blkcg: shoot down blkio_groups on elevator switch

Elevator switch may involve changes to blkcg policies.  Implement
shoot down of blkio_groups.

Combined with the previous bypass updates, the end goal is updating
blkcg core such that it can ensure that blkcg's being affected become
quiescent and don't have any per-blkg data hanging around before
commencing any policy updates.  Until queues are made aware of the
policies that applies to them, as an interim step, all per-policy blkg
data will be shot down.

* blk-throtl doesn't need this change as it can't be disabled for a
  live queue; however, update it anyway as the scheduled blkg
  unification requires this behavior change.  This means that
  blk-throtl configuration will be unnecessarily lost over elevator
  switch.  This oddity will be removed after blkcg learns to associate
  individual policies with request_queues.

* blk-throtl dosen't shoot down root_tg.  This is to ease transition.
  Unified blkg will always have persistent root group and not shooting
  down root_tg for now eases transition to that point by avoiding
  having to update td->root_tg and is safe as blk-throtl can never be
  disabled

-v2: Vivek pointed out that group list is not guaranteed to be empty
     on return from clear function if it raced cgroup removal and
     lost.  Fix it by waiting a bit and retrying.  This kludge will
     soon be removed once locking is updated such that blkg is never
     in limbo state between blkcg and request_queue locks.

     blk-throtl no longer shoots down root_tg to avoid breaking
     td->root_tg.

     Also, Nest queue_lock inside blkio_list_lock not the other way
     around to avoid introduce possible deadlock via blkcg lock.

-v3: blkcg_clear_queue() repositioned and renamed to
     blkg_destroy_all() to increase consistency with later changes.
     cfq_clear_queue() updated to check q->elevator before
     dereferencing it to avoid NULL dereference on not fully
     initialized queues (used by later change).

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

5 files changed, 84 insertions, 5 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 970a717a056f..159aef59589f 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -17,8 +17,9 @@
 #include <linux/err.h>
 #include <linux/blkdev.h>
 #include <linux/slab.h>
-#include "blk-cgroup.h"
 #include <linux/genhd.h>
+#include <linux/delay.h>
+#include "blk-cgroup.h"
 
 #define MAX_KEY_LEN 100
 
@@ -546,6 +547,37 @@ struct blkio_group *blkiocg_lookup_group(struct blkio_cgroup *blkcg, void *key)
 }
 EXPORT_SYMBOL_GPL(blkiocg_lookup_group);
 
+void blkg_destroy_all(struct request_queue *q)
+{
+        struct blkio_policy_type *pol;
+
+        while (true) {
+                bool done = true;
+
+                spin_lock(&blkio_list_lock);
+                spin_lock_irq(q->queue_lock);
+
+                /*
+                 * clear_queue_fn() might return with non-empty group list
+                 * if it raced cgroup removal and lost.  cgroup removal is
+                 * guaranteed to make forward progress and retrying after a
+                 * while is enough.  This ugliness is scheduled to be
+                 * removed after locking update.
+                 */
+                list_for_each_entry(pol, &blkio_list, list)
+                        if (!pol->ops.blkio_clear_queue_fn(q))
+                                done = false;
+
+                spin_unlock_irq(q->queue_lock);
+                spin_unlock(&blkio_list_lock);
+
+                if (done)
+                        break;
+
+                msleep(10);     /* just some random duration I like */
+        }
+}
+
 static void blkio_reset_stats_cpu(struct blkio_group *blkg)
 {
         struct blkio_group_stats_cpu *stats_cpu;
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 355168772f51..e5cfcbd4d2f4 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -203,7 +203,7 @@ extern unsigned int blkcg_get_write_iops(struct blkio_cgroup *blkcg,
                                      dev_t dev);
 
 typedef void (blkio_unlink_group_fn) (void *key, struct blkio_group *blkg);
-
+typedef bool (blkio_clear_queue_fn)(struct request_queue *q);
 typedef void (blkio_update_group_weight_fn) (void *key,
                         struct blkio_group *blkg, unsigned int weight);
 typedef void (blkio_update_group_read_bps_fn) (void * key,
@@ -217,6 +217,7 @@ typedef void (blkio_update_group_write_iops_fn) (void *key,
 
 struct blkio_policy_ops {
         blkio_unlink_group_fn *blkio_unlink_group_fn;
+        blkio_clear_queue_fn *blkio_clear_queue_fn;
         blkio_update_group_weight_fn *blkio_update_group_weight_fn;
         blkio_update_group_read_bps_fn *blkio_update_group_read_bps_fn;
         blkio_update_group_write_bps_fn *blkio_update_group_write_bps_fn;
@@ -233,6 +234,7 @@ struct blkio_policy_type {
 /* Blkio controller policy registration */
 extern void blkio_policy_register(struct blkio_policy_type *);
 extern void blkio_policy_unregister(struct blkio_policy_type *);
+extern void blkg_destroy_all(struct request_queue *q);
 
 static inline char *blkg_path(struct blkio_group *blkg)
 {
@@ -249,6 +251,7 @@ struct blkio_policy_type {
 
 static inline void blkio_policy_register(struct blkio_policy_type *blkiop) { }
 static inline void blkio_policy_unregister(struct blkio_policy_type *blkiop) { }
+static inline void blkg_destroy_all(struct request_queue *q) { }
 
 static inline char *blkg_path(struct blkio_group *blkg) { return NULL; }
 
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 702c0e64e09f..3699ab40d494 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -989,12 +989,17 @@ throtl_destroy_tg(struct throtl_data *td, struct throtl_grp *tg)
         td->nr_undestroyed_grps--;
 }
 
