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blkcg: consolidate blkg creation in blkcg_bio_issue_check()

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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 replaced with
  cfq_lookup_cfqg()which uses blkg_lookup().

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.

v2: Build fixes for !CONFIG_CFQ_GROUP_IOSCHED and
    !CONFIG_BLK_DEV_THROTTLING.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Arianna Avanzini <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
This commit is contained in:
Tejun Heo 2015-08-18 14:55:20 -07:00 • committed by Jens Axboe
commit ae11889636
6 changed files with 57 additions and 99 deletions
Download patch file Download diff file Expand all files Collapse all files

72
block/blk-throttle.c
View file

@ -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