linux-kernel - Re: [PATCH] printk: Add loglevel for "do not print to consoles".

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Date:   Sat, 25 Apr 2020 10:07:45 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Dmitry Safonov <dima@...sta.com>,
        Michal Hocko <mhocko@...e.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".

On 2020/04/25 9:46, Sergey Senozhatsky wrote:
> On (20/04/24 11:42), Tetsuo Handa wrote:
> [..]
>> @@ -19,6 +19,7 @@ static inline int printk_get_level(const char *buffer)
>>  		switch (buffer[1]) {
>>  		case '0' ... '7':
>>  		case 'c':	/* KERN_CONT */
>> +		case 'S':       /* KERN_NO_CONSOLES */
>>  			return buffer[1];
>>  		}
>>  	}
> 
> So this means NO_CONSOLES_AT_ALL, slow + fast ones.

Right.

>                                                     I wonder if this
> wants to be NO_SLOW_CONSOLES instead. Which then brings us to the
> next question - can this be done with per-console loglevel setting?

It is difficult to define what is slow consoles. While netconsole will be
a fast console, we can forward kernel messages via syslog daemon if desired.

NO_SLOW_CONSOLES might be useful for immediate notification like panic().
But KERN_NO_CONSOLES is not for immediate notification like panic().

KERN_NO_CONSOLES is for type of messages where "saved for later analysis" is
important but "printed for immediate notification" is not important.
In other words, KERN_NO_CONSOLES is NOT for dying messages where "printed for
immediate notification" is important.