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

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Date:   Thu, 07 Mar 2019 10:53:48 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     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 2019-03-04, 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)?

No, I am not sure if we can convert all console drivers to atomic
consoles. But I think if we don't have to fear disturbing the system,
the possibilities for such an implementation are greater.

> If there are setups which can be fully !atomic (in terms of console
> output) then we, essentially, have a fully preemptible kthread printk
> implementation.

Correct. I've mentioned in another response[0] some ideas about what
could be done to aid this.

I understand that fully preemptible kthread printing is unacceptable for
you. Since all current console drivers are already irq safe, I'm
wondering if using irq_work to handle the emergency printing for console
drivers without write_atomic() would help. (If the printk caller is in a
context that write() supports, then write() could be called directly.)
This would also demand that the irq-safe requirements for write() are
not relaxed. The printk-kthread might still be faster than irq_work, but
it might increase reliability if an irq_work is triggered as an extra
precaution.

John Ogness

[0] https://lkml.kernel.org/r/87o96p9gtx.fsf@linutronix.de