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Thu, 30 Jul 2020 10:16:02 -0700 (PDT) Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append To: Pavel Begunkov , Kanchan Joshi 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> From: Jens Axboe Message-ID: <65a7e9a6-aede-31ce-705c-b7f94f079112@kernel.dk> Date: Thu, 30 Jul 2020 11:16:01 -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-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Jens Axboe