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

[Pavel Begunkov <asml.silence@gmail.com>]

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?

-- 
Pavel Begunkov