linux-kernel - Re: [PATCH] printk: Add CONFIG_CONSOLE_LOGLEVEL

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Date:   Fri, 25 Jun 2021 13:17:43 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] printk: Add CONFIG_CONSOLE_LOGLEVEL_PANIC

Hi Petr, thanks for looking into this,

On 6/25/21 10:13 AM, Petr Mladek wrote:
> On Tue 2021-06-22 15:33:50, Dmitry Safonov wrote:
[..]
>> @@ -76,8 +76,8 @@ static inline void console_silent(void)
>>  
>>  static inline void console_verbose(void)
>>  {
>> -	if (console_loglevel)
>> -		console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
>> +	if (console_loglevel && (CONFIG_CONSOLE_LOGLEVEL_PANIC > 0))
>> +		console_loglevel = CONFIG_CONSOLE_LOGLEVEL_PANIC;
> 
> console_verbose() is called also in some other situations.
> For example, check_hung_task(), oops_begin(), debug_locks_ff().
> These do not always lead to panic.>
> At minimum, the name is misleading. It should be something
> like CONFIG_CONSOLE_LOGLEVEL_VERBOSE.
>
> But the question is whether we really want to limit the loglevel
> also in the non-panic scenarios. IMHO, it is a bad idea.
>
> A better solution would be to introduce console_verbose_panic()
> and use it only when it is really going to panic. The function
> should also use the lower value only when crash dump is really
> successfully enabled.

Hmm, check_hung_task() calls it only if it's going to panic().
debug_locks_off() AFAICS is called only when there is something bad with
either lockdep itself or locks: they may get freed
[print_freed_lock_bug()] or lock is held on return to userspace
[lockdep_sys_exit()] and so on - when lockdep has to turn off. Arguably,
the situations are somewhat close to panic.
MCE calls it also just before panic.

So, the only left is oops_begin().
I'm not sure what to do about it.
What do you think, should console_verbose() be called only under
panic_on_oops? Or should there be console_unverbose() to return the
loglevel in oops_end()? [that seems quite a bit ugly, considering that
there're already places that temporary save loglevel and adding another
one is ugh]

Renaming console_verbose() to console_verbose_on_panic() or something
sounds good to me - I didn't do it only to keep the patch short.

>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 678c13967580..0c12cafd9d8b 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -61,6 +61,19 @@ config CONSOLE_LOGLEVEL_QUIET
>>  	  will be used as the loglevel. IOW passing "quiet" will be the
>>  	  equivalent of passing "loglevel=<CONSOLE_LOGLEVEL_QUIET>"
>>  
>> +config CONSOLE_LOGLEVEL_PANIC
>> +	int "panic console loglevel (1-15)"
> 
> The range is 1-15 here.
> 
>> +	range 0 15
> 
> But it is 0-15 here. If you use "range 1 15" you should not need the
> check (CONFIG_CONSOLE_LOGLEVEL_PANIC > 0) in the code.
> 
>> +	default "15"
>> +	help
>> +	  loglevel to use in kernel panic or oopses.
>> +
>> +	  Usually in order to provide more debug information on console upon
>> +	  panic, one wants to see everything being printed (loglevel = 15).
>> +	  With an exception to setups with low baudrate on serial console,
>> +	  keeping this value high is a good choice.
>> +	  0 value is to keep the loglevel during panic/oops unchanged.
> 
> The trick with 0 value just makes things more complicated. The default
> "15" does the same job and should be good enough. The hard-coded
> default is good enough for the other CONSOLE_LOGLEVEL_* settings.

Well, "0" is kinda reverse to "15" - it doesn't change loglevel at all.
Actually, the origin purpose of the patch is to have "0" :-)
I thought 0-15 would be better than just off or on to MOTORMOUTH.

Now, looking at it again, I think what may be even better:

: if (console_loglevel && (CONFIG_CONSOLE_LOGLEVEL_PANIC >
console_loglevel))
:       console_loglevel = CONFIG_CONSOLE_LOGLEVEL_PANIC;

that way I can get rid of "0".
What do you think?

Thanks,
          Dmitry