io_uring_prep_timeout() leading to an IO pressure close to 100

[Fiona Ebner]

Dear maintainers,

I'm currently investigating an issue with QEMU causing an IO pressure
value of nearly 100 when io_uring is used for the event loop of a QEMU
iothread (which is the case since QEMU 10.2 if io_uring is enabled
during configuration and available).

The cause seems to be the io_uring_prep_timeout() call that is used for
blocking wait. I attached a minimal reproducer below, which exposes the
issue [0].

This was observed on a kernel based on 7.0-rc6 as well as 6.17.13. I
haven't investigated what happens inside the kernel yet, so I don't know
if it is an accounting issue or within io_uring.

Let me know if you need more information or if I should test something
specific.

Best Regards,
Fiona

[0]:

#include <errno.h>
#include <stdio.h>
#include <liburing.h>

int main(void) {
    int ret;
    struct io_uring ring;
    struct __kernel_timespec ts;
    struct io_uring_sqe *sqe;

    ret = io_uring_queue_init(128, &ring, 0);
    if (ret != 0) {
        printf("Failed to initialize io_uring\n");
        return ret;
    }

    ts = (struct __kernel_timespec){
        .tv_sec = 60,
    };

    sqe = io_uring_get_sqe(&ring);
    if (!sqe) {
        printf("Full sq\n");
        return -1;
    }

    io_uring_prep_timeout(sqe, &ts, 1, 0);
    io_uring_sqe_set_data(sqe, NULL);

    do {
        ret = io_uring_submit_and_wait(&ring, 1);
        printf("got ret %d\n", ret);
    } while (ret == -EINTR);

    io_uring_queue_exit(&ring);

    return 0;
}