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

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Date:   Fri, 30 Jun 2017 22:38:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        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

On (06/30/17 15:16), Petr Mladek wrote:
> Anyway, the handshake during offloading might be pretty
> problematic. To be honest, I do not have much experience
> with it. I have shared some my fears in the other mail[1].
> Jan Kara spent a lot of time on this and probably could
> say more.
> 
> Maybe, we could try to look into the throotling path. Slowing down
> massive printk() callers looks necessary when things gets
> out of control.

throttling, in some form, is already there. I think.

there is a printk_delay() function. which we can silently activate
when things don't look cool anymore. and printk_delay() is already
getting called on every vprintk_emit() entry. the question is -- how
big should be our delay value, and... when do we need to activate
printk_delay()?

when the distance between console_seq and log_next_seq... suggests
that we will drop (overwrite) un-flushed messages sooner than console_seq
reaches log_next_seq? so log_next_seq is closer to log_first_seq than
console_seq to log_next_seq. or something like this... I'm a bit tired
after a long week; need more time to think about it.

the same printk_delay() *may be* can be used in console_unlock(), to
give potential new console_sem owner more time. just an idea.

> I wonder if I could add some counter into task_struct.
> It might be configurable. I am not sure if people would
> want this enabled on production systems where the level
> of messages should be lower anyway.

	-ss