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

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

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!

-- 
Pavel Begunkov