Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Message-ID: <4bcfd2b3-66fd-c5df-a8df-c51a89a0bce4@gmx.de>
Date:   Thu, 7 Apr 2022 22:20:30 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.7.0
Subject: Re: [PATCH printk v2 09/12] printk: add functions to prefer direct
 printing
Content-Language: en-US
To:     John Ogness <john.ogness@linutronix.de>,
        Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Wei Liu <wei.liu@kernel.org>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Marco Elver <elver@google.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Shawn Guo <shawn.guo@linaro.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Phil Auld <pauld@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Wang Qing <wangqing@vivo.com>, rcu@vger.kernel.org
References: <20220405132535.649171-1-john.ogness@linutronix.de>
 <20220405132535.649171-10-john.ogness@linutronix.de> <Yk61Q+w7tWTjh8xl@alley>
 <b746d99e-729f-ca0d-5d30-6d803d1cd833@gmx.de>
 <87zgkxnepg.fsf@jogness.linutronix.de>
 <7fc5853e-dbe6-c2d2-2130-003e3b9777bc@gmx.de>
 <878rsgvews.fsf@jogness.linutronix.de>
From:   Helge Deller <deller@gmx.de>
In-Reply-To: <878rsgvews.fsf@jogness.linutronix.de>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On 4/7/22 22:04, John Ogness wrote:
> On 2022-04-07, Helge Deller <deller@gmx.de> wrote:
>> In my case - while I debug low-level kernel code - I then just need to
>> use pr_warn() or pr_emerg() and get it printed non-threadened. That's
>> sufficient for me.
>
> Actually, no. The loglevel does not determine if a message is direct
> printed or not. By "warn" I was referring to WARN_ON(condition).

Oh, then there was a misunderstanding on my side.

> If you are debugging low-level kernel code, you usually will _want_
> threaded printing. The timestamps match the printk() call, so you will
> get accurate logs. And the runtime performance of your low-level kernel
> code will not be significantly affected by the printk() call.

That really much depends on what you debug.
Currently I'm debugging some CPU hotplug stuff, and I really want my
debug info printed immediately, otherwise it's too late to analyze what's
going wrong. I can imaginge other use cases like trying to find memory
leaks are similiar.

> If for some reason you really want non-threaded printing, the patch we
> are discussing creates new functions to manually activate it:
>
> printk_prefer_direct_enter();
> pr_info("debugging\n");
> printk_prefer_direct_exit();

That's quite unhandy. At least I would have thought that pr_emerg() would
do that call sequence.
Wouldn't it make sense to make pr_emerg() work unthreadened, as it's
used quite seldom and only in special situations...

> But then your pr_info() will cause significant latencies. The timestamp
> would be the same, with or without direct printing.

Yes, but that would be Ok in my case.

Helge