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Fri, 31 Jul 2020 06:42:10 +0000 From: Damien Le Moal To: Kanchan Joshi , Jens Axboe CC: Pavel Begunkov , Kanchan Joshi , "viro@zeniv.linux.org.uk" , "bcrl@kvack.org" , Matthew Wilcox , "hch@infradead.org" , "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 Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append Thread-Topic: [PATCH v4 6/6] io_uring: add support for zone-append Thread-Index: AQHWYdbcc0q15qREKECGO7brHi6zEg== Date: Fri, 31 Jul 2020 06:42:10 +0000 Message-ID: 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> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: gmail.com; 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x-ms-exchange-antispam-messagedata: 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 x-ms-exchange-transport-forked: True Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: wdc.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: MWHPR04MB3758.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 0ce9f775-50d2-4bf4-a8b4-08d8351cd9d2 X-MS-Exchange-CrossTenant-originalarrivaltime: 31 Jul 2020 06:42:10.1183 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: b61c8803-16f3-4c35-9b17-6f65f441df86 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: V4CyTjRU7rYsLZ1QI4Ubv9Tl9o6SyM+LM2otU4GJjBq5i+uD872wjfQX8kTZrpBsTdsy6P0klJBnp5a5QfmdaQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR04MB0878 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020/07/31 3:26, Kanchan Joshi wrote:=0A= > On Thu, Jul 30, 2020 at 11:24 PM Jens Axboe wrote:=0A= >>=0A= >> On 7/30/20 11:51 AM, Kanchan Joshi wrote:=0A= >>> On Thu, Jul 30, 2020 at 11:10 PM Pavel Begunkov wrote:=0A= >>>>=0A= >>>> On 30/07/2020 20:16, Jens Axboe wrote:=0A= >>>>> On 7/30/20 10:26 AM, Pavel Begunkov wrote:=0A= >>>>>> On 30/07/2020 19:13, Jens Axboe wrote:=0A= >>>>>>> On 7/30/20 10:08 AM, Pavel Begunkov wrote:=0A= >>>>>>>> On 27/07/2020 23:34, Jens Axboe wrote:=0A= >>>>>>>>> On 7/27/20 1:16 PM, Kanchan Joshi wrote:=0A= >>>>>>>>>> On Fri, Jul 24, 2020 at 10:00 PM Jens Axboe wr= ote:=0A= >>>>>>>>>>>=0A= >>>>>>>>>>> On 7/24/20 9:49 AM, Kanchan Joshi wrote:=0A= >>>>>>>>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c=0A= >>>>>>>>>>>> index 7809ab2..6510cf5 100644=0A= >>>>>>>>>>>> --- a/fs/io_uring.c=0A= >>>>>>>>>>>> +++ b/fs/io_uring.c=0A= >>>>>>>>>>>> @@ -1284,8 +1301,15 @@ static void __io_cqring_fill_event(stru= ct io_kiocb *req, long res, long cflags)=0A= >>>>>>>>>>>> cqe =3D io_get_cqring(ctx);=0A= >>>>>>>>>>>> if (likely(cqe)) {=0A= >>>>>>>>>>>> WRITE_ONCE(cqe->user_data, req->user_data);=0A= >>>>>>>>>>>> - WRITE_ONCE(cqe->res, res);=0A= >>>>>>>>>>>> - WRITE_ONCE(cqe->flags, cflags);=0A= >>>>>>>>>>>> + if (unlikely(req->flags & REQ_F_ZONE_APPEND)) {= =0A= >>>>>>>>>>>> + if (likely(res > 0))=0A= >>>>>>>>>>>> + WRITE_ONCE(cqe->res64, req->rw.a= ppend_offset);=0A= >>>>>>>>>>>> + else=0A= >>>>>>>>>>>> + WRITE_ONCE(cqe->res64, res);=0A= >>>>>>>>>>>> + } else {=0A= >>>>>>>>>>>> + WRITE_ONCE(cqe->res, res);=0A= >>>>>>>>>>>> + WRITE_ONCE(cqe->flags, cflags);=0A= >>>>>>>>>>>> + }=0A= >>>>>>>>>>>=0A= >>>>>>>>>>> This would be nice to keep out of the fast path, if possible.= =0A= >>>>>>>>>>=0A= >>>>>>>>>> I was thinking of keeping a function-pointer (in io_kiocb) durin= g=0A= >>>>>>>>>> submission. That would have avoided this check......but argument= count=0A= >>>>>>>>>> differs, so it did not add up.