BPF comes to io_uring at last

The kernel's asynchronous io_uring interface maintains two shared ring buffers: a submission queue for sending requests to the kernel, and a completion queue containing the results of those requests. Even with shared memory removing much of the overhead of communicating with user space, there is still some overhead whenever the kernel must switch to user space to give it the opportunity to process completion requests and queue up any subsequent work items. A patch set from Pavel Begunkov minimizes this overhead by letting programmers extend the io_uring event loop with a BPF program that can enqueue additional work in response to completion events. The patch set has been in development for a long time, but has finally been accepted.

To use io_uring, the programmer sets up appropriate shared buffers with io_uring_setup() and mmap() before putting a number of io_uring_sqe (submission queue event) structures in the submission queue. The kernel can be notified of the presence of new events to process in two ways: by setting up a dedicated kernel thread to poll the queue, or by having user space call io_uring_enter() periodically.