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

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Date:   Wed, 31 May 2017 16:30:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...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>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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