Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.

From: Petr Mladek <pmladek@suse.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: David Laight <David.Laight@ACULAB.COM>,
	'Steven Rostedt' <rostedt@goodmis.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.
Date: Fri, 30 Nov 2018 17:01:47 +0100	[thread overview]
Message-ID: <20181130160147.r6idgsr2biy5rwap@pathway.suse.cz> (raw)
In-Reply-To: <1d29f61a-8f36-ab1c-bb92-402ee9ad161d@i-love.sakura.ne.jp>

On Thu 2018-11-29 19:09:26, Tetsuo Handa wrote:
> On 2018/11/28 22:29, David Laight wrote:
> > I also spent a week trying to work out why a customer kernel was
> > locking up - only to finally find out that the distro they were
> > using set 'panic on opps' - making it almost impossible to find
> > out what was happening.

Did the machine rebooted before the messages reached console or
did it produced crash-dump or frozen?

panic() tries relatively hard to flush the messages to the console,
see printk_safe_flush_on_panic() and console_flush_on_panic().
It is less aggressive when crashdump is called. It might deadlock
in console drivers.

Hmm, it might also fail when another CPU is still running and
steals console_lock. We might want to disable
console_trylock_spinning() if the caller is not
panic_cpu.

> On 2018/11/26 13:34, Sergey Senozhatsky wrote:
> > Or... Instead.
> > We can just leave pr_cont() alone for now. And make it possible to
> > reconstruct messages - IOW, inject some info to printk messages. We
> > do this at Samsung (inject CPU number at the beginning of every
> > message. `cat serial.0 | grep "\[1\]"` to grep for all messages from
> > CPU1). Probably this would be the simplest thing.
> 
> Yes, I sent a patch which helps reconstructing messages at
> http://lkml.kernel.org/r/1543045075-3008-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .

All the buffering approaches have problems that cannot be solved
easily. The prefix-based approach looks like the best alternative
at the moment.

Best Regards,
Petr