Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <CAJfpegu3EwbBFTSJiPhm7eMyTK2MzijLUp1gcboOo3meMF_+Qg@mail.gmail.com>
 <D9FAB37B-D059-4137-A115-616237D78640@amacapital.net> <20200715171130.GG12769@casper.infradead.org>
 <7c09f6af-653f-db3f-2378-02dca2bc07f7@gmail.com>
In-Reply-To: <7c09f6af-653f-db3f-2378-02dca2bc07f7@gmail.com>
From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 15 Jul 2020 22:09:54 +0200
Message-ID: <CAJfpegt9=p4uo5U2GXqc-rwqOESzZCWAkGMRTY1r8H6fuXx96g@mail.gmail.com>
Subject: Re: strace of io_uring events?
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Stefano Garzarella <sgarzare@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        strace-devel@lists.strace.io, io-uring@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 15, 2020 at 9:43 PM Pavel Begunkov <asml.silence@gmail.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