Re: [PATCH] block: Improve shared tag set performance

[Bart Van Assche <bvanassche@acm.org>]

On 10/18/23 11:00, Bart Van Assche wrote:
> -/*
> - * For shared tag users, we track the number of currently active users
> - * and attempt to provide a fair share of the tag depth for each of them.
> - */
> -static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
> -				  struct sbitmap_queue *bt)
> -{
> -	unsigned int depth, users;
> -
> -	if (!hctx || !(hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
> -		return true;
> -
> -	/*
> -	 * Don't try dividing an ant
> -	 */
> -	if (bt->sb.depth == 1)
> -		return true;
> -
> -	if (blk_mq_is_shared_tags(hctx->flags)) {
> -		struct request_queue *q = hctx->queue;
> -
> -		if (!test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
> -			return true;
> -	} else {
> -		if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
> -			return true;
> -	}
> -
> -	users = READ_ONCE(hctx->tags->active_queues);
> -	if (!users)
> -		return true;
> -
> -	/*
> -	 * Allow at least some tags
> -	 */
> -	depth = max((bt->sb.depth + users - 1) / users, 4U);
> -	return __blk_mq_active_requests(hctx) < depth;
> -}

(replying to my own email)

Hi Jens,

Do you perhaps remember why the above code was introduced by commit
0d2602ca30e4 ("blk-mq: improve support for shared tags maps") in 2014
(nine years ago)?

Thanks,

Bart.