On Tue, Mar 25, 2025 at 09:49:38PM +0100, ~h0lyalg0rithm wrote: > From: Suraj Shirvankar <surajshirvan...@gmail.com> >
Please include the rationale for this change in the commit description.
This way anyone reading the git log will be able to understand the
intent behind this change. Something like:
IORING_SETUP_SINGLE_ISSUER enables optimizations in the kernel for
applications that only access the io_uring context from one thread.
QEMU calls io_uring_enter(2) from one AioContext, so it is safe to
enable this flag.
> Signed-off-by: Suraj Shirvankar <surajshirvan...@gmail.com>
> ---
> util/fdmon-io_uring.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
`make check` fails with this patch applied because aio_context_new()
(which calls fdmon_io_uring_setup()) is called before the thread is
created. When fdmon_io_uring_wait() is called the io_uring context is
now being used by another thread:
qemu-system-x86_64: ../util/fdmon-io_uring.c:330: fdmon_io_uring_wait:
Assertion `ret >= 0' failed.
Once this hurdle is overcome it should be possible to use
IORING_SETUP_SINGLE_ISSUER. Two ideas:
1. Modify aio_context_new() callers so they create the AioContext inside
the thread.
2. Defer io_uring context creation until it is needed. It's probably
still a good idea to create a temporary io-uring context early during
startup to check that io_uring is available (and then destroy it
right away).
I slightly prefer the first option.
>
> diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c
> index b0d68bdc44..235837abcb 100644
> --- a/util/fdmon-io_uring.c
> +++ b/util/fdmon-io_uring.c
> @@ -324,8 +324,14 @@ static const FDMonOps fdmon_io_uring_ops = {
> bool fdmon_io_uring_setup(AioContext *ctx)
> {
> int ret;
> + unsigned int flags = 0;
>
> - ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring,
> 0);
> + /* This improves performance but can be skipped on old hosts */
> +#ifdef IORING_SETUP_SINGLE_ISSUER
> + flags |= IORING_SETUP_SINGLE_ISSUER
The semicolon is missing at the end of the line.
> +#endif
> +
> + ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring,
> flags);
> if (ret != 0) {
> return false;
> }
> --
> 2.45.3
>
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