blkcg: factor out blkio_group creation

Currently both blk-throttle and cfq-iosched implement their own
blkio_group creation code in throtl_get_tg() and cfq_get_cfqg().  This
patch factors out the common code into blkg_lookup_create(), which
returns ERR_PTR value so that transitional failures due to queue
bypass can be distinguished from other failures.

* New plkio_policy_ops methods blkio_alloc_group_fn() and
  blkio_link_group_fn added.  Both are transitional and will be
  removed once the blkg management code is fully moved into
  blk-cgroup.c.

* blkio_alloc_group_fn() allocates policy-specific blkg which is
  usually a larger data structure with blkg as the first entry and
  intiailizes it.  Note that initialization of blkg proper, including
  percpu stats, is responsibility of blk-cgroup proper.

  Note that default config (weight, bps...) initialization is done
  from this method; otherwise, we end up violating locking order
  between blkcg and q locks via blkcg_get_CONF() functions.

* blkio_link_group_fn() is called under queue_lock and responsible for
  linking the blkg to the queue.  blkcg side is handled by blk-cgroup
  proper.

* The common blkg creation function is named blkg_lookup_create() and
  blkiocg_lookup_group() is renamed to blkg_lookup() for consistency.
  Also, throtl / cfq related functions are similarly [re]named for
  consistency.

This simplifies blkcg policy implementations and enables further
cleanup.

-v2: Vivek noticed that blkg_lookup_create() incorrectly tested
     blk_queue_dead() instead of blk_queue_bypass() leading a user of
     the function ending up creating a new blkg on bypassing queue.
     This is a bug introduced while relocating bypass patches before
     this one.  Fixed.

-v3: ERR_PTR patch folded into this one.  @for_root added to
     blkg_lookup_create() to allow creating root group on a bypassed
     queue during elevator switch.

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, 52 insertions, 103 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index aeeb798d1cda..2ae637b9e80c 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -181,17 +181,25 @@ static void throtl_put_tg(struct throtl_grp *tg)
         call_rcu(&tg->rcu_head, throtl_free_tg);
 }
 
-static void throtl_init_group(struct throtl_grp *tg)
+static struct blkio_group *throtl_alloc_blkio_group(struct request_queue *q,
+                                                    struct blkio_cgroup *blkcg)
 {
+        struct throtl_grp *tg;
+
+        tg = kzalloc_node(sizeof(*tg), GFP_ATOMIC, q->node);
+        if (!tg)
+                return NULL;
+
         INIT_HLIST_NODE(&tg->tg_node);
         RB_CLEAR_NODE(&tg->rb_node);
         bio_list_init(&tg->bio_lists[0]);
         bio_list_init(&tg->bio_lists[1]);
         tg->limits_changed = false;
 
-        /* Practically unlimited BW */
-        tg->bps[0] = tg->bps[1] = -1;
-        tg->iops[0] = tg->iops[1] = -1;
+        tg->bps[READ] = blkcg_get_read_bps(blkcg, tg->blkg.dev);
+        tg->bps[WRITE] = blkcg_get_write_bps(blkcg, tg->blkg.dev);
+        tg->iops[READ] = blkcg_get_read_iops(blkcg, tg->blkg.dev);
+        tg->iops[WRITE] = blkcg_get_write_iops(blkcg, tg->blkg.dev);
 
         /*
          * Take the initial reference that will be released on destroy
@@ -200,14 +208,8 @@ static void throtl_init_group(struct throtl_grp *tg)
          * exit or cgroup deletion path depending on who is exiting first.
          */
         atomic_set(&tg->ref, 1);
-}
 
-/* Should be called with rcu read lock held (needed for blkcg) */
-static void
-throtl_add_group_to_td_list(struct throtl_data *td, struct throtl_grp *tg)
-{
-        hlist_add_head(&tg->tg_node, &td->tg_list);
-        td->nr_undestroyed_grps++;
+        return &tg->blkg;
 }
 
 static void
@@ -246,119 +248,62 @@ throtl_tg_fill_dev_details(struct throtl_data *td, struct throtl_grp *tg)
         spin_unlock_irq(td->queue->queue_lock);
 }
 
