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blkcg: cleanup and make blk_get_rl use blkg_lookup_create

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blk_get_rl is responsible for identifying which request_list a request
should be allocated to. Try get logic was added earlier, but
semantically the logic was not changed.

This patch makes better use of the bio already having a reference to the
blkg in the hot path. The cold path uses a better fallback of
blkg_lookup_create rather than just blkg_lookup and then falling back to
the q->root_rl. If lookup_create fails with anything but -ENODEV, it
falls back to q->root_rl.

A clarifying comment is added to explain why q->root_rl is used rather
than the root blkg's rl.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Dennis Zhou (Facebook) 2018-09-11 14:41:35 -04:00 committed by Jens Axboe
parent f0fcb3ec89
commit e2b0989954
1 changed files with 21 additions and 13 deletions
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34
include/linux/blk-cgroup.h
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@ -586,28 +586,36 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
rcu_read_lock();
blkcg = bio_blkcg(bio);
if (!blkcg)
blkcg = css_to_blkcg(blkcg_css());
if (bio && bio->bi_blkg) {
blkcg = bio->bi_blkg->blkcg;
if (blkcg == &blkcg_root)
goto rl_use_root;
/* bypass blkg lookup and use @q->root_rl directly for root */
blkg_get(bio->bi_blkg);
rcu_read_unlock();
return &bio->bi_blkg->rl;
}
blkcg = css_to_blkcg(blkcg_css());
if (blkcg == &blkcg_root)
goto root_rl;
goto rl_use_root;
/*
* Try to use blkg->rl. blkg lookup may fail under memory pressure
* or if either the blkcg or queue is going away. Fall back to
* root_rl in such cases.
*/
blkg = blkg_lookup(blkcg, q);
if (unlikely(!blkg))
goto root_rl;
blkg = __blkg_lookup_create(blkcg, q);
if (!blkg_try_get(blkg))
goto root_rl;
goto rl_use_root;
rcu_read_unlock();
return &blkg->rl;
root_rl:
/*
* Each blkg has its own request_list, however, the root blkcg
* uses the request_queue's root_rl. This is to avoid most
* overhead for the root blkcg.
*/
rl_use_root:
rcu_read_unlock();
return &q->root_rl;
}