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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe