linux-kernel - Re: [PATCH printk v2 09/12] printk: add functions to prefer direct printing

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Date:   Thu, 7 Apr 2022 22:20:30 +0200
From:   Helge Deller <deller@....de>
To:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Wei Liu <wei.liu@...nel.org>,
        Xiaoming Ni <nixiaoming@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Marco Elver <elver@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Shawn Guo <shawn.guo@...aro.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Phil Auld <pauld@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Wang Qing <wangqing@...o.com>, rcu@...r.kernel.org
Subject: Re: [PATCH printk v2 09/12] printk: add functions to prefer direct
 printing

On 4/7/22 22:04, John Ogness wrote:
> On 2022-04-07, Helge Deller <deller@....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