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
