blk-cgroup: Allow sleeping while dynamically allocating a group

Currently, all the cfq_group or throtl_group allocations happen while
we are holding ->queue_lock and sleeping is not allowed.

Soon, we will move to per cpu stats and also need to allocate the
per group stats. As one can not call alloc_percpu() from atomic
context as it can sleep, we need to drop ->queue_lock, allocate the
group, retake the lock and continue processing.

In throttling code, I check the queue DEAD flag again to make sure
that driver did not call blk_cleanup_queue() in the mean time.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

3 files changed, 205 insertions, 67 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 3fe00a14822a..9e8e297374b9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1550,7 +1550,8 @@ static inline void __generic_make_request(struct bio *bio)
                         goto end_io;
                 }
 
-                blk_throtl_bio(q, &bio);
+                if (blk_throtl_bio(q, &bio))
+                        goto end_io;
 
                 /*
                  * If bio = NULL, bio has been throttled and will be submitted
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index fa9a900c1254..c201967b33cd 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -188,8 +188,40 @@ throtl_add_group_to_td_list(struct throtl_data *td, struct throtl_grp *tg)
         td->nr_undestroyed_grps++;
 }
 
-static struct throtl_grp * throtl_find_alloc_tg(struct throtl_data *td,
-                        struct blkio_cgroup *blkcg)
+static void throtl_init_add_tg_lists(struct throtl_data *td,
+                        struct throtl_grp *tg, struct blkio_cgroup *blkcg)
+{
+        struct backing_dev_info *bdi = &td->queue->backing_dev_info;
+        unsigned int major, minor;
+
+        /* Add group onto cgroup list */
+        sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
+        blkiocg_add_blkio_group(blkcg, &tg->blkg, (void *)td,
+                                MKDEV(major, minor), 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;
+
+        tg = kzalloc_node(sizeof(*tg), GFP_ATOMIC, td->queue->node);
+        if (!tg)
+                return NULL;
+
+        throtl_init_group(tg);
+        return tg;
+}
+
+static struct
+throtl_grp *throtl_find_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
 {
         struct throtl_grp *tg = NULL;
         void *key = td;
@@ -197,12 +229,6 @@ static struct throtl_grp * throtl_find_alloc_tg(struct throtl_data *td,
         unsigned int major, minor;
 
         /*
-         * TODO: Speed up blkiocg_lookup_group() by maintaining a radix
-         * tree of blkg (instead of traversing through hash list all
-         * the time.
-         */
-
-        /*
          * This is the common case when there are no blkio cgroups.
          * Avoid lookup in this case
          */
@@ -215,43 +241,83 @@ static struct throtl_grp * throtl_find_alloc_tg(struct throtl_data *td,
         if (tg && !tg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
                 sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
                 tg->blkg.dev = MKDEV(major, minor);
-                goto done;
         }
 
-        if (tg)
-                goto done;
-
-        tg = kzalloc_node(sizeof(*tg), GFP_ATOMIC, td->queue->node);
-        if (!tg)
-                goto done;
-
-        throtl_init_group(tg);
-
-        /* Add group onto cgroup list */
-        sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-        blkiocg_add_blkio_group(blkcg, &tg->blkg, (void *)td,
-                                MKDEV(major, minor), 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);
-done:
         return tg;
 }
 
+/*
+ * This function returns with queue lock unlocked in case of error, like
+ * request queue is no more
+ */
 static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
 {
-        struct throtl_grp *tg = NULL;
+        struct throtl_grp *tg = NULL, *__tg = NULL;
         struct blkio_cgroup *blkcg;
+        struct request_queue *q = td->queue;
 
         rcu_read_lock();
         blkcg = task_blkio_cgroup(current);
-        tg = throtl_find_alloc_tg(td, blkcg);
-        if (!tg)
+        tg = throtl_find_tg(td, blkcg);
+        if (tg) {
+                rcu_read_unlock();
+                return tg;
+        }
+
+        /*
+         * 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
+         *
+         * Take the request queue reference to make sure queue does not
+         * go away once we return from allocation.
+         */
+        blk_get_queue(q);
+        rcu_read_unlock();
+        spin_unlock_irq(q->queue_lock);
+
+        tg = throtl_alloc_tg(td);
+        /*
+         * We might have slept in group allocation. Make sure queue is not
+         * dead
+         */
+        if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
+                blk_put_queue(q);
+                if (tg)
+                        kfree(tg);
+
+                return ERR_PTR(-ENODEV);
+        }
+        blk_put_queue(q);
+
+        /* Group allocated and queue is still alive. take the lock */
+        spin_lock_irq(q->queue_lock);
+
+        /*
+         * Initialize the new group. After sleeping, read the blkcg again.
+         */
+        rcu_read_lock();
+        blkcg = task_blkio_cgroup(current);
+
+        /*
+         * If some other thread already allocated the group while we were
+         * not holding queue lock, free up the group
+         */
+        __tg = throtl_find_tg(td, blkcg);
+
+        if (__tg) {
+                kfree(tg);
+                rcu_read_unlock();
+                return __tg;
+        }
+
+        /* Group allocation failed. Account the IO to root group */
+        if (!tg) {
                 tg = &td->root_tg;
+                return tg;
+        }
+
+        throtl_init_add_tg_lists(td, tg, blkcg);
         rcu_read_unlock();
         return tg;
 }
@@ -1014,6 +1080,15 @@ int blk_throtl_bio(struct request_queue *q, struct bio **biop)
         spin_lock_irq(q->queue_lock);
         tg = throtl_get_tg(td);
 
