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

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Message-ID: <d66c38d9-dd97-072d-e1a7-949e9573b38d@i-love.sakura.ne.jp>
Date:   Wed, 13 May 2020 20:24:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>, Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.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 2020/05/13 19:49, Michal Hocko wrote:
> On Wed 13-05-20 12:04:13, Petr Mladek wrote:
>> What is so special about  OOM dump task so that it would deserve such
>> complications?
> 
> Nothing really. Except for the potential amount of the output.

"echo t > /proc/sysrq-trigger" from userspace program is another example.

> But as
> you've said there are two ways around that. Disable this output if you
> do not need it or make it a lower loglevel. 

Disable this output for syslog is not acceptable for me, but disable this
output for consoles is preferable for me.

> I simply cannot tell whether somebody considers
> dump_tasks an important information to be printed on consoles.

I don't think dump_tasks() is important information to be printed on consoles.
But since somebody might think dump_tasks() is important information to be
printed on consoles, I suggest switching KERN_NO_CONSOLES using e.g. sysctl.

> 
> If there is any need to control which messages should be routed to which
> backend then the proper solution would be to filter messages per log
> level per backend.

Possible backends will be "zero or more than zero" consoles + "zero or one"
syslog, and KERN_NO_CONSOLES is a method for force selecting "zero" console
backend.

>                    But I have no idea how feasible this is for the
> existing infrastructure - or maybe it already exists...

There is per-console loglevel proposal (which allows selecting "some" console
backends). But it is based on KERN_$LOGLEVEL which is too rough-grained.