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Thu, 30 Jul 2020 10:54:54 -0700 (PDT) Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append To: Kanchan Joshi , Pavel Begunkov Cc: Kanchan Joshi , viro@zeniv.linux.org.uk, bcrl@kvack.org, Matthew Wilcox , Christoph Hellwig , Damien Le Moal , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-aio@kvack.org, io-uring@vger.kernel.org, linux-block@vger.kernel.org, linux-api@vger.kernel.org, SelvaKumar S , Nitesh Shetty , Javier Gonzalez References: <1595605762-17010-1-git-send-email-joshi.k@samsung.com> <1595605762-17010-7-git-send-email-joshi.k@samsung.com> <80d27717-080a-1ced-50d5-a3a06cf06cd3@kernel.dk> <65a7e9a6-aede-31ce-705c-b7f94f079112@kernel.dk> From: Jens Axboe Message-ID: Date: Thu, 30 Jul 2020 11:54:53 -0600 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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On 7/30/20 11:51 AM, Kanchan Joshi wrote: > On Thu, Jul 30, 2020 at 11:10 PM Pavel Begunkov wrote: >> >> On 30/07/2020 20:16, Jens Axboe wrote: >>> On 7/30/20 10:26 AM, Pavel Begunkov wrote: >>>> On 30/07/2020 19:13, Jens Axboe wrote: >>>>> On 7/30/20 10:08 AM, Pavel Begunkov wrote: >>>>>> On 27/07/2020 23:34, Jens Axboe wrote: >>>>>>> On 7/27/20 1:16 PM, Kanchan Joshi wrote: >>>>>>>> On Fri, Jul 24, 2020 at 10:00 PM Jens Axboe wrote: >>>>>>>>> >>>>>>>>> On 7/24/20 9:49 AM, Kanchan Joshi wrote: >>>>>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c >>>>>>>>>> index 7809ab2..6510cf5 100644 >>>>>>>>>> --- a/fs/io_uring.c >>>>>>>>>> +++ b/fs/io_uring.c >>>>>>>>>> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags) >>>>>>>>>> cqe = io_get_cqring(ctx); >>>>>>>>>> if (likely(cqe)) { >>>>>>>>>> WRITE_ONCE(cqe->user_data, req->user_data); >>>>>>>>>> - WRITE_ONCE(cqe->res, res); >>>>>>>>>> - WRITE_ONCE(cqe->flags, cflags); >>>>>>>>>> + if (unlikely(req->flags & REQ_F_ZONE_APPEND)) { >>>>>>>>>> + if (likely(res > 0)) >>>>>>>>>> + WRITE_ONCE(cqe->res64, req->rw.append_offset); >>>>>>>>>> + else >>>>>>>>>> + WRITE_ONCE(cqe->res64, res); >>>>>>>>>> + } else { >>>>>>>>>> + WRITE_ONCE(cqe->res, res); >>>>>>>>>> + WRITE_ONCE(cqe->flags, cflags); >>>>>>>>>> + } >>>>>>>>> >>>>>>>>> This would be nice to keep out of the fast path, if possible. >>>>>>>> >>>>>>>> I was thinking of keeping a function-pointer (in io_kiocb) during >>>>>>>> submission. That would have avoided this check......but argument count >>>>>>>> differs, so it did not add up. >>>>>>> >>>>>>> But that'd grow the io_kiocb just for this use case, which is arguably >>>>>>> even worse. Unless you can keep it in the per-request private data, >>>>>>> but there's no more room there for the regular read/write side. >>>>>>> >>>>>>>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h >>>>>>>>>> index 92c2269..2580d93 100644 >>>>>>>>>> --- a/include/uapi/linux/io_uring.h >>>>>>>>>> +++ b/include/uapi/linux/io_uring.h >>>>>>>>>> @@ -156,8 +156,13 @@ enum { >>>>>>>>>> */ >>>>>>>>>> struct io_uring_cqe { >>>>>>>>>> __u64 user_data; /* sqe->data submission passed back */ >>>>>>>>>> - __s32 res; /* result code for this event */ >>>>>>>>>> - __u32 flags; >>>>>>>>>> + union { >>>>>>>>>> + struct { >>>>>>>>>> + __s32 res; /* result code for this event */ >>>>>>>>>> + __u32 flags; >>>>>>>>>> + }; >>>>>>>>>> + __s64 res64; /* appending offset for zone append */ >>>>>>>>>> + }; >>>>>>>>>> }; >>>>>>>>> >>>>>>>>> Is this a compatible change, both for now but also going forward? You >>>>>>>>> could randomly have IORING_CQE_F_BUFFER set, or any other future flags. >>>>>>>> >>>>>>>> Sorry, I didn't quite understand the concern. CQE_F_BUFFER is not >>>>>>>> used/set for write currently, so it looked compatible at this point. >>>>>>> >>>>>>> Not worried about that, since we won't ever use that for writes. But it >>>>>>> is a potential headache down the line for other flags, if they apply to >>>>>>> normal writes. >>>>>>> >>>>>>>> Yes, no room for future flags for this operation. >>>>>>>> Do you see any other way to enable this support in io-uring? >>>>>>> >>>>>>> Honestly I think the only viable option is as we discussed previously, >>>>>>> pass in a pointer to a 64-bit type where we can copy the additional >>>>>>> completion information to. >>>>>> >>>>>> TBH, I hate the idea of such overhead/latency at times when SSDs can >>>>>> serve writes in less than 10ms. Any chance you measured how long does it >>>>> >>>>> 10us? :-) >>>> >>>> Hah, 10us indeed :) >>>> >>>>> >>>>>> take to drag through task_work? >>>>> >>>>> A 64-bit value copy is really not a lot of overhead... But yes, we'd >>>>> need to push the completion through task_work at that point, as we can't >>>>> do it from the completion side. That's not a lot of overhead, and most >>>>> notably, it's overhead that only affects this particular type. >>>>> >>>>> That's not a bad starting point, and something that can always be >>>>> optimized later if need be. But I seriously doubt it'd be anything to >>>>> worry about. >>>> >>>> I probably need to look myself how it's really scheduled, but if you don't >>>> mind, here is a quick question: if we do work_add(task) when the task is >>>> running in the userspace, wouldn't the work execution wait until the next >>>> syscall/allotted time ends up? >>> >>> It'll get the task to enter the kernel, just like signal delivery. The only >>> tricky part is really if we have a dependency waiting in the kernel, like >>> the recent eventfd fix. >> >> I see, thanks for sorting this out! > > Few more doubts about this (please mark me wrong if that is the case): > > - Task-work makes me feel like N completions waiting to be served by > single task. > Currently completions keep arriving and CQEs would be updated with > result, but the user-space (submitter task) would not be poked. > > - Completion-code will set the task-work. But post that it cannot go > immediately to its regular business of picking cqe and updating > res/flags, as we cannot afford user-space to see the cqe before the > pointer update. So it seems completion-code needs to spawn another > work which will allocate/update cqe after waiting for pointer-update > from task-work? The task work would post the completion CQE for the request after writing the offset. -- Jens Axboe