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: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.h

@@ -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 */