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

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Message-ID: <20171219043658.GD17164@jagdpanzerIV>
Date:   Tue, 19 Dec 2017 13:36:58 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.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 12:46), Steven Rostedt wrote:
> > One question is if we really want to rely on offloading in
> > this case. What if this is printed to debug some stalled
> > system.
> 
> Correct, and this is what I call when debugging hard lockups, and I do
> it from NMI.
[..]
> show_state_filter() is not a normal printk() call. It is used for
> debugging.

just for the record. a side note.

you guys somehow made spectacularly off-target conclusions from the
traces I have provided and decided NOT to concentrate on demonstrated
behavioural patterns, but on, perhaps, process' names (I really should
have renamed i_do_printks to DONALD_TRUMP ;) ) and on how those printk
lines got into the logbuf. like if it mattered. [seriously, why?]. the
point was not in show_state_filter()... the point was - preemption and
things that hand off does. but somehow filling up logbuf when console_sem
owner is preempted is unrealistic if printks are coming from task A
under normal conditions; and it is a completely different story when
the same task A fills up logbuf from OOM while console_sem owner is
preempted. the end result is the same in both cases: it's not task A
that is going to flush logbuf. it's some other task that will have to
do it, possibly being in atomic context. anyway, anyway.

	-ss