linux-kernel - Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

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Date:   Tue, 19 Dec 2017 11:46:10 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

On (12/18/17 21:03), Steven Rostedt wrote:
> > and this is exactly what I'm still observing. i_do_printks-1992 stops
> > printing, while console_sem is owned by another task. Since log_store()
> > much faster than call_console_drivers() AND console_sem owner is getting
> > preempted for unknown period of time, we end up having pending messages
> > in logbuf... and it's kworker/0:1-135 that prints them all.
> > 
> >    systemd-udevd-671   [003] d..3    66.334866: offloading: set console_owner
> >      kworker/0:1-135   [000] d..2    66.335999: offloading: vprintk_emit()->trylock FAIL  will spin? :1
> >     i_do_printks-1992  [002] d..2    66.345474: offloading: vprintk_emit()->trylock FAIL  will spin? :0    x 1100
> >    ...
> >    systemd-udevd-671   [003] d..3    66.345917: offloading: clear console_owner  waiter != NULL :1
> 
> And kworker will still be bounded in what it can print. Yes it may end
> up being the entire buffer, but that should not take longer than a
> watchdog.

not the case on my setup. "1100 messages" is already longer than watchdog.
consoles don't scale. if anyone's console can keep up with 2 printing CPUs,
then let's see what logbuf size that person will set on a system with 1024
CPUs under OOM. I doubt that will be 128KB.

anyway,
before you guys push the patch to printk.git, can we wait for Tejun to
run his tests against it? (or do we have a preemptive "non realistic
tests" conclusion?)

	-ss