[PATCH 05/10] blkcg: consolidate blkg creation in blkcg_bio_issue_check()

Tejun Heo
2015-07-12T18:00:57Z

blkg (blkcg_gq) currently is created by blkcg policies invoking
blkg_lookup_create() which ends up repeating about the same code in
different policies. Theoretically, this can avoid the overhead of
looking and/or creating blkg's if blkcg is enabled but no policy is in
use; however, the cost of blkg lookup / creation is very low
especially if only the root blkcg is in use which is highly likely if
no blkcg policy is in active use - it boils down to a single very
predictable conditional and surrounding RCU protection.

This patch consolidates blkg creation to a new function
blkcg_bio_issue_check() which is called during bio issue from
generic_make_request_checks(). blkcg_bio_issue_check() is now the
only function which tries to create missing blkg's. The subsequent
policy and request_list operations just perform blkg_lookup() and if
missing falls back to the root.

* blk_get_rl() no longer tries to create blkg. It uses blkg_lookup()
instead of blkg_lookup_create().

* blk_throtl_bio() is now called from blkcg_bio_issue_check() with rcu
read locked and blkg already looked up. Both throtl_lookup_tg() and
throtl_lookup_create_tg() are dropped.

* cfq is similarly updated. cfq_lookup_create_cfqg() is dropped and
cfq_get_queue() now directly uses blkg_lookup() and falls back to
oom_cfqg on failure.

This consolidates blkg handling and avoids unnecessary blkg creation
retries under memory pressure. In addition, this provides a common
bio entry point into blkcg where things like common accounting can be
performed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
---
block/blk-cgroup.c | 2 +-
block/blk-core.c | 4 +--
block/blk-throttle.c | 72 ++++------------------------------------------
block/blk.h | 5 ----
block/cfq-iosched.c | 37 ++++--------------------
include/linux/blk-cgroup.h | 40 ++++++++++++++++++++++++--
6 files changed, 52 insertions(+), 108 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c817da0..5bab2a0 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -153,6 +153,7 @@ struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,

return NULL;
}
+EXPORT_SYMBOL_GPL(blkg_lookup_slowpath);

/*
* If @new_blkg is %NULL, this function tries to allocate a new one as
@@ -295,7 +296,6 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
return blkg;
}
}
-EXPORT_SYMBOL_GPL(blkg_lookup_create);

static void blkg_destroy(struct blkcg_gq *blkg)
{
diff --git a/block/blk-core.c b/block/blk-core.c
index 627ed0c..140094f 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1889,8 +1889,8 @@ generic_make_request_checks(struct bio *bio)
*/
create_io_context(GFP_ATOMIC, q->node);

- if (blk_throtl_bio(q, bio))
- return false; /* throttled, will be resubmitted later */
+ if (!blkcg_bio_issue_check(q, bio))
+ return false;

trace_block_bio_queue(q, bio);
return true;
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 900a777..29c22ed 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -182,11 +182,6 @@ static inline struct blkcg_gq *tg_to_blkg(struct throtl_grp *tg)
return pd_to_blkg(&tg->pd);
}

-static inline struct throtl_grp *td_root_tg(struct throtl_data *td)
-{
- return blkg_to_tg(td->queue->root_blkg);
-}
-
/**
* sq_to_tg - return the throl_grp the specified service queue belongs to
* @sq: the throtl_service_queue of interest
@@ -449,39 +444,6 @@ static void throtl_pd_reset_stats(struct blkg_policy_data *pd)
}
}

