linux-kernel - Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread

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Date:   Fri, 7 Apr 2017 14:06:42 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...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>, Len Brown <len.brown@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv2 2/8] printk: introduce printing kernel thread

Hi!

> On (04/07/17 09:21), Pavel Machek wrote:
> > > spin_dump() and trigger_all_cpu_backtrace() result in a bunch of
> > > additional printk()-s so CPU0 has even more job to do in console_unlock(),
> > > while it still holds the contended spin_lock. and so on; there are
> > > many other examples.
> > > 
> > > so should we declare a "we can spend only 2 seconds in direct printk()
> > > and then must offload printing" rule? I don't think it's much better
> > > than a simpler "we always offload, as long as we think it's safe".
> > 
> > I believe we should do the 2 seconds rule. It allows us to print "some
> > messages delayed" message, so at least whoever is trying to debug the
> > crash will have the hints that he needs to look at the printk system.
> 
> do you mean panic()? in panic() we call console_flush_on_panic(),
> which immediately outputs all pending logbuf messages. printk()
> offloading does not happen there.

Not panic(). I have seen many crashes where we had printk(KERN_ERR)
and then hard hang. And the printk() was really important for debugging.

> > "we always offload, as long as we think it's safe" rule does not
> > really work, as printk() can not detect if it is safe or not.
> 
> but "2 seconds" rule has that "as long as we think it's safe" string
> attached as well. just because we do offloading. which is sometimes
> un-safe. so regardless the timeout value (0 seconds or 2 seconds) we
> still need some sort of a hint from the path that issues printk()
> because that path (panic, kexec, sysrq, etc.) knows for sure when
> things are abnormal. printk() is pretty clueless in this regard.
> /* well, I still think that EMERG loglevel thing is not completely
>  broken. */

Well, at least with my solution you know there are messages that were
not printed.

Yes, you'd still want to switch printk_now() for stuff like
panic(). But if you get it wrong (and you will), at least you will see
the "something is missing here" message in the log. 

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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