linux-kernel - Re: [PATCH V8 4/7] blk-mq: introduce .get_budget and .put_budget in blk_mq

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Date:   Fri, 13 Oct 2017 11:32:41 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Cc:     Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>,
        John Garry <john.garry@...wei.com>
Subject: Re: [PATCH V8 4/7] blk-mq: introduce .get_budget and .put_budget in
 blk_mq_ops

On 10/13/2017 11:24 AM, Ming Lei wrote:
> For SCSI devices, there is often per-request-queue depth, which need
> to be respected before queuing one request.
> 
> The current blk-mq always dequeues request first, then calls .queue_rq()
> to dispatch the request to lld. One obvious issue of this way is that I/O
> merge may not be good, because when the per-request-queue depth can't be
> respected, .queue_rq() has to return BLK_STS_RESOURCE, then this request
> has to stay in hctx->dispatch list, and never got chance to participate
> into I/O merge.
> 
> This patch introduces .get_budget and .put_budget callback in blk_mq_ops,
> then we can try to get reserved budget first before dequeuing request.
> Once the budget for queueing I/O can't be satisfied, we don't need to
> dequeue request at all, then I/O merge can get improved a lot.

Still think this should be blk_mq_get_dispatch_budget(), like in the
incremental I sent out. That way you actually know what it is doing,
get_budget() could be anything.

> @@ -2582,6 +2606,9 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
>  	if (!set->ops->queue_rq)
>  		return -EINVAL;
>  
> +	if ((!!set->ops->get_budget) != (!!set->ops->put_budget))
> +		return -EINVAL;

	if (!set->ops->get_budget ^ !set->ops->put_budget)

is cleaner, imho.

-- 
Jens Axboe