linux-kernel - Re: [RFC][PATCH v2 1/2] printk: Make printk() completely async

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Message-Id: <201603072137.CJE87554.HOOVSMOFFLQJtF@I-love.SAKURA.ne.jp>
Date:	Mon, 7 Mar 2016 21:37:12 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	jack@...e.cz, sergey.senozhatsky.work@...il.com
Cc:	akpm@...ux-foundation.org, jack@...e.com, pmladek@...e.com,
	tj@...nel.org, linux-kernel@...r.kernel.org,
	sergey.senozhatsky@...il.com
Subject: Re: [RFC][PATCH v2 1/2] printk: Make printk() completely async

Jan Kara wrote:
> > So for UP systems, we should by default disable async printing anyway I
> > suppose. It is just a pointless overhead. So please just make printk_sync
> > default to true if !CONFIG_SMP.
> > 
> > When IRQs are disabled, you're right we will have a change in behavior. I
> > don't see an easy way of avoiding delaying of printk until IRQs get
> > enabled. I don't want to queue work directly because that creates
> > possibility for lock recursion in queue_work(). And playing some tricks
> > with irq_works isn't easy either - you cannot actually rely on any other
> > CPU doing anything (even a timer tick) because of NOHZ.
> > 
> > So if this will be a problem in practice, using a kthread will probably be
> > the easiest solution.
> 
> Hum, and thinking more about it: Considering that WQ_MEM_RECLAIM workqueues
> create kthread anyway as a rescuer thread, it may be the simplest to just
> go back to using a single kthread for printing. What do you think?
> 

I think a plain kthread is better. printing_work_func() can become a buggy
function. console_unlock() from printing_work_func() can occupy kworker for
many seconds due to looping with cond_resched() (i.e. without calling
schedule_timeout_*()) from schedulable context which can in turn defer
other work items.