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

From: Dmitry Safonov <dima@arista.com>
To: Steven Rostedt <rostedt@goodmis.org>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	linux-kernel@vger.kernel.org, Michal Hocko <mhocko@suse.com>,
	Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".
Date: Fri, 24 Apr 2020 16:52:31 +0100	[thread overview]
Message-ID: <b3129b4a-cd08-c507-4c6a-8b3e24b130d0@arista.com> (raw)
In-Reply-To: <20200424114225.5a3bab7e@gandalf.local.home>

On 4/24/20 4:42 PM, Steven Rostedt wrote:
> On Sat, 25 Apr 2020 00:28:53 +0900
> Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:
>> For those who analyze log files (instead of console output), the point of
>> printk() is to save kernel messages into log files (via userspace syslog
>> daemon).
>>
>> By the way, I think
>>
>>   printk(KERN_NO_CONSOLES "hello\n")
>>
>> is almost same with doing
>>
>>   saved_loglevel = console_loglevel;
>>   console_loglevel = CONSOLE_LOGLEVEL_SILENT;
>>   printk("hello\n");
>>   console_loglevel = saved_loglevel;
>>
>> used by vkdb_printf().
> 
> And both shouldn't be done within the kernel. The "CONSOLE_LOGLEVEL_SILENT"
> if for user decided policy, not the kernel making that policy for the user.

Though, I believe the reverse KERN_UNSUPPRESSED [1] still makes sense
for sysrq/kdb where user expects their interactive messages to appear on
console. And unfortunately, userspace depends on the loglevel of those
messages as it is.
With KERN_UNSUPPRESSED I hope to remove all those console_loglevel
tricks and make it static variable for printk.

[1]: https://lore.kernel.org/lkml/20190528002412.1625-1-dima@arista.com/

Thanks,
          Dmitry