blkcg: consolidate blkg creation in blkcg_bio_issue_check()
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 <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a4429b3..0994f3b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1683,28 +1683,15 @@
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)
+static struct cfq_group *cfq_lookup_cfqg(struct cfq_data *cfqd,
+ struct blkcg *blkcg)
{
- struct request_queue *q = cfqd->queue;
- struct cfq_group *cfqg = NULL;
+ struct blkcg_gq *blkg;
- /* 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;
+ blkg = blkg_lookup(blkcg, cfqd->queue);
+ if (likely(blkg))
+ return blkg_to_cfqg(blkg);
+ return NULL;
}
static void cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg)
@@ -2108,8 +2095,8 @@
{ } /* terminate */
};
#else /* GROUP_IOSCHED */
-static struct cfq_group *cfq_lookup_create_cfqg(struct cfq_data *cfqd,
- struct blkcg *blkcg)
+static struct cfq_group *cfq_lookup_cfqg(struct cfq_data *cfqd,
+ struct blkcg *blkcg)
{
return cfqd->root_group;
}
@@ -3716,7 +3703,7 @@
struct cfq_group *cfqg;
rcu_read_lock();
- cfqg = cfq_lookup_create_cfqg(cfqd, bio_blkcg(bio));
+ cfqg = cfq_lookup_cfqg(cfqd, bio_blkcg(bio));
if (!cfqg) {
cfqq = &cfqd->oom_cfqq;
goto out;