On Thu, Oct 12, 2017 at 12:46:24PM -0600, Jens Axboe wrote: > On 10/12/2017 12:37 PM, 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 one 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 staty 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 we can't get budget for queueing I/O, we don't need to dequeue request > > at all, then I/O merge can get improved a lot. > > I can't help but think that it would be cleaner to just be able to > reinsert the request into the scheduler properly, if we fail to > dispatch it. Bart hinted at that earlier as well. Actually when I start to investigate the issue, the 1st thing I tried is to reinsert, but that way is even worse on qla2xxx. Once request is dequeued, the IO merge chance is decreased a lot. With none scheduler, it becomes not possible to merge because we only try to merge over the last 8 requests. With mq-deadline, when one request is reinserted, another request may be dequeued at the same time. Not mention the cost of acquiring/releasing lock, that work is just doing useless work and wasting CPU. > > With that, you would not need budget functions. > > I don't feel _that_ strongly about it, though, and it is something > we can do down the line as well. I'd just hate having to introduce > budget ops only to kill them a little later. > > If we stick with the budget, then add a parallel blk_mq_get_budget() > like you have a blk_mq_put_budget(), so we don't have to litter the code > with things like: > > > + if (q->mq_ops->get_budget && !q->mq_ops->get_budget(hctx)) > > + return true; > > all over the place. There are also a few places where you don't use > blk_mq_put_budget() but open-code it instead, you should be consistent. OK. -- Ming