-static void throtl_init_add_tg_lists(struct throtl_data *td,
-                        struct throtl_grp *tg, struct blkio_cgroup *blkcg)
+static void throtl_link_blkio_group(struct request_queue *q,
+                                    struct blkio_group *blkg)
 {
-        __throtl_tg_fill_dev_details(td, tg);
-
-        /* Add group onto cgroup list */
-        blkiocg_add_blkio_group(blkcg, &tg->blkg, td->queue,
-                                tg->blkg.dev, BLKIO_POLICY_THROTL);
-
-        tg->bps[READ] = blkcg_get_read_bps(blkcg, tg->blkg.dev);
-        tg->bps[WRITE] = blkcg_get_write_bps(blkcg, tg->blkg.dev);
-        tg->iops[READ] = blkcg_get_read_iops(blkcg, tg->blkg.dev);
-        tg->iops[WRITE] = blkcg_get_write_iops(blkcg, tg->blkg.dev);
-
-        throtl_add_group_to_td_list(td, tg);
-}
-
-/* Should be called without queue lock and outside of rcu period */
-static struct throtl_grp *throtl_alloc_tg(struct throtl_data *td)
-{
-        struct throtl_grp *tg = NULL;
-        int ret;
-
-        tg = kzalloc_node(sizeof(*tg), GFP_ATOMIC, td->queue->node);
-        if (!tg)
-                return NULL;
-
-        ret = blkio_alloc_blkg_stats(&tg->blkg);
+        struct throtl_data *td = q->td;
+        struct throtl_grp *tg = tg_of_blkg(blkg);
 
-        if (ret) {
-                kfree(tg);
-                return NULL;
-        }
+        __throtl_tg_fill_dev_details(td, tg);
 
-        throtl_init_group(tg);
-        return tg;
+        hlist_add_head(&tg->tg_node, &td->tg_list);
+        td->nr_undestroyed_grps++;
 }
 
 static struct
-throtl_grp *throtl_find_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
+throtl_grp *throtl_lookup_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
 {
         struct throtl_grp *tg = NULL;
 
         /*
          * This is the common case when there are no blkio cgroups.
-         * Avoid lookup in this case
-         */
+         * Avoid lookup in this case
+         */
         if (blkcg == &blkio_root_cgroup)
                 tg = td->root_tg;
         else
-                tg = tg_of_blkg(blkiocg_lookup_group(blkcg, td->queue,
-                                                     BLKIO_POLICY_THROTL));
+                tg = tg_of_blkg(blkg_lookup(blkcg, td->queue,
+                                            BLKIO_POLICY_THROTL));
 
         __throtl_tg_fill_dev_details(td, tg);
         return tg;
 }
 
-static struct throtl_grp *throtl_get_tg(struct throtl_data *td,
-                                        struct blkio_cgroup *blkcg)
+static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
+                                                  struct blkio_cgroup *blkcg)
 {
-        struct throtl_grp *tg = NULL, *__tg = NULL;
         struct request_queue *q = td->queue;
-
-        /* no throttling for dead queue */
-        if (unlikely(blk_queue_bypass(q)))
-                return NULL;
-
-        tg = throtl_find_tg(td, blkcg);
-        if (tg)
-                return tg;
-
-        if (!css_tryget(&blkcg->css))
-                return NULL;
-
-        /*
-         * Need to allocate a group. Allocation of group also needs allocation
-         * of per cpu stats which in-turn takes a mutex() and can block. Hence
-         * we need to drop rcu lock and queue_lock before we call alloc.
-         */
-        spin_unlock_irq(q->queue_lock);
-        rcu_read_unlock();
-
-        tg = throtl_alloc_tg(td);
-
-        /* Group allocated and queue is still alive. take the lock */
-        rcu_read_lock();
-        spin_lock_irq(q->queue_lock);
-        css_put(&blkcg->css);
-
-        /* Make sure @q is still alive */
-        if (unlikely(blk_queue_bypass(q))) {
-                kfree(tg);
-                return NULL;
-        }
+        struct throtl_grp *tg = NULL;
 
