Re: [PATCHSET v2 0/3] io_uring support for automatic buffers

Jens Axboe
2020-02-25T16:18:54Z

Ignore this one, sent out a series that hadn't been collapsed yet...
I'll post a v2b in a second.

On 2/25/20 9:04 AM, Jens Axboe wrote:
> With the poll retry based async IO patchset I posted last week, the one
> big missing thing for me was the ability to have automatic buffer
> selection. Generally applications that handle tons of sockets like to
> poll for activity on them, then issue IO when they become ready. This is
> of course at least two system calls, but it also means that it provides
> an application a chance to manage how many IO buffers it needs. With the
> io_uring based polled IO, the application need only issue an
> IORING_OP_RECV (for example, to receive socket data), it doesn't need to
> poll at all. However, this means that the application no longer has an
> opportune moment to select how many IO buffers to keep in flight, it has
> to be equal to what it currently has pending.
>
> I had originally intended to use BPF to provide some means of buffer
> selection, but I had a hard time imagining how life times of the buffer
> could be managed through that. I had a false start today, but Andres
> suggested a nifty approach that also solves the life time issue.
>
> Basically the application registers buffers with the kernel. Each buffer
> is registered with a given group ID, and buffer ID. The buffers are
> organized by group ID, and the application selects a buffer pool based
> on this group ID. One use case might be to group by size. There's an
> opcode for this, IORING_OP_PROVIDE_BUFFERS.
>
> IORING_OP_PROVIDE_BUFFERS takes a start address, length of a buffer, and
> number of buffers. It also provides a group ID with which these buffers
> should be associated, and a starting buffer ID. The buffers are then
> added, and the buffer ID is incremented by 1 for each buffer.
>
> With that, when doing the same IORING_OP_RECV, no buffer is passed in
> with the request. Instead, it's flagged with IOSQE_BUFFER_SELECT, and
> sqe->buf_group is filled in with a valid group ID. When the kernel can
> satisfy the receive, a buffer is selected from the specified group ID
> pool. If none are available, the IO is terminated with -ENOBUFS. On
> success, the buffer ID is passed back through the (CQE) completion
> event. This tells the application what specific buffer was used.
>
> A buffer can be used only once. On completion, the application may
> choose to free it, or register it again with IORING_OP_PROVIDE_BUFFER.
>
> Patches can also be found in the below repo:
>
> https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-buf-select
>
> and they are obviously layered on top of the poll retry rework.
>
> Changes since v1:
> - Cleanup address space
> - Fix locking for async offload issue
> - Add lockdep annotation for uring_lock
> - Verify sqe fields on PROVIDE_BUFFERS prep
> - Fix send/recv kbuf leak on import failure
> - Fix send/recv error handling on -ENOBUFS
> - Change IORING_OP_PROVIDE_BUFFER to PROVIDE_BUFFERS, and allow multiple
> contig buffers in one call
>

--
Jens Axboe