On 8/19/25 6:59 AM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Current depth_updated has some problems:
>
> 1) depth_updated() will be called for each hctx, while all elevators
> will update async_depth for the disk level, this is not related to hctx;
> 2) In blk_mq_update_nr_requests(), if previous hctx update succeed and
> this hctx update failed, q->nr_requests will not be updated, while
> async_depth is already updated with new nr_reqeuests in previous
> depth_updated();
> 3) All elevators are using q->nr_requests to calculate async_depth now,
> however, q->nr_requests is still the old value when depth_updated() is
> called from blk_mq_update_nr_requests();
>
> Fix those problems by:
>
> - pass in request_queue instead of hctx;
> - move depth_updated() after q->nr_requests is updated in
> blk_mq_update_nr_requests();
> - add depth_updated() call in blk_mq_init_sched();
> - remove init_hctx() method for mq-deadline and bfq that is useless now;
>
> Fixes: 77f1e0a52d26 ("bfq: update internal depth state when queue depth changes")
> Fixes: 39823b47bbd4 ("block/mq-deadline: Fix the tag reservation code")
> Fixes: 42e6c6ce03fd ("lib/sbitmap: convert shallow_depth from one word to the whole sbitmap")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> block/bfq-iosched.c | 21 ++++-----------------
> block/blk-mq-sched.c | 3 +++
> block/blk-mq-sched.h | 11 +++++++++++
> block/blk-mq.c | 23 ++++++++++++-----------
> block/elevator.h | 2 +-
> block/kyber-iosched.c | 10 ++++------
> block/mq-deadline.c | 15 ++-------------
> 7 files changed, 37 insertions(+), 48 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 50e51047e1fe..c0c398998aa1 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -7109,9 +7109,10 @@ void bfq_put_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg)
> * See the comments on bfq_limit_depth for the purpose of
> * the depths set in the function. Return minimum shallow depth we'll use.
> */
> -static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
> +static void bfq_depth_updated(struct request_queue *q)
> {
> - unsigned int nr_requests = bfqd->queue->nr_requests;
> + struct bfq_data *bfqd = q->elevator->elevator_data;
> + unsigned int nr_requests = q->nr_requests;
>
> /*
> * In-word depths if no bfq_queue is being weight-raised:
> @@ -7143,21 +7144,8 @@ static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
> bfqd->async_depths[1][0] = max((nr_requests * 3) >> 4, 1U);
> /* no more than ~37% of tags for sync writes (~20% extra tags) */
> bfqd->async_depths[1][1] = max((nr_requests * 6) >> 4, 1U);
> -}
> -
> -static void bfq_depth_updated(struct blk_mq_hw_ctx *hctx)
> -{
> - struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
> - struct blk_mq_tags *tags = hctx->sched_tags;
>
> - bfq_update_depths(bfqd, &tags->bitmap_tags);
> - sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, 1);
> -}
> -
> -static int bfq_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int index)
> -{
> - bfq_depth_updated(hctx);
> - return 0;
> + blk_mq_set_min_shallow_depth(q, 1);
> }
>
> static void bfq_exit_queue(struct elevator_queue *e)
> @@ -7628,7 +7616,6 @@ static struct elevator_type iosched_bfq_mq = {
> .request_merged = bfq_request_merged,
> .has_work = bfq_has_work,
> .depth_updated = bfq_depth_updated,
> - .init_hctx = bfq_init_hctx,
> .init_sched = bfq_init_queue,
> .exit_sched = bfq_exit_queue,
> },
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index e2ce4a28e6c9..bf7dd97422ec 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -585,6 +585,9 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e,
> }
> }
> }
> +
> + if (e->ops.depth_updated)
> + e->ops.depth_updated(q);
> return 0;
>
Overall changes look good. That said, I think it might be cleaner to structure
it this way:
elevator_switch -> blk_mq_init_sched ->init_sched ==> sets async_depth
blk_mq_update_nr_requests ->depth_updated ==> updates async_depth
This way, we don’t need to call ->depth_updated from blk_mq_init_sched.
In summary:
- Avoid calling ->depth_updated during blk_mq_init_sched
- Set async_depth when the elevator is initialized (via ->init_sched)
- Update async_depth when nr_requests is modified through sysfs (via ->depth_updated)
Thanks,
--Nilay
