Date:   Tue, 23 Jan 2018 11:24:36 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On Wed, 24 Jan 2018 01:01:53 +0900
Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> On (01/23/18 10:41), Steven Rostedt wrote:
> [..]
> > We can have more. But if printk is causing printks, that's a major bug.
> > And work queues are not going to fix it, it will just spread out the
> > pain. Have it be 100 printks, it needs to be fixed if it is happening.
> > And having all printks just generate more printks is not helpful. Even
> > if we slow them down. They will still never end.  
> 
> Dropping the messages is not the solution either. The original bug report
> report was - this "locks up my kernel". That's it. That's all people asked
> us to solve.

And throttling the printks would stop the lock up too.

> 
> With WQ we don't lockup the kernel, because we flush printk_safe in
> preemptible context. And people are very much expected to fix the
> misbehaving consoles. But that should not be printk_safe problem.

Right, but now you just made printk safe unreliable to get information
out, because you need to wait for a schedule to occur, and if there's
issues, like a deadlock, that thread will never run. And you just lost
you lockdep splat.

> 
> > A printk causing a printk is a special case, and we need to just show
> > enough to let the user know that its happening, and why printks are
> > being throttled. Yes, we may lose data, but if every printk that goes
> > out causes another printk, then there's going to be so much noise that
> > we wont know what other things went wrong. Honestly, if someone showed
> > me a report where the logs were filled with printks that caused
> > printks, I'd stop right there and tell them that needs to be fixed
> > before we do anything else. And if that recursion is happening because
> > of another problem, I don't want to see the recursion printks. I want
> > to see the printks that show what is causing the recursions.  
> 
> I'll re-read this one tomorrow. Not quite following it.

I'll add more capitals next time ;-)

> 
> > > The problem is - we flush printk_safe too soon and printing CPU ends up
> > > in a lockup - it log_store()-s new messages while it's printing the pending  
> > 
> > No, the problem is that printks are causing more printks. Yes that will
> > make flushing them soon more likely to lock up the system. But that's
> > not the problem. The problem is printks causing printks.  
> 
> Yes. And ignoring those printk()-s by simply dropping them does not fix
> the problem by any means.

How so? If we drop them, then the stuck printk has nothing to print and
will move forward.

I say once you start dropping printks due to recursion, keep dropping
them. For at least a second, to allow them to stop killing the machine.

> 
> > > ones. It's fine to do so when CPU is in preemptible context. Really, we
> > > should not care in printk_safe as long as we don't lockup the kernel. The
> > > misbehaving console must be fixed. If CPU is not in preemptible context then
> > > we do lockup the kernel. Because we flush printk_safe regardless of the
> > > current CPU context. If we will flush printk_safe via WQ then we automatically  
> > 
> > And if we can throttle recursive printks, then we should be able to
> > stop that from happening.  
> 
> pintk_safe was designed to be recursive. It was never designed to be
> used to troubleshoot or debug consoles. But it was designed to be
> recursive - because that's the sort of the problems it was meant to
> handle: recursive printks that would otherwise deadlock us. That's why
> we have it in the first place.

So printk safe is only triggered when at the same context? If we can
guarantee that printk safe is triggered only when its because a printk
is happening at the same context (not because of an interrupt, but
really at the same context, using my context check), then I'm fine with
delaying them to a work queue.

That is, if we have this:

	printk()
		console_lock()
			<interrupt>
				printk()
					add to log buffer
		<print irq printk too>
		console_unlock();


	printk()
		console_lock()
			<console does a printk>
				put in printk safe buffer
				trigger work queue
		console_unlock()
	<work queue>
		flush safe buffer
		printk()

Then I'm fine with that.

I have to look at the latest code. If this is indeed what we have, then
I admit I misunderstood the problem you want to solve.

I only want recursive printks (those that are actually triggered by
doing a printk) to be allowed to be delayed.

Make sense?

-- Steve

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