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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=oFuoJe+oEM0Uix08KBFqSo1gG3LqQuw86Sd2WBM74e0=; b=QP1UCRRe4BYoTmPOfXIHiWP5cglJqlZRHCmII8RXsP386Om2swz8AgDAPvyN//EQbT xt/nYXCMk/B//AtJYCjObxFvAaa3hv2gbhWJ5xRFgJ3OznSFbPr6qk6Xi2PkJraYGdNL 4THngo7VBwgi20B4eKwMjzvYv/JRDnOXioYauVDwNES1z2i0vRmIif5jGVUbOIcKHl6N XpIHGialPXyKKLTMZaNaajEDFZh3Moi4/vdqyXaNiGexGYSuSYMgjG9pAmiX5ufR7v5Q oJTKeHjjPIK6pmlGfy8JJkERWcRbKOOOR2wb7My1TFD30FTaCpnyO1wZL3KOZleWJ1/Z 09Bw== X-Gm-Message-State: AOAM531Sy+dkt/QO/xaQ5NsNoleZciLUbKIFXgnMEH9WxDuxJRmwnBUr lukAI8us1WC7PWubo55FFiSZLiwDs8ojx07sgNbthdHsjq4gng== X-Google-Smtp-Source: ABdhPJxJJqwMHNYpift7oCWRcfUAjrSrb08yLuNE7q/QFrRpEhtUmhuSoC/nJO9RctnF5KT/QXq4R9WLe30T+dmce2E= X-Received: by 2002:adf:ea4f:: with SMTP id j15mr35138244wrn.253.1596131526928; Thu, 30 Jul 2020 10:52:06 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: From: Kanchan Joshi Date: Thu, 30 Jul 2020 23:21:40 +0530 Message-ID: Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append To: Pavel Begunkov Cc: Jens Axboe , 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 Content-Type: text/plain; charset="UTF-8" Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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? -- Joshi