[PATCH 10/11] blkcg: let blkcg core manage per-queue blkg list and counter

From: Tejun Heo
Date: Wed Feb 01 2012 - 16:21:43 EST

With the previous patch to move blkg list heads and counters to
request_queue and blkg, logic to manage them in both policies are
almost identical and can be moved to blkcg core.

This patch moves blkg link logic into blkg_lookup_create(), implements
common blkg unlink code in blkg_destroy(), and updates
blkg_destory_all() so that it's policy specific and can skip root
group. The updated blkg_destroy_all() is now used to both clear queue
for bypassing and elv switching, and release all blkgs on q exit.

This patch introduces a race window where policy [de]registration may
race against queue blkg clearing. This can only be a problem on cfq
unload and shouldn't be a real problem in practice (and we have many
other places where this race already exists). Future patches will
remove these unlikely races.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
---
block/blk-cgroup.c | 72 ++++++++++++++++++++++++++++--------
block/blk-cgroup.h | 15 +++-----
block/blk-throttle.c | 99 +-------------------------------------------------
block/cfq-iosched.c | 98 +++-----------------------------------------------
block/elevator.c | 5 ++-
5 files changed, 71 insertions(+), 218 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 0513ed5..a7f9363 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -596,8 +596,11 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
/* insert */
spin_lock(&blkcg->lock);
swap(blkg, new_blkg);
+
hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
- pol->ops.blkio_link_group_fn(q, blkg);
+ list_add(&blkg->q_node[plid], &q->blkg_list[plid]);
+ q->nr_blkgs[plid]++;
+
spin_unlock(&blkcg->lock);
out:
blkg_free(new_blkg);
@@ -646,36 +649,69 @@ struct blkio_group *blkg_lookup(struct blkio_cgroup *blkcg,
}
EXPORT_SYMBOL_GPL(blkg_lookup);

-void blkg_destroy_all(struct request_queue *q)
+static void blkg_destroy(struct blkio_group *blkg, enum blkio_policy_id plid)
+{
+ struct request_queue *q = blkg->q;
+
+ lockdep_assert_held(q->queue_lock);
+
+ /* Something wrong if we are trying to remove same group twice */
+ WARN_ON_ONCE(list_empty(&blkg->q_node[plid]));
+ list_del_init(&blkg->q_node[plid]);
+
+ WARN_ON_ONCE(q->nr_blkgs[plid] <= 0);
+ q->nr_blkgs[plid]--;
+
+ /*
+ * Put the reference taken at the time of creation so that when all
+ * queues are gone, group can be destroyed.
+ */
+ blkg_put(blkg);
+}
+
+void blkg_destroy_all(struct request_queue *q, enum blkio_policy_id plid,
+ bool destroy_root)
{
- struct blkio_policy_type *pol;
+ struct blkio_group *blkg, *n;

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))
+ list_for_each_entry_safe(blkg, n, &q->blkg_list[plid],
+ q_node[plid]) {
+ /* skip root? */
+ if (!destroy_root && blkg->blkcg == &blkio_root_cgroup)
+ continue;
+
+ /*
+ * If cgroup removal path got to blk_group first
+ * and removed it from cgroup list, then it will
+ * take care of destroying cfqg also.
+ */
+ if (!blkiocg_del_blkio_group(blkg))
+ blkg_destroy(blkg, plid);
+ else
done = false;
+ }

spin_unlock_irq(q->queue_lock);
- spin_unlock(&blkio_list_lock);

+ /*
+ * Group list may not be empty if we 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.
+ */
if (done)
break;

msleep(10); /* just some random duration I like */
}
}
+EXPORT_SYMBOL_GPL(blkg_destroy_all);

static void blkg_rcu_free(struct rcu_head *rcu_head)
{
@@ -1538,11 +1574,13 @@ static int blkiocg_pre_destroy(struct cgroup_subsys *subsys,
* this event.
*/
spin_lock(&blkio_list_lock);
+ spin_lock_irqsave(q->queue_lock, flags);
list_for_each_entry(blkiop, &blkio_list, list) {
if (blkiop->plid != blkg->plid)
continue;
- blkiop->ops.blkio_unlink_group_fn(q, blkg);
+ blkg_destroy(blkg, blkiop->plid);
}
+ spin_unlock_irqrestore(q->queue_lock, flags);
spin_unlock(&blkio_list_lock);
} while (1);

@@ -1684,12 +1722,14 @@ static void blkcg_bypass_start(void)
__acquires(&all_q_mutex)
{
struct request_queue *q;
+ int i;

mutex_lock(&all_q_mutex);

list_for_each_entry(q, &all_q_list, all_q_node) {
blk_queue_bypass_start(q);
- blkg_destroy_all(q);
+ for (i = 0; i < BLKIO_NR_POLICIES; i++)
+ blkg_destroy_all(q, i, false);
}
}