=0A= >>>>>>>>>=0A= >>>>>>>>> But that'd grow the io_kiocb just for this use case, which is arg= uably=0A= >>>>>>>>> even worse. Unless you can keep it in the per-request private dat= a,=0A= >>>>>>>>> but there's no more room there for the regular read/write side.= =0A= >>>>>>>>>=0A= >>>>>>>>>>>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linu= x/io_uring.h=0A= >>>>>>>>>>>> index 92c2269..2580d93 100644=0A= >>>>>>>>>>>> --- a/include/uapi/linux/io_uring.h=0A= >>>>>>>>>>>> +++ b/include/uapi/linux/io_uring.h=0A= >>>>>>>>>>>> @@ -156,8 +156,13 @@ enum {=0A= >>>>>>>>>>>> */=0A= >>>>>>>>>>>> struct io_uring_cqe {=0A= >>>>>>>>>>>> __u64 user_data; /* sqe->data submission passed b= ack */=0A= >>>>>>>>>>>> - __s32 res; /* result code for this event */= =0A= >>>>>>>>>>>> - __u32 flags;=0A= >>>>>>>>>>>> + union {=0A= >>>>>>>>>>>> + struct {=0A= >>>>>>>>>>>> + __s32 res; /* result code for this = event */=0A= >>>>>>>>>>>> + __u32 flags;=0A= >>>>>>>>>>>> + };=0A= >>>>>>>>>>>> + __s64 res64; /* appending offset for zone app= end */=0A= >>>>>>>>>>>> + };=0A= >>>>>>>>>>>> };=0A= >>>>>>>>>>>=0A= >>>>>>>>>>> Is this a compatible change, both for now but also going forwar= d? You=0A= >>>>>>>>>>> could randomly have IORING_CQE_F_BUFFER set, or any other futur= e flags.=0A= >>>>>>>>>>=0A= >>>>>>>>>> Sorry, I didn't quite understand the concern. CQE_F_BUFFER is no= t=0A= >>>>>>>>>> used/set for write currently, so it looked compatible at this po= int.=0A= >>>>>>>>>=0A= >>>>>>>>> Not worried about that, since we won't ever use that for writes. = But it=0A= >>>>>>>>> is a potential headache down the line for other flags, if they ap= ply to=0A= >>>>>>>>> normal writes.=0A= >>>>>>>>>=0A= >>>>>>>>>> Yes, no room for future flags for this operation.=0A= >>>>>>>>>> Do you see any other way to enable this support in io-uring?=0A= >>>>>>>>>=0A= >>>>>>>>> Honestly I think the only viable option is as we discussed previo= usly,=0A= >>>>>>>>> pass in a pointer to a 64-bit type where we can copy the addition= al=0A= >>>>>>>>> completion information to.=0A= >>>>>>>>=0A= >>>>>>>> TBH, I hate the idea of such overhead/latency at times when SSDs c= an=0A= >>>>>>>> serve writes in less than 10ms. Any chance you measured how long d= oes it=0A= >>>>>>>=0A= >>>>>>> 10us? :-)=0A= >>>>>>=0A= >>>>>> Hah, 10us indeed :)=0A= >>>>>>=0A= >>>>>>>=0A= >>>>>>>> take to drag through task_work?=0A= >>>>>>>=0A= >>>>>>> A 64-bit value copy is really not a lot of overhead... But yes, we'= d=0A= >>>>>>> need to push the completion through task_work at that point, as we = can't=0A= >>>>>>> do it from the completion side. That's not a lot of overhead, and m= ost=0A= >>>>>>> notably, it's overhead that only affects this particular type.=0A= >>>>>>>=0A= >>>>>>> That's not a bad starting point, and something that can always be= =0A= >>>>>>> optimized later if need be. But I seriously doubt it'd be anything = to=0A= >>>>>>> worry about.=0A= >>>>>>=0A= >>>>>> I probably need to look myself how it's really scheduled, but if you= don't=0A= >>>>>> mind, here is a quick question: if we do work_add(task) when the tas= k is=0A= >>>>>> running in the userspace, wouldn't the work execution wait until the= next=0A= >>>>>> syscall/allotted time ends up?=0A= >>>>>=0A= >>>>> It'll get the task to enter the kernel, just like signal delivery. Th= e only=0A= >>>>> tricky part is really if we have a dependency waiting in the kernel, = like=0A= >>>>> the recent eventfd fix.