-static void throtl_release_tgs(struct throtl_data *td)
+static bool throtl_release_tgs(struct throtl_data *td, bool release_root)
 {
         struct hlist_node *pos, *n;
         struct throtl_grp *tg;
+        bool empty = true;
 
         hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
+                /* skip root? */
+                if (!release_root && tg == td->root_tg)
+                        continue;
+
                 /*
                  * If cgroup removal path got to blk_group first and removed
                  * it from cgroup list, then it will take care of destroying
@@ -1002,7 +1007,10 @@ static void throtl_release_tgs(struct throtl_data *td)
                  */
                 if (!blkiocg_del_blkio_group(&tg->blkg))
                         throtl_destroy_tg(td, tg);
+                else
+                        empty = false;
         }
+        return empty;
 }
 
 /*
@@ -1029,6 +1037,20 @@ void throtl_unlink_blkio_group(void *key, struct blkio_group *blkg)
         spin_unlock_irqrestore(td->queue->queue_lock, flags);
 }
 
+static bool throtl_clear_queue(struct request_queue *q)
+{
+        lockdep_assert_held(q->queue_lock);
+
+        /*
+         * Clear tgs but leave the root one alone.  This is necessary
+         * because root_tg is expected to be persistent and safe because
+         * blk-throtl can never be disabled while @q is alive.  This is a
+         * kludge to prepare for unified blkg.  This whole function will be
+         * removed soon.
+         */
+        return throtl_release_tgs(q->td, false);
+}
+
 static void throtl_update_blkio_group_common(struct throtl_data *td,
                                 struct throtl_grp *tg)
 {
@@ -1097,6 +1119,7 @@ static void throtl_shutdown_wq(struct request_queue *q)
 static struct blkio_policy_type blkio_policy_throtl = {
         .ops = {
                 .blkio_unlink_group_fn = throtl_unlink_blkio_group,
+                .blkio_clear_queue_fn = throtl_clear_queue,
                 .blkio_update_group_read_bps_fn =
                                         throtl_update_blkio_group_read_bps,
                 .blkio_update_group_write_bps_fn =
@@ -1282,7 +1305,7 @@ void blk_throtl_exit(struct request_queue *q)
         throtl_shutdown_wq(q);
 
         spin_lock_irq(q->queue_lock);
-        throtl_release_tgs(td);
+        throtl_release_tgs(td, true);
 
         /* If there are other groups */
         if (td->nr_undestroyed_grps > 0)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 72680a6715fc..61693d3404d0 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1225,10 +1225,11 @@ static void cfq_destroy_cfqg(struct cfq_data *cfqd, struct cfq_group *cfqg)
         cfq_put_cfqg(cfqg);
 }
 
-static void cfq_release_cfq_groups(struct cfq_data *cfqd)
+static bool cfq_release_cfq_groups(struct cfq_data *cfqd)
 {
         struct hlist_node *pos, *n;
         struct cfq_group *cfqg;
+        bool empty = true;
 
         hlist_for_each_entry_safe(cfqg, pos, n, &cfqd->cfqg_list, cfqd_node) {
                 /*
@@ -1238,7 +1239,10 @@ static void cfq_release_cfq_groups(struct cfq_data *cfqd)
                  */
                 if (!cfq_blkiocg_del_blkio_group(&cfqg->blkg))
                         cfq_destroy_cfqg(cfqd, cfqg);
+                else
+                        empty = false;
         }
+        return empty;
 }
 
 /*
@@ -1265,6 +1269,19 @@ static void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
         spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
 }
 
+static struct elevator_type iosched_cfq;
+
+static bool cfq_clear_queue(struct request_queue *q)
+{
+        lockdep_assert_held(q->queue_lock);
+
+        /* shoot down blkgs iff the current elevator is cfq */
+        if (!q->elevator || q->elevator->type != &iosched_cfq)
+                return true;
+
+        return cfq_release_cfq_groups(q->elevator->elevator_data);
+}
+
 #else /* GROUP_IOSCHED */
 static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
 {
@@ -3875,6 +3892,7 @@ static struct elevator_type iosched_cfq = {
 static struct blkio_policy_type blkio_policy_cfq = {
         .ops = {
                 .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,
         },
         .plid = BLKIO_POLICY_PROP,
diff --git a/block/elevator.c b/block/elevator.c
index 0bdea0ed03a3..8c7561fd2c79 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -38,6 +38,7 @@
 #include <trace/events/block.h>
 
 #include "blk.h"
+#include "blk-cgroup.h"
 
 static DEFINE_SPINLOCK(elv_list_lock);
 static LIST_HEAD(elv_list);
@@ -894,6 +895,8 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
         ioc_clear_queue(q);
         spin_unlock_irq(q->queue_lock);
 
+        blkg_destroy_all(q);
+
         /* allocate, init and register new elevator */
         err = -ENOMEM;
         q->elevator = elevator_alloc(q, new_e);