On Fri, Oct 13, 2017 at 10:19:04AM -0600, Jens Axboe wrote: > On 10/13/2017 10:07 AM, Ming Lei wrote: > > 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. > > Looks like you are right, I had initially misread that as just checking > the busy count. But you are actually getting the count at that point, > so it should be solid. > > >>> 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. > > Ugh, that is low. > > OK, I think we should just roll with this and see how far we can go. I'll > apply it for 4.15. OK, I have some update, will post a new version soon. Thanks Ming
