Re: printk meeting at LPC

From: John Ogness <john.ogness@linutronix.de>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Brendan Higgins <brendanhiggins@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>, Paul Turner <pjt@google.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Prarit Bhargava <prarit@redhat.com>
Subject: Re: printk meeting at LPC
Date: Wed, 18 Sep 2019 09:33:01 +0200	[thread overview]
Message-ID: <87lfumnjo2.fsf@linutronix.de> (raw)
In-Reply-To: <20190918023654.GB15380@jagdpanzerIV> (Sergey Senozhatsky's message of "Wed, 18 Sep 2019 11:36:54 +0900")

On 2019-09-18, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> 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.
>>> 
>>> sysrq over serial?
>>> 
>>> offloading this to kthread may be unreliable.
>> 
>> But we also talked about an "emergency flush" which will not wait for
>> the kthreads to finish and just output everything it can find in the
>> printk buffers (expecting that the consoles have an "emergency"
>> handler. We can add a sysrq to do an emergency flush.

The problem with only a flush here is that the sysrq output may not fit
in the ringbuffer (ftrace, for example). It probably makes more sense to
have a switch to enter/exit "synchronous state", where all atomic
consoles are flushed upon enter and all future printk's are synchronous
on atomic consoles until exit.

I expect sysrq to be the only valid use of "synchronous state" other
than oops/panic. Although I suppose PeterZ would like a boot argument to
always run the consoles in this state.

> I agree that when consoles have ->atomic_write then it surely makes
> sense to switch to emergency mode. I like the emergency state
> approach, but I'm not sure how it can be completely invisible to the
> rest of the system.  Quoting John:
>
>     : Unlike oops_in_progress, this state will not be visible to
>     : anything outside of the printk infrastructure.
>
> For instance, tty/sysrq must be able to switch printk emergency
> on/off.

The switch/flush _will_ be visible. But not the state. So, for example,
it won't be possible for some random driver to determine if we are in an
emergency state. (Well, I don't know if oops_in_progress will really
disappear. But at least the printk/console stuff will no longer rely on
it.)

> We also figured out that some PM (hibernation/suspend/etc.) stages
> (very early and/or very late ones) [2] also should have printk in
> emergency mode, plus some other parts of the kernel [3].
>
> [1] https://lore.kernel.org/lkml/20170815025625.1977-4-sergey.senozhatsky@gmail.com/
> [2] https://lore.kernel.org/lkml/20170815025625.1977-7-sergey.senozhatsky@gmail.com/
> [3] https://lore.kernel.org/lkml/20170815025625.1977-8-sergey.senozhatsky@gmail.com/

Thanks for bringing up that RFC thread again. I haven't looked at it in
over a year. I will go through it again to see if there is anything I've
overlooked. Particularly the suspend stuff.

John Ogness