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

From: Petr Mladek <pmladek@suse.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Michal Hocko <mhocko@kernel.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org, Dmitry Safonov <dima@arista.com>,
	Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".
Date: Thu, 14 May 2020 18:26:12 +0200	[thread overview]
Message-ID: <20200514162612.GR17734@linux-b0ei> (raw)
In-Reply-To: <7af6fc77-986a-8a6a-ea93-b807db44413c@i-love.sakura.ne.jp>

On Thu 2020-05-14 20:23:55, Tetsuo Handa wrote:
> On 2020/05/14 17:00, Petr Mladek wrote:
> > On Wed 2020-05-13 21:59:23, Tetsuo Handa wrote:
> >> On 2020/05/13 21:19, Petr Mladek wrote:
> >>> On Wed 2020-05-13 20:24:24, Tetsuo Handa wrote:
> >>>> 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?
> >>>  
> >>>> 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.
> >>>
> >>> You might achieve the same with DEBUG loglevel. Or do I miss anything?
> >>
> >> Use of KERN_DEBUG affects userspace syslog daemon. We will have to ask administrators
> >> to configure syslog daemon not to filter KERN_DEBUG messages. And administrators will
> >> be bothered by needless KERN_DEBUG messages. Also,
> > 
> > What about using KERN_INFO then? Is there still the same problem?
> 
> dump_tasks() is already using KERN_INFO, and the same problem remains. KERN_INFO cannot
> prevent printk() from printing to consoles unless console loglevel is tuned. And tuning
> console loglevel can prevent all KERN_INFO from printing to consoles. What I want is a
> method for allowing administrators to control whether to print each message to consoles.
> Such method will be by definition controlled via "+ loglevel assigned to each message".

This does not make much sense to me. KERN_NO_CONSOLES would be another
global flag. If you enable/disable its functionality, it would affect
all strings with this flag (not only the ones used by OOM killer).

I am not going to comment the rest. We are going in circles and I do
not know how to better explain my concerns.

> Given that said, if KERN_NO_CONSOLES is not acceptable, I have to again
> battle against Michal Hocko regarding how to offload OOM-related messages,
> like Sergey Senozhatsky estimated
> 
>   "I'd say that lockless logbuf probably will land sometime around 5.8+.
>   Async printk() - unknown."

One problem there is a lack of reviewers.

Best Regards
Petr