+        if (IS_ERR(tg)) {
+                if (PTR_ERR(tg) == -ENODEV) {
+                        /*
+                         * Queue is gone. No queue lock held here.
+                         */
+                        return -ENODEV;
+                }
+        }
+
         if (tg->nr_queued[rw]) {
                 /*
                  * There is already another bio queued in same dir. No
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index e2e6719832e1..606020fe93f3 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1016,28 +1016,47 @@ void cfq_update_blkio_group_weight(void *key, struct blkio_group *blkg,
         cfqg->needs_update = true;
 }
 
-static struct cfq_group * cfq_find_alloc_cfqg(struct cfq_data *cfqd,
-                struct blkio_cgroup *blkcg)
+static void cfq_init_add_cfqg_lists(struct cfq_data *cfqd,
+                        struct cfq_group *cfqg, struct blkio_cgroup *blkcg)
 {
-        struct cfq_group *cfqg = NULL;
-        void *key = cfqd;
-        int i, j;
-        struct cfq_rb_root *st;
         struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
         unsigned int major, minor;
 
-        cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
-        if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
+        /*
+         * Add group onto cgroup list. It might happen that bdi->dev is
+         * not initialized yet. Initialize this new group without major
+         * and minor info and this info will be filled in once a new thread
+         * comes for IO.
+         */
+        if (bdi->dev) {
                 sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                cfqg->blkg.dev = MKDEV(major, minor);
-                goto done;
-        }
-        if (cfqg)
-                goto done;
+                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg,
+                                        (void *)cfqd, MKDEV(major, minor));
+        } else
+                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg,
+                                        (void *)cfqd, 0);
+
+        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)
+{
+        struct cfq_group *cfqg = NULL;
+        int i, j;
+        struct cfq_rb_root *st;
 
         cfqg = kzalloc_node(sizeof(*cfqg), GFP_ATOMIC, cfqd->queue->node);
         if (!cfqg)
-                goto done;
+                return NULL;
 
         for_each_cfqg_st(cfqg, i, j, st)
                 *st = CFQ_RB_ROOT;
@@ -1050,28 +1069,31 @@ static struct cfq_group * cfq_find_alloc_cfqg(struct cfq_data *cfqd,
          * or cgroup deletion path depending on who is exiting first.
          */
         cfqg->ref = 1;
+        return cfqg;
+}
+
+static struct cfq_group *
+cfq_find_cfqg(struct cfq_data *cfqd, struct blkio_cgroup *blkcg)
+{
+        struct cfq_group *cfqg = NULL;
+        void *key = cfqd;
+        struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
+        unsigned int major, minor;
 
         /*
-         * Add group onto cgroup list. It might happen that bdi->dev is
-         * not initialized yet. Initialize this new group without major
-         * and minor info and this info will be filled in once a new thread
-         * comes for IO. See code above.
+         * This is the common case when there are no blkio cgroups.
+         * Avoid lookup in this case
          */
-        if (bdi->dev) {
-                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
-                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg, (void *)cfqd,
-                                        MKDEV(major, minor));
-        } else
-                cfq_blkiocg_add_blkio_group(blkcg, &cfqg->blkg, (void *)cfqd,
-                                        0);
-
-        cfqd->nr_blkcg_linked_grps++;
-        cfqg->weight = blkcg_get_weight(blkcg, cfqg->blkg.dev);
+        if (blkcg == &blkio_root_cgroup)
+                cfqg = &cfqd->root_group;
+        else
+                cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
 
-        /* Add group on cfqd list */
-        hlist_add_head(&cfqg->cfqd_node, &cfqd->cfqg_list);
+        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);
+        }
 
-done:
         return cfqg;
 }
 
@@ -1082,13 +1104,53 @@ done:
 static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
 {
         struct blkio_cgroup *blkcg;
-        struct cfq_group *cfqg = NULL;
+        struct cfq_group *cfqg = NULL, *__cfqg = NULL;
+        struct request_queue *q = cfqd->queue;
+
+        rcu_read_lock();
+        blkcg = task_blkio_cgroup(current);
+        cfqg = cfq_find_cfqg(cfqd, blkcg);
+        if (cfqg) {
+                rcu_read_unlock();
+                return cfqg;
+        }
+
+        /*
+         * 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);
+
+        spin_lock_irq(q->queue_lock);
 
         rcu_read_lock();
         blkcg = task_blkio_cgroup(current);
-        cfqg = cfq_find_alloc_cfqg(cfqd, blkcg);
+
+        /*
+         * 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) {
+                kfree(cfqg);
+                rcu_read_unlock();
+                return __cfqg;
+        }
+
         if (!cfqg)
                 cfqg = &cfqd->root_group;
+
+        cfq_init_add_cfqg_lists(cfqd, cfqg, blkcg);
         rcu_read_unlock();
         return cfqg;
 }