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

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Date:   Tue, 12 Mar 2019 16:15:55 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...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@...il.com>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On 2019-03-12, Petr Mladek <pmladek@...e.com> wrote:
> Per-console kthread sounds interesting but there is the problem with
> reliability. I mean that kthread need not get scheduled.
>
> Some of these problems might get solved by the per-console loglevel
> patchset.
>
> Sigh, any feature might be useful in some situation. But we always
> have to consider the cost and the gain. I wonder how common is
> to actively use two consoles at the same time and what would
> be the motivation.

The following is from the linux-serial mailing list:
"Re: Serial console is causing system lock-up" [0]

I'm responding to it here because it really belongs in this thread.

On 2019-03-12, Petr Mladek <pmladek@...e.com> wrote:
> On Tue 2019-03-12 09:17:49, John Ogness wrote:
>> The current printk implementation is handling all console printing as
>> best effort. Trying hard enough to dramatically affect the system, but
>> not trying hard enough to guarantee success.
>
> I agree that direct output is more reliable. It might be very useful
> for debugging some types of problems. The question is if it is worth
> the cost (code complexity, serializing CPUs == slowing down the
> entire system).
>
> But it is is possible that a reasonable offloading (in the direction
> of last Sergey's approach) might be a better deal.
>
>
> I suggest the following way forward (separate patchsets):
>
>     1. Replace log buffer (least controversial thing)

Yes. I will post a series that only implements the ringbuffer using your
simplified API. That will be enough to remove printk_safe and actually
does most of the work of updating devkmsg, kmsg_dump, and syslog.

>     2. Reliable offload to kthread (would be useful anyway)

Yes. I would like to implement per-console kthreads for this series. I
think the advantages are obvious. For PREEMPT_RT the offloading will
need to be always active. (PREEMPT_RT cannot call the console->write()
from atomic contexts.) But I think this would be acceptable at first. It
would certainly be better than what PREEMPT_RT is doing now.

>     3. Atomic consoles (a lot of tricky code, might not be
> 		worth the effort)

I think this will be necessary. PREEMPT_RT cannot support reliable
emergency console messages without it. And for kernel developers this is
also very helpful. People like PeterZ are using their own patches
because the mainline kernel is not providing this functionality.

The decision about _when_ to use it is still in the air. But I guess
we'll worry about that when we get that far. There's enough to do until
then.

> Could we agree on this?

I think this is a sane path forward. Thank you for providing some
direction here.

John Ogness

[0] https://marc.info/?l=linux-serial&m=155239250721543