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index ae96f19..83ce5fa 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -196,11 +196,6 @@ struct blkio_group {
};

typedef void (blkio_init_group_fn)(struct blkio_group *blkg);
-typedef void (blkio_link_group_fn)(struct request_queue *q,
- struct blkio_group *blkg);
-typedef void (blkio_unlink_group_fn)(struct request_queue *q,
- struct blkio_group *blkg);
-typedef bool (blkio_clear_queue_fn)(struct request_queue *q);
typedef void (blkio_update_group_weight_fn)(struct request_queue *q,
struct blkio_group *blkg, unsigned int weight);
typedef void (blkio_update_group_read_bps_fn)(struct request_queue *q,
@@ -214,9 +209,6 @@ typedef void (blkio_update_group_write_iops_fn)(struct request_queue *q,

struct blkio_policy_ops {
blkio_init_group_fn *blkio_init_group_fn;
- blkio_link_group_fn *blkio_link_group_fn;
- 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;
@@ -238,7 +230,8 @@ extern void blkcg_exit_queue(struct request_queue *q);
/* 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);
+extern void blkg_destroy_all(struct request_queue *q,
+ enum blkio_policy_id plid, bool destroy_root);

/**
* blkg_to_pdata - get policy private data
@@ -319,7 +312,9 @@ static inline void blkcg_drain_queue(struct request_queue *q) { }
static inline void blkcg_exit_queue(struct request_queue *q) { }
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 void blkg_destroy_all(struct request_queue *q,
+ enum blkio_policy_id plid,
+ bool destory_root) { }

static inline void *blkg_to_pdata(struct blkio_group *blkg,
struct blkio_policy_type *pol) { return NULL; }
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index c15d383..1329412 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -157,14 +157,6 @@ static void throtl_init_blkio_group(struct blkio_group *blkg)
tg->iops[WRITE] = -1;
}

-static void throtl_link_blkio_group(struct request_queue *q,
- struct blkio_group *blkg)
-{
- list_add(&blkg->q_node[BLKIO_POLICY_THROTL],
- &q->blkg_list[BLKIO_POLICY_THROTL]);
- q->nr_blkgs[BLKIO_POLICY_THROTL]++;
-}
-
static struct
throtl_grp *throtl_lookup_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
{
@@ -813,89 +805,6 @@ throtl_schedule_delayed_work(struct throtl_data *td, unsigned long delay)
}
}

-static void
-throtl_destroy_tg(struct throtl_data *td, struct throtl_grp *tg)
-{
- struct blkio_group *blkg = tg_to_blkg(tg);
-
- /* Something wrong if we are trying to remove same group twice */
- WARN_ON_ONCE(list_empty(&blkg->q_node[BLKIO_POLICY_THROTL]));
-
- list_del_init(&blkg->q_node[BLKIO_POLICY_THROTL]);
-
- /*
- * Put the reference taken at the time of creation so that when all
- * queues are gone, group can be destroyed.
- */
- blkg_put(tg_to_blkg(tg));
- td->queue->nr_blkgs[BLKIO_POLICY_THROTL]--;
-}
-
-static bool throtl_release_tgs(struct throtl_data *td, bool release_root)
-{
- struct request_queue *q = td->queue;
- struct blkio_group *blkg, *n;
- bool empty = true;
-
- list_for_each_entry_safe(blkg, n, &q->blkg_list[BLKIO_POLICY_THROTL],
- q_node[BLKIO_POLICY_THROTL]) {
- struct throtl_grp *tg = blkg_to_tg(blkg);
-
- /* 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
- * cfqg also.
- */
- if (!blkiocg_del_blkio_group(blkg))
- throtl_destroy_tg(td, tg);
- else
- empty = false;
- }
- return empty;
-}
-
-/*
- * Blk cgroup controller notification saying that blkio_group object is being
- * delinked as associated cgroup object is going away. That also means that
- * no new IO will come in this group. So get rid of this group as soon as
- * any pending IO in the group is finished.
- *
- * This function is called under rcu_read_lock(). @q is the rcu protected
- * pointer. That means @q is a valid request_queue pointer as long as we
- * are rcu read lock.
- *
- * @q was fetched from blkio_group under blkio_cgroup->lock. That means
- * it should not be NULL as even if queue was going away, cgroup deltion
- * path got to it first.
- */
-void throtl_unlink_blkio_group(struct request_queue *q,
- struct blkio_group *blkg)
-{
- unsigned long flags;
-
- spin_lock_irqsave(q->queue_lock, flags);
- throtl_destroy_tg(q->td, blkg_to_tg(blkg));
- spin_unlock_irqrestore(q->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)
{
@@ -960,9 +869,6 @@ static void throtl_shutdown_wq(struct request_queue *q)
static struct blkio_policy_type blkio_policy_throtl = {
.ops = {
.blkio_init_group_fn = throtl_init_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 =
throtl_update_blkio_group_read_bps,
.blkio_update_group_write_bps_fn =
@@ -1148,12 +1054,11 @@ void blk_throtl_exit(struct request_queue *q)

throtl_shutdown_wq(q);

- spin_lock_irq(q->queue_lock);
- throtl_release_tgs(td, true);
+ 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];
-
spin_unlock_irq(q->queue_lock);

/*
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index c13e376..7093309 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1045,14 +1045,6 @@ static void cfq_update_blkio_group_weight(struct request_queue *q,
cfqg->needs_update = true;
}

