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

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Date:	Sat, 12 Mar 2016 14:01:20 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jan Kara <jack@...e.cz>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	akpm@...ux-foundation.org, jack@...e.com, pmladek@...e.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v2 1/2] printk: Make printk() completely async

Hello Tejun,

On (03/11/16 12:22), Tejun Heo wrote:
> On Tue, Mar 08, 2016 at 07:21:52PM +0900, Sergey Senozhatsky wrote:
> > I'd personally prefer to go with the "less dependency" option -- a dedicated
> > kthread, I think. mostly for the sake of simplicity. I agree with the point
> > that console_unlock() has unpredictable execution time, and in general case
> > we would have a busy kworker (or sleeping in console_lock() or doing
> > cond_resched()) and an idle extra WQ_RESCUER kthread, with activation rules
> > that don't depend on printk. printk with dedicated printk-kthread seems
> > easier to control. how does it sound?
> 
> I don't think it makes sense to avoid workqueue for execution latency.
> The only case which can matter is the rescuer case and as I wrote
> before the system is already in an extremely high latency mode by the
> time rescuer is needed, so it's unlikely to make noticeable
> differences.
>
> However, I agree that using kthread is a good idea here just to reduce
> the amount of dependency as prink working even during complex failures
> is important.  workqueue itself is fairly complex and it also requires
> timer and task creation to work correctly for proper operation.
> That's a lot of extra dependency.

Thanks!

I agree that, in some cases (if not in most of them) the "value" of printk()
output is inversely proportional to the system health -- the worst the state,
the more attention people pay to printk() output; so a simpler solution here
gives more confidence.

	-ss