         /*
-         * If some other thread already allocated the group while we were
-         * not holding queue lock, free up the group
+         * This is the common case when there are no blkio cgroups.
+         * Avoid lookup in this case
          */
-        __tg = throtl_find_tg(td, blkcg);
+        if (blkcg == &blkio_root_cgroup) {
+                tg = td->root_tg;
+        } else {
+                struct blkio_group *blkg;
 
-        if (__tg) {
-                kfree(tg);
-                return __tg;
-        }
+                blkg = blkg_lookup_create(blkcg, q, BLKIO_POLICY_THROTL, false);
 
-        /* Group allocation failed. Account the IO to root group */
-        if (!tg) {
-                tg = td->root_tg;
-                return tg;
+                /* if %NULL and @q is alive, fall back to root_tg */
+                if (!IS_ERR(blkg))
+                        tg = tg_of_blkg(blkg);
+                else if (!blk_queue_dead(q))
+                        tg = td->root_tg;
         }
 
-        throtl_init_add_tg_lists(td, tg, blkcg);
+        __throtl_tg_fill_dev_details(td, tg);
         return tg;
 }
 
@@ -1107,6 +1052,8 @@ static void throtl_shutdown_wq(struct request_queue *q)
 
 static struct blkio_policy_type blkio_policy_throtl = {
         .ops = {
+                .blkio_alloc_group_fn = throtl_alloc_blkio_group,
+                .blkio_link_group_fn = throtl_link_blkio_group,
                 .blkio_unlink_group_fn = throtl_unlink_blkio_group,
                 .blkio_clear_queue_fn = throtl_clear_queue,
                 .blkio_update_group_read_bps_fn =
@@ -1141,7 +1088,7 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
          */
         rcu_read_lock();
         blkcg = task_blkio_cgroup(current);
-        tg = throtl_find_tg(td, blkcg);
+        tg = throtl_lookup_tg(td, blkcg);
         if (tg) {
                 throtl_tg_fill_dev_details(td, tg);
 
@@ -1157,7 +1104,7 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
          * IO group
          */
         spin_lock_irq(q->queue_lock);
-        tg = throtl_get_tg(td, blkcg);
+        tg = throtl_lookup_create_tg(td, blkcg);
         if (unlikely(!tg))
                 goto out_unlock;
 
@@ -1252,6 +1199,7 @@ void blk_throtl_drain(struct request_queue *q)
 int blk_throtl_init(struct request_queue *q)
 {
         struct throtl_data *td;
+        struct blkio_group *blkg;
 
         td = kzalloc_node(sizeof(*td), GFP_KERNEL, q->node);
         if (!td)
@@ -1262,13 +1210,17 @@ int blk_throtl_init(struct request_queue *q)
         td->limits_changed = false;
         INIT_DELAYED_WORK(&td->throtl_work, blk_throtl_work);
 
-        /* alloc and Init root group. */
+        q->td = td;
         td->queue = q;
 
+        /* alloc and init root group. */
         rcu_read_lock();
         spin_lock_irq(q->queue_lock);
 
-        td->root_tg = throtl_get_tg(td, &blkio_root_cgroup);
+        blkg = blkg_lookup_create(&blkio_root_cgroup, q, BLKIO_POLICY_THROTL,
+                                  true);
+        if (!IS_ERR(blkg))
+                td->root_tg = tg_of_blkg(blkg);
 
         spin_unlock_irq(q->queue_lock);
         rcu_read_unlock();
@@ -1277,9 +1229,6 @@ int blk_throtl_init(struct request_queue *q)
                 kfree(td);
                 return -ENOMEM;
         }
-
-        /* Attach throtl data to request queue */
-        q->td = td;
         return 0;
 }