-static void cfq_link_blkio_group(struct request_queue *q,
- struct blkio_group *blkg)
-{
- list_add(&blkg->q_node[BLKIO_POLICY_PROP],
- &q->blkg_list[BLKIO_POLICY_PROP]);
- q->nr_blkgs[BLKIO_POLICY_PROP]++;
-}
-
static void cfq_init_blkio_group(struct blkio_group *blkg)
{
struct cfq_group *cfqg = blkg_to_cfqg(blkg);
@@ -1096,84 +1088,6 @@ static void cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg)
blkg_get(cfqg_to_blkg(cfqg));
}

-static void cfq_destroy_cfqg(struct cfq_data *cfqd, struct cfq_group *cfqg)
-{
- struct blkio_group *blkg = cfqg_to_blkg(cfqg);
-
- /* Something wrong if we are trying to remove same group twice */
- BUG_ON(list_empty(&blkg->q_node[BLKIO_POLICY_PROP]));
-
- list_del_init(&blkg->q_node[BLKIO_POLICY_PROP]);
-
- BUG_ON(cfqd->queue->nr_blkgs[BLKIO_POLICY_PROP] <= 0);
- cfqd->queue->nr_blkgs[BLKIO_POLICY_PROP]--;
-
- /*
- * Put the reference taken at the time of creation so that when all
- * queues are gone, group can be destroyed.
- */
- blkg_put(cfqg_to_blkg(cfqg));
-}
-
-static bool cfq_release_cfq_groups(struct cfq_data *cfqd)
-{
- struct request_queue *q = cfqd->queue;
- struct blkio_group *blkg, *n;
- bool empty = true;
-
- list_for_each_entry_safe(blkg, n, &q->blkg_list[BLKIO_POLICY_PROP],
- q_node[BLKIO_POLICY_PROP]) {
- /*
- * If cgroup removal path got to blk_group first and removed
- * it from cgroup list, then it will take care of destroying
- * cfqg also.
- */
- if (!cfq_blkiocg_del_blkio_group(blkg))
- cfq_destroy_cfqg(cfqd, blkg_to_cfqg(blkg));
- else
- empty = false;
- }
- return empty;
-}
-
-/*
- * Blk cgroup controller notification saying that blkio_group object is being
- * delinked as associated cgroup object is going away. That also means that
- * no new IO will come in this group. So get rid of this group as soon as
- * any pending IO in the group is finished.
- *
- * This function is called under rcu_read_lock(). key is the rcu protected
- * pointer. That means @q is a valid request_queue pointer as long as we
- * are rcu read lock.
- *
- * @q was fetched from blkio_group under blkio_cgroup->lock. That means
- * it should not be NULL as even if elevator was exiting, cgroup deltion
- * path got to it first.
- */
-static void cfq_unlink_blkio_group(struct request_queue *q,
- struct blkio_group *blkg)
-{
- struct cfq_data *cfqd = q->elevator->elevator_data;
- unsigned long flags;
-
- spin_lock_irqsave(q->queue_lock, flags);
- cfq_destroy_cfqg(cfqd, blkg_to_cfqg(blkg));
- spin_unlock_irqrestore(q->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_lookup_create_cfqg(struct cfq_data *cfqd,
struct blkio_cgroup *blkcg)
@@ -1186,8 +1100,6 @@ cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg) {
cfqq->cfqg = cfqg;
}

-static void cfq_release_cfq_groups(struct cfq_data *cfqd) {}
-
#endif /* GROUP_IOSCHED */

/*
@@ -3553,14 +3465,17 @@ static void cfq_exit_queue(struct elevator_queue *e)
__cfq_slice_expired(cfqd, cfqd->active_queue, 0);

cfq_put_async_queues(cfqd);
- cfq_release_cfq_groups(cfqd);
+
+ spin_unlock_irq(q->queue_lock);
+
+ blkg_destroy_all(q, BLKIO_POLICY_PROP, true);

/*
* If there are groups which we could not unlink from blkcg list,
* wait for a rcu period for them to be freed.
*/
+ spin_lock_irq(q->queue_lock);
wait = q->nr_blkgs[BLKIO_POLICY_PROP];
-
spin_unlock_irq(q->queue_lock);

cfq_shutdown_timer_wq(cfqd);
@@ -3799,9 +3714,6 @@ static struct elevator_type iosched_cfq = {
static struct blkio_policy_type blkio_policy_cfq = {
.ops = {
.blkio_init_group_fn = cfq_init_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,
},
.plid = BLKIO_POLICY_PROP,
diff --git a/block/elevator.c b/block/elevator.c
index d6116af..4599615 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -923,7 +923,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
{
struct elevator_queue *old = q->elevator;
bool registered = old->registered;
- int err;
+ int i, err;

/*
* Turn on BYPASS and drain all requests w/ elevator private data.
@@ -942,7 +942,8 @@ 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);
+ for (i = 0; i < BLKIO_NR_POLICIES; i++)
+ blkg_destroy_all(q, i, false);

/* allocate, init and register new elevator */
err = -ENOMEM;
--
1.7.7.3

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