linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Date:   Fri, 30 Jun 2017 23:28:51 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On (06/30/17 09:33), Steven Rostedt wrote:
[..]
> > "something sometime in the future" is equal to "no one".
> > 
> > we must stay and continue printing. because it gives the right
> > answer - "current process and right now. until someone else
> > (+printk_kthread) takes over".
> 
> Would it be acceptable to have a user knob that allows for it not to
> happen? That is, let the user of the kernel decide if they care about
> critical prints or not? If a knob says, "only print X, then offload"
> would that be allowed. Of course the default would be "only print ALL
> OF IT" to keep the current behavior.

I'm sorry, I'm not sure I got your question right (I need some rest
probably).

I guess the question was, a knob that would determine what happens after
current wakes up printk_kthread -- does it stay in console_unlock() and
wait for new console_sem owner, printing the messages in the meantime,
or goes all in and expects printk_kthread or anything else to lock
console_sem at some point and flush the remaining messages. is that
correct? we can do this (well, I'm absolutely not in position to say
"we can't do this" :) ). I guess the sort of a problem we have now is
that we can't guarantee that wake_up() will actually wake_up printk_kthread.
but if user requests it, then well... it might be easier to adjust watchdog
timeout value ;) just kidding. or may be I misunderstood your question.

or... another silly idea... shall we start touching the lockup
watchdog per-cpu counters on the CPU that has:

a) woken up printk_kthread
b) but is still in console_unlock() waiting for the new console_sem owner?

...

or we, may be, can add a new sysrq that would flush logbuf messages...
the same way as console_flush_on_panic() does. just in case if wake_up
didn't work.... or is there already a sysrq for that?

	-ss

> A lot of times the console isn't recorded to debug hard lock ups. I
> know most desktops running a GUI do not. When ever my workstation locks
> up, and it has no serial, I don't get to see the dmesg at all. In this
> situation, I don't care if the prints are offloaded or not.
> 
> -- Steve
>