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Tue, 21 Dec 2021 07:33:14 -0800 (PST) Received: from [192.168.1.30] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id e9sm10238415ilm.44.2021.12.21.07.33.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Dec 2021 07:33:14 -0800 (PST) Subject: Re: [PATCH 4/4] nvme: add support for mq_ops->queue_rqs() To: Max Gurtovoy , Christoph Hellwig Cc: io-uring@vger.kernel.org, linux-nvme@lists.infradead.org, Hannes Reinecke , Oren Duer References: <20211215162421.14896-1-axboe@kernel.dk> <9b4202b4-192a-6611-922e-0b837e2b97c3@nvidia.com> <5f249c03-5cb2-9978-cd2c-669c0594d1c0@kernel.dk> <3474493a-a04d-528c-7565-f75db5205074@nvidia.com> <87e3a197-e8f7-d8d6-85b6-ce05bf1f35cd@kernel.dk> <5ee0e257-651a-ec44-7ca3-479438a737fb@nvidia.com> <01f9ce91-d998-c823-f2f2-de457625021e@nvidia.com> <573bbe72-d232-6063-dd34-2e12d8374594@kernel.dk> <4fbf2936-8e4c-9c04-e5a9-10eae387b562@nvidia.com> <6ca82929-7e70-be15-dcbb-1e68a02dd933@kernel.dk> <92c5065e-dc2a-9e3f-404a-64c6e22624b7@kernel.dk> <5b001e0f-cec0-112d-533a-d71684eb1d2e@nvidia.com> <7cbaa97b-4d79-82ca-d037-7d47f089ba8a@kernel.dk> <460ad0f5-7fb5-85c4-2718-20eeefe2050c@nvidia.com> From: Jens Axboe Message-ID: Date: Tue, 21 Dec 2021 08:33:13 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <460ad0f5-7fb5-85c4-2718-20eeefe2050c@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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 >>>>>>>>>> >>>>>>>>>> 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? -- Jens Axboe