linux-kernel - strace of io

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Message-ID: <CAJfpegu3EwbBFTSJiPhm7eMyTK2MzijLUp1gcboOo3meMF_+Qg@mail.gmail.com>
Date:   Wed, 15 Jul 2020 13:12:09 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     strace-devel@...ts.strace.io, io-uring@...r.kernel.org
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: strace of io_uring events?

Hi,

This thread is to discuss the possibility of stracing requests
submitted through io_uring.   I'm not directly involved in io_uring
development, so I'm posting this out of  interest in using strace on
processes utilizing io_uring.

io_uring gives the developer a way to bypass the syscall interface,
which results in loss of information when tracing.  This is a strace
fragment on  "io_uring-cp" from liburing:

io_uring_enter(5, 40, 0, 0, NULL, 8)    = 40
io_uring_enter(5, 1, 0, 0, NULL, 8)     = 1
io_uring_enter(5, 1, 0, 0, NULL, 8)     = 1
...

What really happens are read + write requests.  Without that
information the strace output is mostly useless.

This loss of information is not new, e.g. calls through the vdso or
futext fast paths are also invisible to strace.  But losing filesystem
I/O calls are a major blow, imo.

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?

Is there some existing tracing infrastructure that strace could use to
get async completion events?  Should we be introducing one?

Thanks,
Miklos