linux-kernel - Re: [PATCH v10 1/2] printk: Make printk() completely async

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Date:	Thu, 7 Apr 2016 18:48:51 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jan Kara <jack@...e.com>, Petr Mladek <pmladek@...e.com>,
	Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async

Hello,

On (04/06/16 10:27), Jan Kara wrote:
[..]
> > Well, it's good that we have this.
> > 
> > It would be better if it was runtime-controllable - changing boot
> > parameters is a bit of a pain.  In fact with this approach, your
> > zillions-of-scsi-disks scenario becomes less problematic: do the async
> > offloading during the boot process then switch back to the more
> > reliable sync printing late in boot.
> 
> Doing this should be relatively easy. It would be userspace's decision
> whether they want more reliable or faster printk. Sounds fine with me.

I can add it as a separate patch to the series. should be quite trivial.

I have [minor] concerns, though. I can see how, for example, user space
can decide what logging level it wants '1 4 4 7' or anything else, but
how can user space decide what printk implementation it wants to use?

I'm more or less positive not to back-port that `synchronous RW' patch
to the kernels that I use; just because I don't want to give this freedom
to people, sync printk is something I'm trying to run away from.

> > This gets normal scheduling policy, so a spinning userspace SCHED_FIFO
> > task will block printk for ever.  This seems bad.
> 
> I have to research this a bit but won't the SCHED_FIFO task that has
> potentially unbounded amount of work lockup the CPU even though it does
> occasional cond_resched()?

depending on `watchdog_thresh' value, it can take something like 20+
seconds before watchdog will notice softlockup.
so I'm setting printk kthread prio to `MAX_RT_PRIO - 1' as of now,
just in case.

I think I'll leave printk kthread init as a late_initcall. probably
would prefer core/arch/device init calls to happen in sync printk mode.

	-ss