-static struct throtl_grp *throtl_lookup_tg(struct throtl_data *td,
- struct blkcg *blkcg)
-{
- return blkg_to_tg(blkg_lookup(blkcg, td->queue));
-}
-
-static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
- struct blkcg *blkcg)
-{
- struct request_queue *q = td->queue;
- struct throtl_grp *tg = NULL;
-
- /*
- * This is the common case when there are no blkcgs. Avoid lookup
- * in this case
- */
- if (blkcg == &blkcg_root) {
- tg = td_root_tg(td);
- } else {
- struct blkcg_gq *blkg;
-
- blkg = blkg_lookup_create(blkcg, q);
-
- /* if %NULL and @q is alive, fall back to root_tg */
- if (!IS_ERR(blkg))
- tg = blkg_to_tg(blkg);
- else
- tg = td_root_tg(td);
- }
-
- return tg;
-}
-
static struct throtl_grp *
throtl_rb_first(struct throtl_service_queue *parent_sq)
{
@@ -1403,46 +1365,26 @@ static struct blkcg_policy blkcg_policy_throtl = {
.pd_reset_stats_fn = throtl_pd_reset_stats,
};

-bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
+bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
+ struct bio *bio)
{
- struct throtl_data *td = q->td;
struct throtl_qnode *qn = NULL;
- struct throtl_grp *tg;
+ struct throtl_grp *tg = blkg_to_tg(blkg ?: q->root_blkg);
struct throtl_service_queue *sq;
bool rw = bio_data_dir(bio);
- struct blkcg *blkcg;
bool throttled = false;

+ WARN_ON_ONCE(!rcu_read_lock_held());
+
/* see throtl_charge_bio() */
- if (bio->bi_rw & REQ_THROTTLED)
+ if ((bio->bi_rw & REQ_THROTTLED) || !tg->has_rules[rw])
goto out;

- /*
- * A throtl_grp pointer retrieved under rcu can be used to access
- * basic fields like stats and io rates. If a group has no rules,
- * just update the dispatch stats in lockless manner and return.
- */
- rcu_read_lock();
- blkcg = bio_blkcg(bio);
- tg = throtl_lookup_tg(td, blkcg);
- if (tg) {
- if (!tg->has_rules[rw]) {
- throtl_update_dispatch_stats(tg_to_blkg(tg),
- bio->bi_iter.bi_size, bio->bi_rw);
- goto out_unlock_rcu;
- }
- }
-
- /*
- * Either group has not been allocated yet or it is not an unlimited
- * IO group
- */
spin_lock_irq(q->queue_lock);

if (unlikely(blk_queue_bypass(q)))
goto out_unlock;

- tg = throtl_lookup_create_tg(td, blkcg);
sq = &tg->service_queue;

while (true) {
@@ -1507,8 +1449,6 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)

out_unlock:
spin_unlock_irq(q->queue_lock);
-out_unlock_rcu:
- rcu_read_unlock();
out:
/*
* As multiple blk-throtls may stack in the same issue path, we
diff --git a/block/blk.h b/block/blk.h
index 026d959..d905b26 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -266,15 +266,10 @@ static inline struct io_context *create_io_context(gfp_t gfp_mask, int node)
* Internal throttling interface
*/
#ifdef CONFIG_BLK_DEV_THROTTLING
-extern bool blk_throtl_bio(struct request_queue *q, struct bio *bio);
extern void blk_throtl_drain(struct request_queue *q);
extern int blk_throtl_init(struct request_queue *q);
extern void blk_throtl_exit(struct request_queue *q);
#else /* CONFIG_BLK_DEV_THROTTLING */
-static inline bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
-{
- return false;
-}
static inline void blk_throtl_drain(struct request_queue *q) { }
static inline int blk_throtl_init(struct request_queue *q) { return 0; }
static inline void blk_throtl_exit(struct request_queue *q) { }
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a4429b3..b73e353 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1683,30 +1683,6 @@ static void cfq_pd_reset_stats(struct blkg_policy_data *pd)
cfqg_stats_reset(&cfqg->dead_stats);
}

