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Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org, Dmitry Safonov <dima@arista.com>,
        Yafang Shao <laoar.shao@gmail.com>
References: <20200424024239.63607-1-penguin-kernel@I-love.SAKURA.ne.jp>
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 <20200428121828.GP28637@dhcp22.suse.cz>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <b4d74234-8009-9ffd-011f-bd5d1a4b85f6@i-love.sakura.ne.jp>
Date:   Tue, 28 Apr 2020 22:11:19 +0900
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On 2020/04/28 21:18, Michal Hocko wrote:
> On Tue 28-04-20 20:33:21, Tetsuo Handa wrote:
>> On 2020/04/27 15:21, Sergey Senozhatsky wrote:
>>>> 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.
>>>
>>> per-console loglevel is a user configurable parameter.
>>> KERN_NO_CONSOLES is a hard-coded policy.
>>
>> But given that whether to use KERN_NO_CONSOLES is configurable via e.g. sysctl,
>> KERN_NO_CONSOLES will become a user configurable parameter. What's still wrong?
> 
> How do I as a kernel developer know that KERN_NO_CONSOLES should be
> used? In other words, how can I assume what a user will consider
> important on the console?
> 

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).

Consoles which are limited by "rows" * "columns" size should receive (ideally) up to
a few lines of messages. Printing more lines on such consoles disturbs users using
such consoles. Let alone printing one thousand lines of messages (e.g. dump_tasks())
on such consoles. Printing one thousand lines of messages on read-only consoles (e.g.
netconsole) might be fine, but non-urgent messages can afford waiting for userspace
to do more sophisticated handling (e.g. evaluate and filter).

Even without per-console loglevel feature, we can reduce spam messages for consoles
by using KERN_NO_CONSOLES. For example, send only the summary part of OOM-killer and
allocation failure messages to both consoles and log files (by not using KERN_NO_CONSOLES),
and send the non-summary part to only log files (by using KERN_NO_CONSOLES with 
KERN_$LEVEL used for summary part in order to make sure that userspace daemon will
save both the summary and non-summary parts).