Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

[Ming Lei]

On Fri, Oct 13, 2017 at 08:44:23AM -0600, Jens Axboe wrote:
> On 10/12/2017 06:19 PM, Ming Lei wrote:
> > 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.
> 
> I don't care too much about 'none'. If perfect merging is crucial for
> getting to the performance level you want on the hardware you are using,
> you should not be using 'none'. 'none' will work perfectly fine for NVMe
> etc style devices, where we are not dependent on merging to the same
> extent that we are on other devices.
> 
> mq-deadline reinsertion will be expensive, that's in the nature of that
> beast. It's basically the same as a normal request inserition.  So for
> that, we'd have to be a bit careful not to run into this too much. Even
> with a dumb approach, it should only happen 1 out of N times, where N is
> the typical point at which the device will return STS_RESOURCE. The
> reinsertion vs dequeue should be serialized with your patch to do that,
> at least for the single queue mq-deadline setup. In fact, I think your
> approach suffers from that same basic race, in that the budget isn't a
> hard allocation, it's just a hint. It can change from the time you check
> it, and when you go and dispatch the IO, if you don't serialize that
> part. So really should be no different in that regard.

In case of SCSI, the .get_buget is done as atomic counting,
and it is completely effective to avoid unnecessary dequeue, please take
a look at patch 6.

> 
> > Not mention the cost of acquiring/releasing lock, that work
> > is just doing useless work and wasting CPU.
> 
> Sure, my point is that if it doesn't happen too often, it doesn't really
> matter. It's not THAT expensive.

Actually it is in hot path, for example, lpfc and qla2xx's queue depth is 3,
it is quite easy to trigger STS_RESOURCE.


Thanks,
Ming