linux-kernel - Re: strace of io

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Date:   Wed, 15 Jul 2020 22:09:54 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        strace-devel@...ts.strace.io, io-uring@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: strace of io_uring events?

On Wed, Jul 15, 2020 at 9:43 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> To clear details for those who are not familiar with io_uring:
>
> io_uring has a pair of queues, submission (SQ) and completion queues (CQ),
> both shared between kernel and user spaces. The userspace submits requests
> by filling a chunk of memory in SQ. The kernel picks up SQ entries in
> (syscall io_uring_enter) or asynchronously by polling SQ.
>
> CQ entries are filled by the kernel completely asynchronously and
> in parallel. Some users just poll CQ to get them, but also have a way
> to wait for them.
>
> >>>
> >>> What do people think?
> >>>
> >>> From what I can tell, listing the submitted requests on
> >>> io_uring_enter() would not be hard.  Request completion is
> >>> asynchronous, however, and may not require  io_uring_enter() syscall.
> >>> Am I correct?
>
> Both, submission and completion sides may not require a syscall.

Okay.

> >>> Is there some existing tracing infrastructure that strace could use to
> >>> get async completion events?  Should we be introducing one?
>
> There are static trace points covering all needs.

This needs to be unprivileged, or its usefulness is again compromized.

>
> And if not used the whole thing have to be zero-overhead. Otherwise
> there is perf, which is zero-overhead, and this IMHO won't fly.

Obviously it needs to be zero overhead if not tracing.

What won't fly?

Thanks,
Miklos