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

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Date:   Tue, 19 Dec 2017 15:31:47 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        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 Tue 19-12-17 10:24:55, Sergey Senozhatsky wrote:
> On (12/18/17 20:08), Steven Rostedt wrote:
> > > ... do you guys read my emails? which part of the traces I have provided
> > > suggests that there is any improvement?
> > 
> > The traces I've seen from you were from non-realistic scenarios.
> > But I have hit issues with printk()s happening that cause one CPU to do all
> > the work, where my patch would fix that. Those are the scenarios I'm
> > talking about.
> 
> any hints about what makes your scenario more realistic than mine?

Well, Tetsuo had some semi-realistic scenario where alloc stall messages
managed to stall other printk callers (namely oom handler). I am saying
sem-realistic because he is testing OOM throughput with an unrealistic
workload which itself is not very real on production systems. The
essential thing here is that many processes might be in the allocation
path and any printk there could just swamp some unrelated printk caller
and cause hard to debug problems. Steven's patch should address that
with a relatively simple lock handover. I was pessimistic this would
work sufficiently well but it survived Tetsuo's testing IIRC so it
sounds good enough to me.
-- 
Michal Hocko
SUSE Labs