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, 40 insertions, 91 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 1c3f41b9d5dd..acef564578c3 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1048,10 +1048,12 @@ static void cfq_update_blkio_group_weight(struct request_queue *q,
         cfqg->needs_update = true;
 }
 
-static void cfq_init_add_cfqg_lists(struct cfq_data *cfqd,
-                        struct cfq_group *cfqg, struct blkio_cgroup *blkcg)
+static void cfq_link_blkio_group(struct request_queue *q,
+                                 struct blkio_group *blkg)
 {
-        struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
+        struct cfq_data *cfqd = q->elevator->elevator_data;
+        struct backing_dev_info *bdi = &q->backing_dev_info;
+        struct cfq_group *cfqg = cfqg_of_blkg(blkg);
         unsigned int major, minor;
 
         /*
@@ -1062,34 +1064,26 @@ static void cfq_init_add_cfqg_lists(struct cfq_data *cfqd,
          */
         if (bdi->dev) {
                 sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg,
-                                        cfqd->queue, MKDEV(major, minor));
-        } else
-                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg,
-                                        cfqd->queue, 0);
+                blkg->dev = MKDEV(major, minor);
+        }
 
         cfqd->nr_blkcg_linked_grps++;
-        cfqg->weight = blkcg_get_weight(blkcg, cfqg->blkg.dev);
 
         /* Add group on cfqd list */
         hlist_add_head(&cfqg->cfqd_node, &cfqd->cfqg_list);
 }
 
-/*
- * Should be called from sleepable context. No request queue lock as per
- * cpu stats are allocated dynamically and alloc_percpu needs to be called
- * from sleepable context.
- */
-static struct cfq_group * cfq_alloc_cfqg(struct cfq_data *cfqd)
+static struct blkio_group *cfq_alloc_blkio_group(struct request_queue *q,
+                                                 struct blkio_cgroup *blkcg)
 {
         struct cfq_group *cfqg;
-        int ret;
 
-        cfqg = kzalloc_node(sizeof(*cfqg), GFP_ATOMIC, cfqd->queue->node);
+        cfqg = kzalloc_node(sizeof(*cfqg), GFP_ATOMIC, q->node);
         if (!cfqg)
                 return NULL;
 
         cfq_init_cfqg_base(cfqg);
+        cfqg->weight = blkcg_get_weight(blkcg, cfqg->blkg.dev);
 
         /*
          * Take the initial reference that will be released on destroy
@@ -1099,90 +1093,38 @@ static struct cfq_group * cfq_alloc_cfqg(struct cfq_data *cfqd)
          */
         cfqg->ref = 1;
 
-        ret = blkio_alloc_blkg_stats(&cfqg->blkg);
-        if (ret) {
-                kfree(cfqg);
-                return NULL;
-        }
-
-        return cfqg;
-}
-
-static struct cfq_group *
-cfq_find_cfqg(struct cfq_data *cfqd, struct blkio_cgroup *blkcg)
-{
-        struct cfq_group *cfqg = NULL;
-        struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
-        unsigned int major, minor;
-
-        /*
-         * This is the common case when there are no blkio cgroups.
-         * Avoid lookup in this case
-         */
-        if (blkcg == &blkio_root_cgroup)
-                cfqg = cfqd->root_group;
-        else
-                cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, cfqd->queue,
-                                                         BLKIO_POLICY_PROP));
-
-        if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
-                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                cfqg->blkg.dev = MKDEV(major, minor);
-        }
-
-        return cfqg;
+        return &cfqg->blkg;
 }
 
 /*
  * Search for the cfq group current task belongs to. request_queue lock must
  * be held.
  */
-static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd,
-                                      struct blkio_cgroup *blkcg)
+static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
+                                                struct blkio_cgroup *blkcg)
 {
-        struct cfq_group *cfqg = NULL, *__cfqg = NULL;
         struct request_queue *q = cfqd->queue;
+        struct backing_dev_info *bdi = &q->backing_dev_info;
+        struct cfq_group *cfqg = NULL;
 
