Re: [PATCH 4/4] nvme: add support for mq_ops->queue_rqs()

Max Gurtovoy
2021-12-21T16:08:34Z

On 12/21/2021 5:33 PM, Jens Axboe wrote:
> On 12/21/21 8:29 AM, Max Gurtovoy wrote:
>> On 12/21/2021 5:23 PM, Jens Axboe wrote:
>>> On 12/21/21 3:20 AM, Max Gurtovoy wrote:
>>>> On 12/20/2021 8:58 PM, Jens Axboe wrote:
>>>>> On 12/20/21 11:48 AM, Max Gurtovoy wrote:
>>>>>> On 12/20/2021 6:34 PM, Jens Axboe wrote:
>>>>>>> On 12/20/21 8:29 AM, Max Gurtovoy wrote:
>>>>>>>> On 12/20/2021 4:19 PM, Jens Axboe wrote:
>>>>>>>>> On 12/20/21 3:11 AM, Max Gurtovoy wrote:
>>>>>>>>>> On 12/19/2021 4:48 PM, Jens Axboe wrote:
>>>>>>>>>>> On 12/19/21 5:14 AM, Max Gurtovoy wrote:
>>>>>>>>>>>> On 12/16/2021 7:16 PM, Jens Axboe wrote:
>>>>>>>>>>>>> On 12/16/21 9:57 AM, Max Gurtovoy wrote:
>>>>>>>>>>>>>> On 12/16/2021 6:36 PM, Jens Axboe wrote:
>>>>>>>>>>>>>>> On 12/16/21 9:34 AM, Max Gurtovoy wrote:
>>>>>>>>>>>>>>>> On 12/16/2021 6:25 PM, Jens Axboe wrote:
>>>>>>>>>>>>>>>>> On 12/16/21 9:19 AM, Max Gurtovoy wrote:
>>>>>>>>>>>>>>>>>> On 12/16/2021 6:05 PM, Jens Axboe wrote:
>>>>>>>>>>>>>>>>>>> On 12/16/21 9:00 AM, Max Gurtovoy wrote:
>>>>>>>>>>>>>>>>>>>> On 12/16/2021 5:48 PM, Jens Axboe wrote:
>>>>>>>>>>>>>>>>>>>>> On 12/16/21 6:06 AM, Max Gurtovoy wrote:
>>>>>>>>>>>>>>>>>>>>>> On 12/16/2021 11:08 AM, Christoph Hellwig wrote:
>>>>>>>>>>>>>>>>>>>>>>> On Wed, Dec 15, 2021 at 09:24:21AM -0700, Jens Axboe wrote:
>>>>>>>>>>>>>>>>>>>>>>>> + spin_lock(&nvmeq->sq_lock);
>>>>>>>>>>>>>>>>>>>>>>>> + while (!rq_list_empty(*rqlist)) {
>>>>>>>>>>>>>>>>>>>>>>>> + struct request *req = rq_list_pop(rqlist);
>>>>>>>>>>>>>>>>>>>>>>>> + struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
>>>>>>>>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>>>>>>>>> + memcpy(nvmeq->sq_cmds + (nvmeq->sq_tail << nvmeq->sqes),
>>>>>>>>>>>>>>>>>>>>>>>> + absolute_pointer(&iod->cmd), sizeof(iod->cmd));
>>>>>>>>>>>>>>>>>>>>>>>> + if (++nvmeq->sq_tail == nvmeq->q_depth)
>>>>>>>>>>>>>>>>>>>>>>>> + nvmeq->sq_tail = 0;
>>>>>>>>>>>>>>>>>>>>>>> So this doesn't even use the new helper added in patch 2? I think this
>>>>>>>>>>>>>>>>>>>>>>> should call nvme_sq_copy_cmd().
>>>>>>>>>>>>>>>>>>>>>> I also noticed that.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> So need to decide if to open code it or use the helper function.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> Inline helper sounds reasonable if you have 3 places that will use it.
>>>>>>>>>>>>>>>>>>>>> Yes agree, that's been my stance too :-)
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>> The rest looks identical to the incremental patch I posted, so I guess
>>>>>>>>>>>>>>>>>>>>>>> the performance degration measured on the first try was a measurement
>>>>>>>>>>>>>>>>>>>>>>> error?
>>>>>>>>>>>>>>>>>>>>>> giving 1 dbr for a batch of N commands sounds good idea. Also for RDMA host.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> But how do you moderate it ? what is the batch_sz <--> time_to_wait
>>>>>>>>>>>>>>>>>>>>>> algorithm ?
>>>>>>>>>>>>>>>>>>>>> The batching is naturally limited at BLK_MAX_REQUEST_COUNT, which is 32
>>>>>>>>>>>>>>>>>>>>> in total. I do agree that if we ever made it much larger, then we might
>>>>>>>>>>>>>>>>>>>>> want to cap it differently. But 32 seems like a pretty reasonable number
>>>>>>>>>>>>>>>>>>>>> to get enough gain from the batching done in various areas, while still
>>>>>>>>>>>>>>>>>>>>> not making it so large that we have a potential latency issue. That
>>>>>>>>>>>>>>>>>>>>> batch count is already used consistently for other items too (like tag
>>>>>>>>>>>>>>>>>>>>> allocation), so it's not specific to just this one case.
>>>>>>>>>>>>>>>>>>>> I'm saying that the you can wait to the batch_max_count too long and it
>>>>>>>>>>>>>>>>>>>> won't be efficient from latency POV.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> So it's better to limit the block layar to wait for the first to come: x
>>>>>>>>>>>>>>>>>>>> usecs or batch_max_count before issue queue_rqs.
>>>>>>>>>>>>>>>>>>> There's no waiting specifically for this, it's just based on the plug.
>>>>>>>>>>>>>>>>>>> We just won't do more than 32 in that plug. This is really just an
>>>>>>>>>>>>>>>>>>> artifact of the plugging, and if that should be limited based on "max of
>>>>>>>>>>>>>>>>>>> 32 or xx time", then that should be done there.
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> But in general I think it's saner and enough to just limit the total
>>>>>>>>>>>>>>>>>>> size. If we spend more than xx usec building up the plug list, we're
>>>>>>>>>>>>>>>>>>> doing something horribly wrong. That really should not happen with 32
