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Subject[PATCH 11/21] blkio: Take care of cgroup deletion and cfq group reference counting
Date
o One can choose to change elevator or delete a cgroup. Implement group
reference counting so that both elevator exit and cgroup deletion can
take place gracefully.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Nauman Rafique <nauman@google.com>
---
block/blk-cgroup.c | 66 ++++++++++++++++++++++++++++++++++-
block/blk-cgroup.h | 1 +
block/cfq-iosched.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 160 insertions(+), 2 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 4f6afd7..0426ab6 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -13,6 +13,8 @@
#include <linux/ioprio.h>
#include "blk-cgroup.h"

+extern void cfq_unlink_blkio_group(void *, struct blkio_group *);
+
struct blkio_cgroup blkio_root_cgroup = { .weight = 2*BLKIO_WEIGHT_DEFAULT };

struct blkio_cgroup *cgroup_to_blkio_cgroup(struct cgroup *cgroup)
@@ -28,14 +30,43 @@ void blkiocg_add_blkio_group(struct blkio_cgroup *blkcg,

spin_lock_irqsave(&blkcg->lock, flags);
rcu_assign_pointer(blkg->key, key);
+ blkg->blkcg_id = css_id(&blkcg->css);
hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
spin_unlock_irqrestore(&blkcg->lock, flags);
}

+static void __blkiocg_del_blkio_group(struct blkio_group *blkg)
+{
+ hlist_del_init_rcu(&blkg->blkcg_node);
+ blkg->blkcg_id = 0;
+}
+
+/*
+ * returns 0 if blkio_group was still on cgroup list. Otherwise returns 1
+ * indicating that blk_group was unhashed by the time we got to it.
+ */
int blkiocg_del_blkio_group(struct blkio_group *blkg)
{
- /* Implemented later */
- return 0;
+ struct blkio_cgroup *blkcg;
+ unsigned long flags;
+ struct cgroup_subsys_state *css;
+ int ret = 1;
+
+ rcu_read_lock();
+ css = css_lookup(&blkio_subsys, blkg->blkcg_id);
+ if (!css)
+ goto out;
+
+ blkcg = container_of(css, struct blkio_cgroup, css);
+ spin_lock_irqsave(&blkcg->lock, flags);
+ if (!hlist_unhashed(&blkg->blkcg_node)) {
+ __blkiocg_del_blkio_group(blkg);
+ ret = 0;
+ }
+ spin_unlock_irqrestore(&blkcg->lock, flags);
+out:
+ rcu_read_unlock();
+ return ret;
}

/* called under rcu_read_lock(). */
@@ -97,8 +128,39 @@ static int blkiocg_populate(struct cgroup_subsys *subsys, struct cgroup *cgroup)
static void blkiocg_destroy(struct cgroup_subsys *subsys, struct cgroup *cgroup)
{
struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
+ unsigned long flags;
+ struct blkio_group *blkg;
+ void *key;

+ rcu_read_lock();
+remove_entry:
+ spin_lock_irqsave(&blkcg->lock, flags);
+
+ if (hlist_empty(&blkcg->blkg_list)) {
+ spin_unlock_irqrestore(&blkcg->lock, flags);
+ goto done;
+ }
+
+ blkg = hlist_entry(blkcg->blkg_list.first, struct blkio_group,
+ blkcg_node);
+ key = rcu_dereference(blkg->key);
+ __blkiocg_del_blkio_group(blkg);
+
+ spin_unlock_irqrestore(&blkcg->lock, flags);
+
+ /*
+ * This blkio_group is being unlinked as associated cgroup is going
+ * away. Let all the IO controlling policies know about this event.
+ *
+ * Currently this is static call to one io controlling policy. Once
+ * we have more policies in place, we need some dynamic registration
+ * of callback function.
+ */
+ cfq_unlink_blkio_group(key, blkg);
+ goto remove_entry;
+done:
free_css_id(&blkio_subsys, &blkcg->css);
+ rcu_read_unlock();
kfree(blkcg);
}

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index ba5703f..cd50a2f 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -26,6 +26,7 @@ struct blkio_group {
/* An rcu protected unique identifier for the group */
void *key;
struct hlist_node blkcg_node;
+ unsigned short blkcg_id;
};

#define BLKIO_WEIGHT_MIN 100
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 3706f19..af3d569 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -191,6 +191,7 @@ struct cfq_group {
struct blkio_group blkg;
#ifdef CONFIG_CFQ_GROUP_IOSCHED
struct hlist_node cfqd_node;
+ atomic_t ref;
#endif
};

@@ -926,6 +927,14 @@ cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
*st = CFQ_RB_ROOT;
RB_CLEAR_NODE(&cfqg->rb_node);

+ /*
+ * Take the initial reference that will be released on destroy
+ * This can be thought of a joint reference by cgroup and
+ * elevator which will be dropped by either elevator exit
+ * or cgroup deletion path depending on who is exiting first.
+ */
+ atomic_set(&cfqg->ref, 1);
+
/* Add group onto cgroup list */
blkiocg_add_blkio_group(blkcg, &cfqg->blkg, (void *)cfqd);

