linux-kernel - Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

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Date:   Wed, 06 Mar 2019 23:22:57 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

On 2019-03-06, Petr Mladek <pmladek@...e.com> wrote:
>> _Both_ categories are important for the user, but their requirements
>> are different:
>> 
>>    informational: non-disturbing
>>    emergency:     reliable
>
> Isn't this already handled by the console_level?
>
> The informational messages can be reliably read via syslog, /dev/kmsg.
> They are related to the normal works when the system works well.
>
> The emergency messages (errors, warnings) are printed in emergency
> situations. They are printed as reliably as possible to the console
> because the userspace might not be reliable enough.

I've never viewed console_level this way. _If_ console_level really is
supposed to define the emergency/informational boundary, all
informational messages are supposed to be handled by userspace, and
console printing's main objective is reliability... then I would change
my proposal such that:

- if a console supports write_atomic(), _all_ console printing for that
  console would use write_atomic()

- only consoles without write_atomic() will be printing via the
  printk-kthread(s)

IMO, for consoles with write_atomic(), this would increase reliability
over the current mainline implementation. It would also simplify
write_atomic() implementations because they would no longer need to
synchronize against write().

For those consoles that cannot implement write_atomic() (vt and
netconsole come to mind), or as a transition period until remaining
console drivers have implemented write_atomic(), these would use the
"fallback" of printing fully preemptively in their own kthread using
write().

Does this better align with the concept of the console_loglevel and the
purpose of console printing?

John Ogness