>>>>>>>>>>>>>>>>>>> requests, and we'll never eg wait on requests if we're out of tags. That
>>>>>>>>>>>>>>>>>>> will result in a plug flush to begin with.
>>>>>>>>>>>>>>>>>> I'm not aware of the plug. I hope to get to it soon.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> My concern is if the user application submitted only 28 requests and
>>>>>>>>>>>>>>>>>> then you'll wait forever ? or for very long time.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I guess not, but I'm asking how do you know how to batch and when to
>>>>>>>>>>>>>>>>>> stop in case 32 commands won't arrive anytime soon.
>>>>>>>>>>>>>>>>> The plug is in the stack of the task, so that condition can never
>>>>>>>>>>>>>>>>> happen. If the application originally asks for 32 but then only submits
>>>>>>>>>>>>>>>>> 28, then once that last one is submitted the plug is flushed and
>>>>>>>>>>>>>>>>> requests are issued.
>>>>>>>>>>>>>>>> So if I'm running fio with --iodepth=28 what will plug do ? send batches
>>>>>>>>>>>>>>>> of 28 ? or 1 by 1 ?
>>>>>>>>>>>>>>> --iodepth just controls the overall depth, the batch submit count
>>>>>>>>>>>>>>> dictates what happens further down. If you run queue depth 28 and submit
>>>>>>>>>>>>>>> one at the time, then you'll get one at the time further down too. Hence
>>>>>>>>>>>>>>> the batching is directly driven by what the application is already
>>>>>>>>>>>>>>> doing.
>>>>>>>>>>>>>> I see. Thanks for the explanation.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> So it works only for io_uring based applications ?
>>>>>>>>>>>>> It's only enabled for io_uring right now, but it's generically available
>>>>>>>>>>>>> for anyone that wants to use it... Would be trivial to do for aio, and
>>>>>>>>>>>>> other spots that currently use blk_start_plug() and has an idea of how
>>>>>>>>>>>>> many IOs will be submitted
>>>>>>>>>>>> Can you please share an example application (or is it fio patches) that
>>>>>>>>>>>> can submit batches ? The same that was used to test this patchset is
>>>>>>>>>>>> fine too.
>>>>>>>>>>>>
>>>>>>>>>>>> I would like to test it with our NVMe SNAP controllers and also to
>>>>>>>>>>>> develop NVMe/RDMA queue_rqs code and test the perf with it.
>>>>>>>>>>> You should just be able to use iodepth_batch with fio. For my peak
>>>>>>>>>>> testing, I use t/io_uring from the fio repo. By default, it'll run QD of
>>>>>>>>>>> and do batches of 32 for complete and submit. You can just run:
>>>>>>>>>>>
>>>>>>>>>>> t/io_uring <dev or file>
>>>>>>>>>>>
>>>>>>>>>>> maybe adding -p0 for IRQ driven rather than polled IO.
>>>>>>>>>> I used your block/for-next branch and implemented queue_rqs in NVMe/RDMA
>>>>>>>>>> but it was never called using the t/io_uring test nor fio with
>>>>>>>>>> iodepth_batch=32 flag with io_uring engine.
>>>>>>>>>>
>>>>>>>>>> Any idea what might be the issue ?
>>>>>>>>>>
>>>>>>>>>> I installed fio from sources..
>>>>>>>>> The two main restrictions right now are a scheduler and shared tags, are
>>>>>>>>> you using any of those?
>>>>>>>> No.
>>>>>>>>
>>>>>>>> But maybe I'm missing the .commit_rqs callback. is it mandatory for this
>>>>>>>> feature ?
>>>>>>> I've only tested with nvme pci which does have it, but I don't think so.
>>>>>>> Unless there's some check somewhere that makes it necessary. Can you
>>>>>>> share the patch you're currently using on top?
>>>>>> The attached POC patches apply cleanly on block/for-next branch
>>>>> Looks reasonable to me from a quick glance. Not sure why you're not
>>>>> seeing it hit, maybe try and instrument
>>>>> block/blk-mq.c:blk_mq_flush_plug_list() and find out why it isn't being
>>>>> called? As mentioned, no elevator or shared tags, should work for
>>>>> anything else basically.
>>>> Yes. I saw that the blk layer converted the original non-shared tagset
>>>> of NVMe/RDMA to a shared one because of the nvmf connect request queue
>>>> that is using the same tagset (uses only the reserved tag).
>>>>
>>>> So I guess this is the reason that the I couldn't reach the new code of
>>>> queue_rqs.
>>>>
>>>> The question is how we can overcome this ?
>>> Do we need to mark it shared for just the reserved tags? I wouldn't
>>> think so...
>> We don't mark it. The block layer does it in blk_mq_add_queue_tag_set:
>>
>> if (!list_empty(&set->tag_list) &&
>> !(set->flags & BLK_MQ_F_TAG_QUEUE_SHARED))
> Yes, that's what I meant, do we need to mark it as such for just the
> reserved tags?

I'm afraid it doesn't related only to reserved tags.

If you have nvme device with 2 namespaces it will get to this code and
mark it as shared set. And then the queue_rqs() won't be called for NVMe
PCI as well.

>