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

[Jens Axboe]

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@xxxxxxxxx> 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.

-- 
Jens Axboe