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blkcg: drop unnecessary RCU locking 

Now that blkg additions / removals are always done under both q and
blkcg locks, the only places RCU locking is necessary are
blkg_lookup[_create]() for lookup w/o blkcg lock.  This patch drops
unncessary RCU locking replacing it with plain blkcg locking as
necessary.

* blkiocg_pre_destroy() already perform proper locking and don't need
  RCU.  Dropped.

* blkio_read_blkg_stats() now uses blkcg->lock instead of RCU read
  lock.  This isn't a hot path.

* Now unnecessary synchronize_rcu() from queue exit paths removed.
  This makes q->nr_blkgs unnecessary.  Dropped.

* RCU annotation on blkg->q removed.

-v2: Vivek pointed out that blkg_lookup_create() still needs to be
     called under rcu_read_lock().  Updated.

-v3: After the update, stats_lock locking in blkio_read_blkg_stats()
     shouldn't be using _irq variant as it otherwise ends up enabling
     irq while blkcg->lock is locked.  Fixed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
hifive-unleashed-5.1
Tejun Heo 2012-03-05 13:15:22 -08:00 committed by Jens Axboe
parent 9f13ef678e
commit c875f4d025
5 changed files with 12 additions and 74 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
block/blk-cgroup.c
View File

@ -500,7 +500,7 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
return NULL;
spin_lock_init(&blkg->stats_lock);
rcu_assign_pointer(blkg->q, q);
blkg->q = q;
INIT_LIST_HEAD(&blkg->q_node);
blkg->blkcg = blkcg;
blkg->refcnt = 1;
@ -611,7 +611,6 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
list_add(&blkg->q_node, &q->blkg_list);
q->nr_blkgs++;
spin_unlock(&blkcg->lock);
out:
@ -648,9 +647,6 @@ static void blkg_destroy(struct blkio_group *blkg)
list_del_init(&blkg->q_node);
hlist_del_init_rcu(&blkg->blkcg_node);
WARN_ON_ONCE(q->nr_blkgs <= 0);
q->nr_blkgs--;
/*
* Put the reference taken at the time of creation so that when all
* queues are gone, group can be destroyed.
@ -1232,8 +1228,9 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
struct hlist_node *n;
uint64_t cgroup_total = 0;
rcu_read_lock();
hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node) {
spin_lock_irq(&blkcg->lock);
hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node) {
const char *dname = blkg_dev_name(blkg);
int plid = BLKIOFILE_POLICY(cft->private);
@ -1243,15 +1240,16 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
cgroup_total += blkio_get_stat_cpu(blkg, plid,
cb, dname, type);
} else {
spin_lock_irq(&blkg->stats_lock);
spin_lock(&blkg->stats_lock);
cgroup_total += blkio_get_stat(blkg, plid,
cb, dname, type);
spin_unlock_irq(&blkg->stats_lock);
spin_unlock(&blkg->stats_lock);
}
}
if (show_total)
cb->fill(cb, "Total", cgroup_total);
rcu_read_unlock();
spin_unlock_irq(&blkcg->lock);
return 0;
}
@ -1583,28 +1581,24 @@ static int blkiocg_pre_destroy(struct cgroup_subsys *subsys,
{
struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
rcu_read_lock();
spin_lock_irq(&blkcg->lock);
while (!hlist_empty(&blkcg->blkg_list)) {
struct blkio_group *blkg = hlist_entry(blkcg->blkg_list.first,
struct blkio_group, blkcg_node);
struct request_queue *q = rcu_dereference(blkg->q);
struct request_queue *q = blkg->q;
if (spin_trylock(q->queue_lock)) {
blkg_destroy(blkg);
spin_unlock(q->queue_lock);
} else {
spin_unlock_irq(&blkcg->lock);
rcu_read_unlock();
cpu_relax();
rcu_read_lock();
spin_lock(&blkcg->lock);
}
}
spin_unlock_irq(&blkcg->lock);
rcu_read_unlock();
return 0;
}

4
block/blk-cgroup.h
View File

@ -176,8 +176,8 @@ struct blkg_policy_data {
};
struct blkio_group {
/* Pointer to the associated request_queue, RCU protected */
struct request_queue __rcu *q;
/* Pointer to the associated request_queue */
struct request_queue *q;
struct list_head q_node;
struct hlist_node blkcg_node;
struct blkio_cgroup *blkcg;

33
block/blk-throttle.c
View File

@ -1046,39 +1046,8 @@ int blk_throtl_init(struct request_queue *q)
void blk_throtl_exit(struct request_queue *q)
{
struct throtl_data *td = q->td;
bool wait;
BUG_ON(!td);
BUG_ON(!q->td);
throtl_shutdown_wq(q);
/* If there are other groups */
spin_lock_irq(q->queue_lock);
wait = q->nr_blkgs;
spin_unlock_irq(q->queue_lock);
/*
* Wait for tg_to_blkg(tg)->q accessors to exit their grace periods.
* Do this wait only if there are other undestroyed groups out
* there (other than root group). This can happen if cgroup deletion
* path claimed the responsibility of cleaning up a group before
* queue cleanup code get to the group.
*
* Do not call synchronize_rcu() unconditionally as there are drivers
* which create/delete request queue hundreds of times during scan/boot
* and synchronize_rcu() can take significant time and slow down boot.
*/
if (wait)
synchronize_rcu();
/*
* Just being safe to make sure after previous flush if some body did
* update limits through cgroup and another work got queued, cancel
* it.
*/
throtl_shutdown_wq(q);
kfree(q->td);
}

24
block/cfq-iosched.c
View File

@ -3449,7 +3449,6 @@ static void cfq_exit_queue(struct elevator_queue *e)
{
struct cfq_data *cfqd = e->elevator_data;
struct request_queue *q = cfqd->queue;
bool wait = false;
cfq_shutdown_timer_wq(cfqd);
@ -3462,31 +3461,8 @@ static void cfq_exit_queue(struct elevator_queue *e)
spin_unlock_irq(q->queue_lock);
#ifdef CONFIG_BLK_CGROUP
/*
* 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;
spin_unlock_irq(q->queue_lock);
#endif
cfq_shutdown_timer_wq(cfqd);
/*
* Wait for cfqg->blkg->key accessors to exit their grace periods.
* Do this wait only if there are other unlinked groups out
* there. This can happen if cgroup deletion path claimed the
* responsibility of cleaning up a group before queue cleanup code
* get to the group.
*
* Do not call synchronize_rcu() unconditionally as there are drivers
* which create/delete request queue hundreds of times during scan/boot
* and synchronize_rcu() can take significant time and slow down boot.
*/
if (wait)
synchronize_rcu();
#ifndef CONFIG_CFQ_GROUP_IOSCHED
kfree(cfqd->root_group);
#endif

1
include/linux/blkdev.h
View File

@ -365,7 +365,6 @@ struct request_queue {
#ifdef CONFIG_BLK_CGROUP
/* XXX: array size hardcoded to avoid include dependency (temporary) */
struct list_head blkg_list;
int nr_blkgs;
#endif
struct queue_limits limits;