linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141207182420.GG25340@linux.vnet.ibm.com>
Date:	Sun, 7 Dec 2014 10:24:20 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Dâniel Fraga <fragabr@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Sun, Dec 07, 2014 at 09:58:14AM -0500, Sasha Levin wrote:
> On 12/05/2014 01:15 PM, Linus Torvalds wrote:
> > On Fri, Dec 5, 2014 at 7:03 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
> >>
> >> Yes, it's going to a serial line, but it's only about 100 lines/second on
> >> average. I wouldn't expect it to cause anything to hang!
> > 
> > A regular 16650 serial chip? Running at 115kbps, I assume? So that's
> > about 11kB/s.
> > 
> > And the serial console is polling, since it can't sleep or depend on interrupts.
> > 
> > At a average line length of what, 40 characters? At less than 300
> > lines/s, you'd be using up 100% of one CPU. And since the printouts
> > are serialized, that would be all other CPU's too..
> > 
> > 100 lines/s _average_ means that I can easily see it be 300lines/s for a while.
> > 
> > So yeah. The serial console is simply not designed to handle
> > continuous output. It's for the "occasional" stuff.
> > 
> > The fact that your rcu lockups go away when you make the fault
> > injection be quiet makes me really suspect this is related.
> 
> The lockups themselves "go away", but looking closer at the log
> without those extra prints, I'm seeing:
> 
> [ 1458.700070] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [ 1458.700133]  (detected by 19, t=30502 jiffies, g=12293, c=12292, q=0)
> [ 1458.702764] INFO: Stall ended before state dump start
> 
> Quite often.
> 
> Maybe the extra prints were just a catalyst?

Is anything else being printed about the time that these message show
up?  Or is this the only output for the 40,000 jiffies preceding this
message?

							Thanx, Paul

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