496fb780

block/blk-cgroup.c

+44 −8
Original line number Diff line number Diff line
@@ -142,11 +142,21 @@ static struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg, 
				      struct request_queue *q)

{

	struct blkcg_gq *blkg;

	struct hlist_node *n;



	hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node)

		if (blkg->q == q)

	blkg = rcu_dereference(blkcg->blkg_hint);

	if (blkg && blkg->q == q)

		return blkg;



	/*

	 * Hint didn't match.  Look up from the radix tree.  Note that we

	 * may not be holding queue_lock and thus are not sure whether

	 * @blkg from blkg_tree has already been removed or not, so we

	 * can't update hint to the lookup result.  Leave it to the caller.

	 */

	blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);

	if (blkg && blkg->q == q)

		return blkg;



	return NULL;

}
@@ -179,9 +189,12 @@ static struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg, 
	WARN_ON_ONCE(!rcu_read_lock_held());

	lockdep_assert_held(q->queue_lock);



	/* lookup and update hint on success, see __blkg_lookup() for details */

	blkg = __blkg_lookup(blkcg, q);

	if (blkg)

	if (blkg) {

		rcu_assign_pointer(blkcg->blkg_hint, blkg);

		return blkg;

	}



	/* blkg holds a reference to blkcg */

	if (!css_tryget(&blkcg->css))
@@ -194,12 +207,24 @@ static struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg, 
		goto err_put;



	/* insert */

	ret = radix_tree_preload(GFP_ATOMIC);

	if (ret)

		goto err_free;



	spin_lock(&blkcg->lock);

	ret = radix_tree_insert(&blkcg->blkg_tree, q->id, blkg);

	if (likely(!ret)) {

		hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);

		list_add(&blkg->q_node, &q->blkg_list);

	}

	spin_unlock(&blkcg->lock);

	return blkg;



	radix_tree_preload_end();



	if (!ret)

		return blkg;

err_free:

	blkg_free(blkg);

err_put:

	css_put(&blkcg->css);

	return ERR_PTR(ret);
@@ -229,9 +254,19 @@ static void blkg_destroy(struct blkcg_gq *blkg) 
	/* Something wrong if we are trying to remove same group twice */

	WARN_ON_ONCE(list_empty(&blkg->q_node));

	WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));



	radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);

	list_del_init(&blkg->q_node);

	hlist_del_init_rcu(&blkg->blkcg_node);



	/*

	 * Both setting lookup hint to and clearing it from @blkg are done

	 * under queue_lock.  If it's not pointing to @blkg now, it never

	 * will.  Hint assignment itself can race safely.

	 */

	if (rcu_dereference_raw(blkcg->blkg_hint) == blkg)

		rcu_assign_pointer(blkcg->blkg_hint, NULL);



	/*

	 * Put the reference taken at the time of creation so that when all

	 * queues are gone, group can be destroyed.
@@ -593,6 +628,7 @@ static struct cgroup_subsys_state *blkcg_create(struct cgroup *cgroup) 
	blkcg->id = atomic64_inc_return(&id_seq); /* root is 0, start from 1 */

done:

	spin_lock_init(&blkcg->lock);

	INIT_RADIX_TREE(&blkcg->blkg_tree, GFP_ATOMIC);

	INIT_HLIST_HEAD(&blkcg->blkg_list);



	return &blkcg->css;

block/blk-cgroup.h

+6 −0
Original line number Diff line number Diff line
@@ -16,6 +16,7 @@ 
#include <linux/cgroup.h>

#include <linux/u64_stats_sync.h>

#include <linux/seq_file.h>

#include <linux/radix-tree.h>



/* Max limits for throttle policy */

#define THROTL_IOPS_MAX		UINT_MAX
@@ -37,9 +38,14 @@ enum blkg_rwstat_type { 
	BLKG_RWSTAT_TOTAL = BLKG_RWSTAT_NR,

};



struct blkcg_gq;



struct blkcg {

	struct cgroup_subsys_state	css;

	spinlock_t			lock;



	struct radix_tree_root		blkg_tree;

	struct blkcg_gq			*blkg_hint;

	struct hlist_head		blkg_list;



	/* for policies to test whether associated blkcg has changed */