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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

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))

>