-/*
- * Search for the cfq group current task belongs to. request_queue lock must
- * be held.
- */
-static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
- struct blkcg *blkcg)
-{
- struct request_queue *q = cfqd->queue;
- struct cfq_group *cfqg = NULL;
-
- /* avoid lookup for the common case where there's no blkcg */
- if (blkcg == &blkcg_root) {
- cfqg = cfqd->root_group;
- } else {
- struct blkcg_gq *blkg;
-
- blkg = blkg_lookup_create(blkcg, q);
- if (!IS_ERR(blkg))
- cfqg = blkg_to_cfqg(blkg);
- }
-
- return cfqg;
-}
-
static void cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg)
{
cfqq->cfqg = cfqg;
@@ -2108,12 +2084,6 @@ static struct cftype cfq_blkcg_files[] = {
{ } /* terminate */
};
#else /* GROUP_IOSCHED */
-static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
- struct blkcg *blkcg)
-{
- return cfqd->root_group;
-}
-
static inline void
cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg) {
cfqq->cfqg = cfqg;
@@ -3711,16 +3681,19 @@ cfq_get_queue(struct cfq_data *cfqd, bool is_sync, struct cfq_io_cq *cic,
{
int ioprio_class = IOPRIO_PRIO_CLASS(cic->ioprio);
int ioprio = IOPRIO_PRIO_DATA(cic->ioprio);
+ struct blkcg_gq *blkg;
struct cfq_queue **async_cfqq = NULL;
struct cfq_queue *cfqq;
struct cfq_group *cfqg;

rcu_read_lock();
- cfqg = cfq_lookup_create_cfqg(cfqd, bio_blkcg(bio));
- if (!cfqg) {
+
+ blkg = blkg_lookup(bio_blkcg(bio), cfqd->queue);
+ if (!blkg) {
cfqq = &cfqd->oom_cfqq;
goto out;
}
+ cfqg = blkg_to_cfqg(blkg);

if (!is_sync) {
if (!ioprio_valid(cic->ioprio)) {
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 0609bce..ca9432f 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -421,8 +421,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
* or if either the blkcg or queue is going away. Fall back to
* root_rl in such cases.
*/
- blkg = blkg_lookup_create(blkcg, q);
- if (unlikely(IS_ERR(blkg)))
+ blkg = blkg_lookup(blkcg, q);
+ if (unlikely(!blkg))
goto root_rl;

blkg_get(blkg);
@@ -636,6 +636,39 @@ static inline void blkg_rwstat_merge(struct blkg_rwstat *to,
u64_stats_update_end(&to->syncp);
}

+#ifdef CONFIG_BLK_DEV_THROTTLING
+extern bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
+ struct bio *bio);
+#else
+static inline bool blk_throtl_bio(struct request_queue *q, struct blkgc_gq *blkg,
+ struct bio *bio) { return false; }
+#endif
+
+static inline bool blkcg_bio_issue_check(struct request_queue *q,
+ struct bio *bio)
+{
+ struct blkcg *blkcg;
+ struct blkcg_gq *blkg;
+ bool throtl = false;
+
+ rcu_read_lock();
+ blkcg = bio_blkcg(bio);
+
+ blkg = blkg_lookup(blkcg, q);
+ if (unlikely(!blkg)) {
+ spin_lock_irq(q->queue_lock);
+ blkg = blkg_lookup_create(blkcg, q);
+ if (IS_ERR(blkg))
+ blkg = NULL;
+ spin_unlock_irq(q->queue_lock);
+ }
+
+ throtl = blk_throtl_bio(q, blkg, bio);
+
+ rcu_read_unlock();
+ return !throtl;
+}
+
#else /* CONFIG_BLK_CGROUP */

struct blkcg {
@@ -689,6 +722,9 @@ static inline void blk_put_rl(struct request_list *rl) { }
static inline void blk_rq_set_rl(struct request *rq, struct request_list *rl) { }
static inline struct request_list *blk_rq_rl(struct request *rq) { return &rq->q->root_rl; }

+static inline bool blkcg_bio_issue_check(struct request_queue *q,
+ struct bio *bio) { return true; }
+
#define blk_queue_for_each_rl(rl, q) \
for ((rl) = &(q)->root_rl; (rl); (rl) = NULL)

--
2.4.3