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Date:	Sat, 13 Dec 2014 15:41:52 -0500
From:	Dave Jones <davej@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Sat, Dec 13, 2014 at 10:04:08AM -0800, Paul E. McKenney wrote:
 > On Sat, Dec 13, 2014 at 11:59:15AM -0500, Dave Jones wrote:
 > > On Fri, Dec 12, 2014 at 11:14:06AM -0800, Linus Torvalds wrote:
 > >  > On Fri, Dec 12, 2014 at 10:54 AM, Dave Jones <davej@...hat.com> wrote:
 > >  > >
 > >  > > Something that's still making me wonder if it's some kind of hardware
 > >  > > problem is the non-deterministic nature of this bug.
 > >  > 
 > >  > I'd expect it to be a race condition, though. Which can easily cause
 > >  > these kinds of issues, and the timing will be pretty random even if
 > >  > the load is very regular.
 > >  > 
 > >  > And we know that the scheduler has an integer overflow under Sasha's
 > >  > loads, although I didn't hear anything from Ingo and friends about it.
 > >  > Ingo/Peter, you were cc'd on that report, where at least one of the
 > >  > multiplcations in wake_affine() ended up overflowing..
 > >  > 
 > >  > Some scheduler thing that overflows only under heavy load, and screws
 > >  > up scheduling could easily account for the RCU thread thing. I see it
 > >  > *less* easily accounting for DaveJ's case, though, because the
 > >  > watchdog is running at RT priority,  and the scheduler would have to
 > >  > screw up much more to then not schedule an RT task, but..
 > >  > 
 > >  > I'm also not sure if the bug ever happens with preemption disabled.
 > > 
 > > Bah, so I see some watchdog traces with preemption off, and that then
 > > taints the kernel, and the fuzzing stops.  I'll hack something up
 > > so it ignores the taint and keeps going. All I really care about here
 > > is the "machine hangs completely" case, which the trace below didn't
 > > hit..
 > > 
 > > (back to fuzzing almost everything, not just lsetxattr btw)
 > 
 > Hmmm...  This one looks like the RCU grace-period kthread is getting
 > starved: "idle=b4c/0/0".  Is this running with the "dangerous" patch
 > that sets these kthreads to RT priority?

sorry, no. Ran out of time yesterday. I'll try and get to applying that
later this evening if I get chance.

	Dave
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