linux-kernel - Re: [RFC][PATCH 0/4] printk: introduce printing kernel thread

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Date:   Sat, 25 Mar 2017 09:18:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 0/4] printk: introduce printing kernel thread

On (03/24/17 15:43), Petr Mladek wrote:
[..]
> > we have that thing which we call "old printk" mode, which is not
> > really informative. and my proposal is rename "old" mode and use
> > "printk rescue" mode instead. because we switch to that mode when
> > we are trying to "rescue" kernel logs. so the API can be something
> > like
> > 		printk_rescue_on()
> > 		printk_rescue_off()
> 
> Sounds good to me. Slight problem is that off() does not cause
> stopping the mode if we are nested.
> 
> Just one more attempt inspired by this:
> 
> 		printk_emergency_begin()
> 		printk_emergency_end()
> 
> Note that we actually start this mode automatically also
> with pr_emerg() message.

good. printk_emergency sounds OK to me.

[..]
> > and we've got everything we need to have such a feedback in
> > vprintk_emit():
> > 
> > 	a) console is not suspended so console_unlock() can call console drivers
> > 	b) printk_kthread != NULL
> > 	c) we are not in enforced rescue/emergency mode
> > 	d) `log_next_seq' moves forward (always `true', we are in vprintk_emit())
> > 	e) `console_seq' stands still
> > 
> > so we can have an automatic rescue mode fallback in vprintk_emit().
> > if (a)-(e) are true then we give up on waking up printk_kthread,
> > switch to rescue mode and attempt to console_trylock() directly from
> > vprintk_emit(). the part that sucks here is that we need to give
> > printk_kthread some time to catch up. for instance, if (e) is true
> > for the past 50 invocations of vprintk_emit(), IOW:
> > 
> > 	- we added 50 lines to printk
> > 	- none have been printed on the serial console
> >
> > then we
> > 	- declare rescue
> > 	- do console_trylock() instead of wake_up() //unless in deferred vprintk_emit()
> 
> I am not sure if we are able to distinguish a flood of messages
> from a real emergency situation.

that's one of problems. yes.


> Well, there is a handful of annotated locations at the moment.
> I would start thinking of an automatic detection once we have
> more of them and have more data for a good heuristic.

so my 'automatic emergency' switch is for another case. when we
wake_up() printk_kthread, but it never prints anything. could be
caused by a missing printk_emergency annotation, or something else.


> I still would like to see the kernel parameter/sysfs knob
> that would allow to force the rescue/emergency mode all
> the time ;-)

sure.

	-ss