linux-kernel - Re: printk meeting at LPC

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Date:   Sun, 15 Sep 2019 15:47:26 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>, Paul Turner <pjt@...gle.com>,
        Prarit Bhargava <prarit@...hat.com>
Subject: Re: printk meeting at LPC

On 2019-09-13, Daniel Vetter <daniel.vetter@...ll.ch> wrote:
>> 2. A kernel thread will be created for each registered console, each
>> responsible for being the sole printers to their respective
>> consoles. With this, console printing is _fully_ decoupled from
>> printk() callers.
>
> Is the plan to split the console_lock up into a per-console thing? Or
> postponed for later on?

AFAICT, the only purpose for a console_lock would be to synchronize
between the console printing kthread and some other component that wants
to write to that same device. So a per-console console_lock should be
the proper solution. However, I will look into the details. My main
concerns about this are the suspend/resume logic and the code sitting
behind /dev/console. I will share details once I've sorted it all out.

>> 6. A new may-sleep function pr_flush() will be made available to wait
>> for all previously printk'd messages to be output on all consoles
>> before proceeding. For example:
>>
>>     pr_cont("Running test ABC... ");
>>     pr_flush();
>>
>>     do_test();
>>
>>     pr_cont("PASSED\n");
>>     pr_flush();
>
> Just crossed my mind: Could/should we lockdep-annotate pr_flush (take
> a lockdep map in there that we also take around the calls down into
> console drivers in each of the console printing kthreads or something
> like that)? Just to avoid too many surprises when people call pr_flush
> from within gpu drivers and wonder why it doesn't work so well.

Why would it not work so well? Basically the task calling pr_flush()
will monitor the lockless iterators of the various consoles until _all_
have hit/passed the latest sequence number from the time of the call.

> Although with this nice plan we'll take the modeset paths fully out of
> the printk paths (even for normal outputs) I hope, so should be a lot
> more reasonable.

You will be running in your own preemptible kthread, so any paths you
take should be safe.

> From gpu perspective this all sounds extremely good and first
> realistic plan that might lead us to an actually working bsod on
> linux.

Are you planning on basing the bsod stuff on write_atomic() (which is
used after entering an emergency state) or on the kmsg_dump facility? I
would expect kmsg_dump might be more appropriate, unless there are
concerns that the machine will die before getting that far (i.e. there
is a lot that happens between when an OOPS begins and when kmsg_dumpers
are invoked).

John Ogness