=0A= >>>>=0A= >>>> I see, thanks for sorting this out!=0A= >>>=0A= >>> Few more doubts about this (please mark me wrong if that is the case):= =0A= >>>=0A= >>> - Task-work makes me feel like N completions waiting to be served by=0A= >>> single task.=0A= >>> Currently completions keep arriving and CQEs would be updated with=0A= >>> result, but the user-space (submitter task) would not be poked.=0A= >>>=0A= >>> - Completion-code will set the task-work. But post that it cannot go=0A= >>> immediately to its regular business of picking cqe and updating=0A= >>> res/flags, as we cannot afford user-space to see the cqe before the=0A= >>> pointer update. So it seems completion-code needs to spawn another=0A= >>> work which will allocate/update cqe after waiting for pointer-update=0A= >>> from task-work?=0A= >>=0A= >> The task work would post the completion CQE for the request after=0A= >> writing the offset.=0A= > =0A= > Got it, thank you for making it simple.=0A= > Overall if I try to put the tradeoffs of moving to indirect-offset=0A= > (compared to current scheme)=96=0A= > =0A= > Upside:=0A= > - cqe res/flags would be intact, avoids future-headaches as you mentioned= =0A= > - short-write cases do not have to be failed in lower-layers (as=0A= > cqe->res is there to report bytes-copied)=0A= =0A= I personally think it is a super bad idea to allow short asynchronous appen= d=0A= writes. The interface should allow the async zone append write to proceed o= nly=0A= and only if it can be stuffed entirely into a single BIO which necessarilly= will=0A= be a single request on the device side. Otherwise, the application would ha= ve no=0A= guarantees as to where a split may happen, and since this is zone append, t= he=0A= next async append will not leave any hole to complete a previous short writ= e.=0A= This will wreak the structure of the application data.=0A= =0A= For the sync case, this is fine. The application can just issue a new appen= d=0A= write with the remaining unwritten data from the previous append write. But= in=0A= the async case, if one write =3D=3D one data record (e.g. a key-value tuple= for an=0A= SSTable in an LSM tree), then allowing a short write will destroy the recor= d:=0A= the partial write will be garbage data that will need garbage collection...= =0A= =0A= > =0A= > Downside:=0A= > - We may not be able to use RWF_APPEND, and need exposing a new=0A= > type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this=0A= > sounds outrageous, but is it OK to have uring-only flag which can be=0A= > combined with RWF_APPEND?=0A= =0A= Why ? Where is the problem ? O_APPEND/RWF_APPEND is currently meaningless f= or=0A= raw block device accesses. We could certainly define a meaning for these in= the=0A= context of zoned block devices.=0A= =0A= I already commented on the need for first defining an interface (flags etc)= and=0A= its semantic (e.g. do we allow short zone append or not ? What happens for= =0A= regular files ? etc). Did you read my comment ? We really need to first agr= ee on=0A= something to clarify what needs to be done.=0A= =0A= =0A= > - Expensive compared to sending results in cqe itself. But I agree=0A= > that this may not be major, and only for one type of write.=0A= > =0A= > =0A= =0A= =0A= -- =0A= Damien Le Moal=0A= Western Digital Research=0A=