linux-kernel - Re: [PATCH 1/4] printk: Introduce per-console loglevel setting

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Date:   Wed, 20 Mar 2019 16:37:29 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Calvin Owens <calvinowens@...com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH 1/4] printk: Introduce per-console loglevel setting

On Thu 2019-03-14 23:12:49, Tetsuo Handa wrote:
> Petr Mladek wrote:
> > It might be even more straightforward when the per-console value
> > defines the effective console level. I mean the following semantic:
> > 
> >    + "console_loglevel" would define the default loglevel used
> >      by consoles at runtime.
> > 
> >    + the per-console loglevel could override the default
> >      console_loglevel.
> > 
> >    + We would need a custom handler for the sysctl "console_loglevel".
> >      It would write the given value to the global console_loglevel
> >      variable and for all already registered consoles (con->loglevel).
> 
> But some functions change console_loglevel without sysctl (e.g.
> console_verbose() when reporting hung tasks and panic()). Should
> con->loglevel be changed (which might result in too much messages to
> slow consoles) when console_loglevel changes?

It is about the semantic. We either want to set a hard limit
for each console or we want to set per-console loglevel that
will get used in normal situations.

I prefer the 2nd semantic. IMHO, console_verbose() should be
used only in situations when people really want to see all
lines, for example, panic, sysrq output when the machine
looks deadlocked, ignore_loglevel is set. I believe that they
want to see them even on the slow consoles that are
there exactly for debugging these critical situations.

Best Regards,
Petr