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

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Eric Biederman <ebiederm@xmission.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>, Pavel Machek <pavel@ucw.cz>,
	Andreas Mohr <andi@lisas.de>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread
Date: Wed, 31 May 2017 16:30:59 +0900	[thread overview]
Message-ID: <20170531073058.GD7672@jagdpanzerIV.localdomain> (raw)
In-Reply-To: <20170529121235.GG3608@quack2.suse.cz>

Hello Jan,

On (05/29/17 14:12), Jan Kara wrote:
[..]
> Actually I had something very similar in old versions of my patch set. And
> it didn't work very well. The problem was that e.g. sometimes scheduler
> decided that printk kthread should run on the same CPU as the process
> currently doing printing and in such case printk kthread never took over
> printing and the machine locked up due to heavy printing.

hm, interesting.

> > First, the real time priority is questionable on its own. Logging
> > is important but the real time priority is dangerous. Any "flood"
> > of messages will starve all other processes with normal priority.
> > It is better than a softlockup but it might cause problems as well.
> 
> Processes with real-time priority should have well bounded runtime (in
> miliseconds). Printk kthread doesn't have such bounded runtime so it should
> not be a real time process as it could hog the CPU it is running on...

yeah, I can easily make it a normal prio task. at the same time
printk_kthread has 'soft' limits on its execution. it's under the
same constraints as the rest of the processes that do printing.
there can be a random RT task doing console_trylock()->console_unlock(),
so we still can hog CPUs. but, yeah, I don't want printk_kthread to be
special.

> So I think what Petr suggests below is better. Keep normal priority, print
> something to console from the process doing printk() and just wake up
> printk kthread and hope it can print the rest. It is not ideal but unless
> there's a flood of messages there is no regression to current state.

hm, this is very close to what I do in my patch. with some additional
guarantess. because people mostly want to have good old printk. that
let's hope part basically doesn't work when it's needed the most. we
had a ton of cases of lost messages in serial logs.

I replied in more details in another mail.

	-ss