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

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Date:   Wed, 29 Apr 2020 01:23:15 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Dmitry Safonov <dima@...sta.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".

On 2020/04/29 0:45, Michal Hocko wrote:
> On Tue 28-04-20 22:11:19, Tetsuo Handa wrote:
>> Existing KERN_$LEVEL allows a user to determine whether he/she wants that message
>> to be printed on consoles (even if it spams his/her operation doing on consoles), and
>> at the same time constrains that user whether that message is saved to log files.
>> KERN_NO_CONSOLES allows a user to control whether he/she wants that message to be
>> saved to log files (without spamming his/her operation doing on consoles).
> 
> I understand that. But how do I know whether the user considers the
> particular information important enough to be dumped on the console.
> This sounds like a policy in the kernel to me.

I'm still unable to understand your question.

>                                                I simply cannot forsee
> any console configuration to tell whether my information is going to
> swamp the console to no use or not.

Neither can I.

>                                     Compare that to KERN_$LEVEL instead.
> I know that an information is of low/high importance. It is the user
> policy to decide and base some filtering on top of that priority.

Whether to use KERN_NO_CONSOLES is not per-importance basis but per-content basis.

Since both pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n", ...) from dump_tasks() and
pr_info("oom-kill:constraint=%s,nodemask=%*pbl", ...) from dump_oom_summary() use KERN_INFO importance,
existing KERN_$LEVEL-based approach cannot handle these messages differently. Since changing the former to
e.g. KERN_DEBUG will cause userspace to discard the messages, we effectively can't change KERN_$LEVEL.
If the kernel allows the former to use KERN_NO_CONSOLES in addition to KERN_INFO, the administrator can
select from two choices: printing "both the former and the latter" or "only the latter" to consoles.