Re: [PATCH 3/4] block: move queue enter logic into blk_mq_submit_bio()
From: Christoph Hellwig <hch@infradead.org>
Date: 2021-11-04 09:10:12
On Wed, Nov 03, 2021 at 12:32:21PM -0600, Jens Axboe wrote:
Retain the old logic for the fops based submit, but for our internal blk_mq_submit_bio(), move the queue entering logic into the core function itself.
Can you explain the why? I guess you want to skip the extra reference for the cached requests now that they already have one. But please state that, and explain why it is a fix, as to me it just seems like another little optimization.
We need to be a bit careful if going into the scheduler, as a scheduler or queue mappings can arbitrarily change before we have entered the queue. Have the bio scheduler mapping do that separately, it's a very cheap operation compared to actually doing merging locking and lookups.
So just don't do the merges for cache requets and side step this extra bio_queue_enter for that case?
- if (unlikely(bio_queue_enter(bio) != 0)) - return; - if (!submit_bio_checks(bio) || !blk_crypto_bio_prep(&bio)) - goto queue_exit; + return;
This is broken, we really ant the submit checks under freeze protection to make sure the parameters can't be changed underneath us.
+static inline bool blk_mq_queue_enter(struct request_queue *q, struct bio *bio)
+{
+ if (!blk_try_enter_queue(q, false) && bio_queue_enter(bio))
+ return false;
+ return true;
+}This looks weird, as blk_try_enter_queue is already called by bio_queue_enter.
quoted hunk ↗ jump to hunk
} else { struct blk_mq_alloc_data data = { .q = q,@@ -2528,6 +2534,11 @@ void blk_mq_submit_bio(struct bio *bio) .cmd_flags = bio->bi_opf, }; + if (unlikely(!blk_mq_queue_enter(q, bio))) + return; + + rq_qos_throttle(q, bio); +
At some point the code in this !cached branch really needs to move into a helper..