linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141207194304.GA17810@linux.vnet.ibm.com>
Date:	Sun, 7 Dec 2014 11:43:04 -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 10:24:20AM -0800, Paul E. McKenney wrote:
> 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?

And could you please build with CONFIG_RCU_CPU_STALL_INFO=y and try
this again?

							Thanx, Paul

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