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

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Date:   Thu, 7 Apr 2022 15:35:53 +0200
From:   Helge Deller <deller@....de>
To:     Petr Mladek <pmladek@...e.com>,
        John Ogness <john.ogness@...utronix.de>
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 11:56, Petr Mladek wrote:
> On Tue 2022-04-05 15:31:32, John Ogness wrote:
>> Once kthread printing is available, console printing will no longer
>> occur in the context of the printk caller. However, there are some
>> special contexts where it is desirable for the printk caller to
>> directly print out kernel messages. Using pr_flush() to wait for
>> threaded printers is only possible if the caller is in a sleepable
>> context and the kthreads are active. That is not always the case.
>>
>> Introduce printk_prefer_direct_enter() and printk_prefer_direct_exit()
>> functions to explicitly (and globally) activate/deactivate preferred
>> direct console printing. The term "direct console printing" refers to
>> printing to all enabled consoles from the context of the printk
>> caller. The term "prefer" is used because this type of printing is
>> only best effort. If the console is currently locked or other
>> printers are already actively printing, the printk caller will need
>> to rely on the other contexts to handle the printing.
>>
>> This preferred direct printing is how all printing is currently
>> handled (unless explicitly deferred).
>>
>> When kthread printing is introduced, there may be some unanticipated
>> problems due to kthreads being unable to flush important messages.
>> In order to minimize such risks, preferred direct printing is
>> activated for the primary important messages when the system
>> experiences general types of major errors. These are:
>>
>>  - emergency reboot/shutdown
>>  - cpu and rcu stalls
>>  - hard and soft lockups
>>  - hung tasks
>>  - warn
>>  - sysrq

I agree that those needs direct printing.
Did you maybe checked how much (e.g. in percentage) of a normal log is then
still left to be printed threadened?  Just in case it's not much,
is there still a benefit to introduce threadened printing?

Or another example, e.g. when running on a slow console (the best
testcase I know of is a 9600 baud serial port), where some printks
are threadened. Then a warn printk() has to be shown, doesn't it mean
all threadened printks have to be flushed (waited for) until
the warning can be shown. Will there then still be a measureable time benefit?

Just wondering...

Helge