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

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Message-ID: <20160307144051.GB835@swordfish>
Date:	Mon, 7 Mar 2016 23:40:51 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	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

On (03/07/16 11:52), 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.

ok, thanks.

> 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.

yes. I thought about some sort of PRINTK_IPI on smp, and queue work from
PRINTK_IPI. which is a bit insane.

> So if this will be a problem in practice, using a kthread will probably be
> the easiest solution.

probably kthread is the way to go then. Tetsuo's bad experience with
workqueues also sounds a bit alarming.

I'll post a new patch soon.

	-ss