-        cfqg = cfq_find_cfqg(cfqd, blkcg);
-        if (cfqg)
-                return cfqg;
-
-        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.
-         *
-         * Not taking any queue reference here and assuming that queue is
-         * around by the time we return. CFQ queue allocation code does
-         * the same. It might be racy though.
-         */
-        rcu_read_unlock();
-        spin_unlock_irq(q->queue_lock);
-
-        cfqg = cfq_alloc_cfqg(cfqd);
+        /* avoid lookup for the common case where there's no blkio cgroup */
+        if (blkcg == &blkio_root_cgroup) {
+                cfqg = cfqd->root_group;
+        } else {
+                struct blkio_group *blkg;
 
-        spin_lock_irq(q->queue_lock);
-        rcu_read_lock();
-        css_put(&blkcg->css);
+                blkg = blkg_lookup_create(blkcg, q, BLKIO_POLICY_PROP, false);
+                if (!IS_ERR(blkg))
+                        cfqg = cfqg_of_blkg(blkg);
+        }
 
-        /*
-         * If some other thread already allocated the group while we were
-         * not holding queue lock, free up the group
-         */
-        __cfqg = cfq_find_cfqg(cfqd, blkcg);
+        if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
+                unsigned int major, minor;
 
-        if (__cfqg) {
-                kfree(cfqg);
-                return __cfqg;
+                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
+                cfqg->blkg.dev = MKDEV(major, minor);
         }
 
-        if (!cfqg)
-                cfqg = cfqd->root_group;
-
-        cfq_init_add_cfqg_lists(cfqd, cfqg, blkcg);
         return cfqg;
 }
 
@@ -1294,8 +1236,8 @@ static bool cfq_clear_queue(struct request_queue *q)
 }
 
 #else /* GROUP_IOSCHED */
-static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd,
-                                      struct blkio_cgroup *blkcg)
+static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
+                                                struct blkio_cgroup *blkcg)
 {
         return cfqd->root_group;
 }
@@ -2887,7 +2829,8 @@ retry:
 
         blkcg = task_blkio_cgroup(current);
 
-        cfqg = cfq_get_cfqg(cfqd, blkcg);
+        cfqg = cfq_lookup_create_cfqg(cfqd, blkcg);
+
         cic = cfq_cic_lookup(cfqd, ioc);
         /* cic always exists here */
         cfqq = cic_to_cfqq(cic, is_sync);
@@ -3694,6 +3637,7 @@ static void cfq_exit_queue(struct elevator_queue *e)
 static int cfq_init_queue(struct request_queue *q)
 {
         struct cfq_data *cfqd;
+        struct blkio_group *blkg __maybe_unused;
         int i;
 
         cfqd = kmalloc_node(sizeof(*cfqd), GFP_KERNEL | __GFP_ZERO, q->node);
@@ -3711,7 +3655,10 @@ static int cfq_init_queue(struct request_queue *q)
         rcu_read_lock();
         spin_lock_irq(q->queue_lock);
 
-        cfqd->root_group = cfq_get_cfqg(cfqd, &blkio_root_cgroup);
+        blkg = blkg_lookup_create(&blkio_root_cgroup, q, BLKIO_POLICY_PROP,
+                                  true);
+        if (!IS_ERR(blkg))
+                cfqd->root_group = cfqg_of_blkg(blkg);
 
         spin_unlock_irq(q->queue_lock);
         rcu_read_unlock();
@@ -3897,6 +3844,8 @@ static struct elevator_type iosched_cfq = {
 #ifdef CONFIG_CFQ_GROUP_IOSCHED
 static struct blkio_policy_type blkio_policy_cfq = {
         .ops = {
+                .blkio_alloc_group_fn =         cfq_alloc_blkio_group,
+                .blkio_link_group_fn =          cfq_link_blkio_group,
                 .blkio_unlink_group_fn =        cfq_unlink_blkio_group,
                 .blkio_clear_queue_fn = cfq_clear_queue,
                 .blkio_update_group_weight_fn = cfq_update_blkio_group_weight,