Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Kanchan Joshi <joshiiitr@gmail.com>]

On Thu, Jul 30, 2020 at 11:24 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 7/30/20 11:51 AM, Kanchan Joshi wrote:
> > On Thu, Jul 30, 2020 at 11:10 PM Pavel Begunkov <asml.silence@gmail.com> 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 <axboe@kernel.dk> 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.

Got it, thank you for making it simple.
Overall if I try to put the tradeoffs of moving to indirect-offset
(compared to current scheme)–

Upside:
- cqe res/flags would be intact, avoids future-headaches as you mentioned
- short-write cases do not have to be failed in lower-layers (as
cqe->res is there to report bytes-copied)

Downside:
- We may not be able to use RWF_APPEND, and need exposing a new
type/flag (RWF_INDIRECT_OFFSET etc.) user-space. Not sure if this
sounds outrageous, but is it OK to have uring-only flag which can be
combined with RWF_APPEND?
-  Expensive compared to sending results in cqe itself. But I agree
that this may not be major, and only for one type of write.


-- 
Joshi