linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Date:   Mon, 4 Mar 2019 14:23:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        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@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On (02/13/19 15:43), John Ogness wrote:
> On 2019-02-13, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
> >> - A dedicated kernel thread is created for printing to all consoles in
> >>   a fully preemptible context.
> >
> > How do you handle sysrq-<foo> printouts on systems which can't
> > schedule printk-kthread?
> 
> If those sysrq printouts are at the emergency loglevel (which most are),
> then they are printed immediately to the emergency consoles. This has
> already proved useful for our own kernel debugging work. For example,
> currently sysrq-z for very large traces result in messages being dropped
> because of printk buffer overflows. But with the emergency console we
> always see the full trace buffer.

Are we sure that all systems will always have ->atomic console(-s)
enabled? Is it possible to convert all console drivers to ->atomic?
fbcon, for instance (with scrolling and font scaling, etc)? If there
are setups which can be fully !atomic (in terms of console output)
then we, essentially, have a fully preemptible kthread printk
implementation.

> Because you have already done so much work and experimentation with
> printk-kthreads, I feel like many of your comments are related to your
> kthread work in this area. Really the big design change I make with my
> printk-kthread is that it is only for non-critical messages. For
> anything critical, users should rely on an emergency console.

Fair point. Well maybe my printk-kthread comments are not utterly
unreasonable, but who knows :)

	-ss