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.
> } 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..