@@ -962,7 +971,77 @@ static void cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg)
cfqg = &cfqq->cfqd->root_group;

cfqq->cfqg = cfqg;
+ /* cfqq reference on cfqg */
+ atomic_inc(&cfqq->cfqg->ref);
+}
+
+static void cfq_put_cfqg(struct cfq_group *cfqg)
+{
+ struct cfq_rb_root *st;
+ int i, j;
+
+ BUG_ON(atomic_read(&cfqg->ref) <= 0);
+ if (!atomic_dec_and_test(&cfqg->ref))
+ return;
+ for_each_cfqg_st(cfqg, i, j, st)
+ BUG_ON(!RB_EMPTY_ROOT(&st->rb) || st->active != NULL);
+ kfree(cfqg);
+}
+
+static void cfq_destroy_cfqg(struct cfq_data *cfqd, struct cfq_group *cfqg)
+{
+ /* Something wrong if we are trying to remove same group twice */
+ BUG_ON(hlist_unhashed(&cfqg->cfqd_node));
+
+ hlist_del_init(&cfqg->cfqd_node);
+
+ /*
+ * Put the reference taken at the time of creation so that when all
+ * queues are gone, group can be destroyed.
+ */
+ cfq_put_cfqg(cfqg);
+}
+
+static void cfq_release_cfq_groups(struct cfq_data *cfqd)
+{
+ struct hlist_node *pos, *n;
+ struct cfq_group *cfqg;
+
+ hlist_for_each_entry_safe(cfqg, pos, n, &cfqd->cfqg_list, cfqd_node) {
+ /*
+ * 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(&cfqg->blkg))
+ cfq_destroy_cfqg(cfqd, cfqg);
+ }
}
+
+/*
+ * 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 "key" is a valid cfq_data pointer as long as we are rcu
+ * read lock.
+ *
+ * "key" 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.
+ */
+void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
+{
+ unsigned long flags;
+ struct cfq_data *cfqd = key;
+
+ spin_lock_irqsave(cfqd->queue->queue_lock, flags);
+ cfq_destroy_cfqg(cfqd, cfqg_of_blkg(blkg));
+ spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
+}
+
#else /* GROUP_IOSCHED */
static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd, int create)
{
@@ -973,6 +1052,9 @@ 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) {}
+static inline void cfq_put_cfqg(struct cfq_group *cfqg) {}
+
#endif /* GROUP_IOSCHED */

/*
@@ -2174,11 +2256,13 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
* task holds one reference to the queue, dropped when task exits. each rq
* in-flight on this queue also holds a reference, dropped when rq is freed.
*
+ * Each cfq queue took a reference on the parent group. Drop it now.
* queue lock must be held here.
*/
static void cfq_put_queue(struct cfq_queue *cfqq)
{
struct cfq_data *cfqd = cfqq->cfqd;
+ struct cfq_group *cfqg;

BUG_ON(atomic_read(&cfqq->ref) <= 0);

@@ -2188,6 +2272,7 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
cfq_log_cfqq(cfqd, cfqq, "put_queue");
BUG_ON(rb_first(&cfqq->sort_list));
BUG_ON(cfqq->allocated[READ] + cfqq->allocated[WRITE]);
+ cfqg = cfqq->cfqg;

if (unlikely(cfqd->active_queue == cfqq)) {
__cfq_slice_expired(cfqd, cfqq, 0);
@@ -2196,6 +2281,7 @@ static void cfq_put_queue(struct cfq_queue *cfqq)

BUG_ON(cfq_cfqq_on_rr(cfqq));
kmem_cache_free(cfq_pool, cfqq);
+ cfq_put_cfqg(cfqg);
}

/*
@@ -3371,11 +3457,15 @@ static void cfq_exit_queue(struct elevator_queue *e)
}

cfq_put_async_queues(cfqd);
+ cfq_release_cfq_groups(cfqd);
+ blkiocg_del_blkio_group(&cfqd->root_group.blkg);

spin_unlock_irq(q->queue_lock);

cfq_shutdown_timer_wq(cfqd);

+ /* Wait for cfqg->blkg->key accessors to exit their grace periods. */
+ synchronize_rcu();
kfree(cfqd);
}

@@ -3403,6 +3493,11 @@ static void *cfq_init_queue(struct request_queue *q)
cfqg->weight = 2*BLKIO_WEIGHT_DEFAULT;

#ifdef CONFIG_CFQ_GROUP_IOSCHED
+ /*
+ * Take a reference to root group which we never drop. This is just
+ * to make sure that cfq_put_cfqg() does not try to kfree root group
+ */
+ atomic_set(&cfqg->ref, 1);
blkiocg_add_blkio_group(&blkio_root_cgroup, &cfqg->blkg, (void *)cfqd);
#endif
/*
--
1.6.2.5


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