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

[Kanchan Joshi <joshiiitr@gmail.com>]

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?


-- 
Joshi