linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <54846B06.8050906@oracle.com>
Date:	Sun, 07 Dec 2014 09:58:14 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Dâniel Fraga <fragabr@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

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?


Thanks,
Sasha
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