linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Date:   Fri, 30 Jun 2017 20:57:11 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     sergey.senozhatsky.work@...il.com, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org, jack@...e.cz,
        akpm@...ux-foundation.org, peterz@...radead.org, rjw@...ysocki.net,
        ebiederm@...ssion.com, gregkh@...uxfoundation.org, jslaby@...e.com,
        pavel@....cz, andi@...as.de, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On (06/30/17 19:18), Tetsuo Handa wrote:
> Sergey Senozhatsky wrote:
> >         if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
> >                 wake_up_process(printk_kthread);
> >                 return true;
> >         }
> 
> Please avoid memory allocations when trying to print something.
> __GFP_DIRECT_RECLAIM allocations (e.g. GFP_KERNEL) can sleep for
> unpredictable duration. Allocations without __GFP_NOWARN will cause
> e.g. memory allocation failure messages. Even with __GFP_NOWARN,
> some messages might be still printed (e.g. serious problem).

wow... dammit, what a stupid mistake. you are 100% right, thanks!
it's sooo unsafe and dumb, console_unlock() and, thus, offloading
can happen from IRQ.

thanks again.

> > I'm still thinking about Steven's proposals; but we will need offloading
> > anyways, so the bits we are talking about here are important regardless
> > the direction printk design will take, I think.
> 
> Is there a chance that printk() waits for only data queued by that printk()
> call (exception will be printk() from NMI).

hm, I don't think this can be done easily... consider

	console_lock();
	printk();
	printk();
	...			-> this guys will wait forever. nothing
				   flushes the logbuf.
	printk();
	console_unlock();


> If we carry penalty for printk() (charge delay according to amount of
> data queued by that printk()), users will stop doing stupid flooding
> with printk() based on an assumption that offloaded kernel thread will
> manage magically with guarantee of being printed out (i.e. users has
> to become careful).

ratelimiting is on my list. but it's a bit tricky... what should happen
if one does

	CPU1

	-> IRQ
		printk()
			console_unlock();
		printk()
			console_unlock();
		...
		printk()
			ratelimit();
			console_unlock();

	-> NMI panic()


need to think more.

	-ss