linux-kernel - Re: [PATCH 2/2] Subject: printk: Don't trap random context in infinite log

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Message-ID: <20171108143322.GC4164960@devbig577.frc2.facebook.com>
Date:   Wed, 8 Nov 2017 06:33:36 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] Subject: printk: Don't trap random context in
 infinite log_buf flush

Hello,

On Wed, Nov 08, 2017 at 02:29:35PM +0900, Sergey Senozhatsky wrote:
> > The following is a completely untested patch to show how we can put
> > the console in full sync mode, just the general idea.  I'm a bit
> > skeptical we really wanna do this given that we already (with or
> > without the patch) stay sync for most of these events due to the way
> > we go async, but, yeah, if we wanna do that, we can do that.
> 
> we've been going in a slightly different direction in printk-kthread.
> 
> we keep printk sync by default [as opposed to previous "immediately
> offload" approach]. people asked for it, some people demanded it. we
> offload to printk-kthread only when we detect that this particular
> task on this particular CPU has been doing printing (without rescheduling)
> for 1/2 of watchdog threshold value. IOW, if we see that we are heading
> towards the lockup limit then we offload. otherwise - we let it loop in
> console_unlock().

FWIW, I'm still a bit skeptical whether that's the right direction.
People asking for it doesn't indicate that the flipside isn't worse.
Hmm... I think my impression is mostly from the fact that throughout
the years I can only count few times where loss of printk messages was
an actual issue.  The system getting slowed down / locking up from
printk message deluge was *far* more prevalent.

Thanks.

-- 
tejun