blkcg: unify blkg's for blkcg policies

Currently, blkg is per cgroup-queue-policy combination.  This is
unnatural and leads to various convolutions in partially used
duplicate fields in blkg, config / stat access, and general management
of blkgs.

This patch make blkg's per cgroup-queue and let them serve all
policies.  blkgs are now created and destroyed by blkcg core proper.
This will allow further consolidation of common management logic into
blkcg core and API with better defined semantics and layering.

As a transitional step to untangle blkg management, elvswitch and
policy [de]registration, all blkgs except the root blkg are being shot
down during elvswitch and bypass.  This patch adds blkg_root_update()
to update root blkg in place on policy change.  This is hacky and racy
but should be good enough as interim step until we get locking
simplified and switch over to proper in-place update for all blkgs.

-v2: Root blkgs need to be updated on elvswitch too and blkg_alloc()
     comment wasn't updated according to the function change.  Fixed.
     Both pointed out by Vivek.

-v3: v2 updated blkg_destroy_all() to invoke update_root_blkg_pd() for
     all policies.  This freed root pd during elvswitch before the
     last queue finished exiting and led to oops.  Directly invoke
     update_root_blkg_pd() only on BLKIO_POLICY_PROP from
     cfq_exit_queue().  This also is closer to what will be done with
     proper in-place blkg update.  Reported by Vivek.

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

1 files changed, 3 insertions, 6 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 132941260e58..e35ee7aeea69 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -167,7 +167,7 @@ throtl_grp *throtl_lookup_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
         if (blkcg == &blkio_root_cgroup)
                 return td->root_tg;
 
-        return blkg_to_tg(blkg_lookup(blkcg, td->queue, BLKIO_POLICY_THROTL));
+        return blkg_to_tg(blkg_lookup(blkcg, td->queue));
 }
 
 static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
@@ -704,8 +704,7 @@ static void throtl_process_limit_change(struct throtl_data *td)
 
         throtl_log(td, "limits changed");
 
-        list_for_each_entry_safe(blkg, n, &q->blkg_list[BLKIO_POLICY_THROTL],
-                                 q_node[BLKIO_POLICY_THROTL]) {
+        list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
                 struct throtl_grp *tg = blkg_to_tg(blkg);
 
                 if (!tg->limits_changed)
@@ -1054,11 +1053,9 @@ void blk_throtl_exit(struct request_queue *q)
 
         throtl_shutdown_wq(q);
 
-        blkg_destroy_all(q, BLKIO_POLICY_THROTL, true);
-
         /* If there are other groups */
         spin_lock_irq(q->queue_lock);
-        wait = q->nr_blkgs[BLKIO_POLICY_THROTL];
+        wait = q->nr_blkgs;
         spin_unlock_irq(q->queue_lock);
 
         /*