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From:   John Ogness <john.ogness@linutronix.de>
To:     Helge Deller <deller@gmx.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
Subject: Re: [PATCH printk v2 09/12] printk: add functions to prefer direct
 printing
In-Reply-To: <7fc5853e-dbe6-c2d2-2130-003e3b9777bc@gmx.de>
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>
Date:   Thu, 07 Apr 2022 22:10:19 +0206
Message-ID: <878rsgvews.fsf@jogness.linutronix.de>
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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).

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.

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();

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

John