linux-kernel - Re: [PATCH/RFC] timer: fix deadlock on cpu hotplug

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Date:	Mon, 11 Oct 2010 15:51:52 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH/RFC] timer: fix deadlock on cpu hotplug

On Mon, Oct 11, 2010 at 02:31:03PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-09-23 at 15:31 +0200, Heiko Carstens wrote:
> > On Wed, Sep 22, 2010 at 04:29:36PM +0200, Peter Zijlstra wrote:
> > > On Wed, 2010-09-22 at 11:22 +0200, Peter Zijlstra wrote:
> > > 
> > > > The idea was to move it to a class of its own above SCHED_FIFO.
> > > > 
> > > > I'll try and get something done, but I'm heading out to LinuxCon.JP
> > > > soon.
> > > 
> > > Something like the below, it seems to boot, build a kernel and hotplug.
> > 
> > Thanks for the fast patch. Yes, it works. Sort of:
> > 
> > ------------[ cut here ]------------
> > WARNING: at kernel/kthread.c:182
> > [...]
> > ..and crashes afterwards ;)
> > 
> > Btw. I also tried to boot 2.6.35.5 with your patch and it hangs just at
> > the beginning. I also had to apply 5e3d20a68f63fc5a310687d81956c3b96e488b84
> > "init: Remove the BKL from startup code" to make the machine boot again,
> > which was a bit surprising.
> 
> Does something like the below (on top of the previous patch) work?

Yes, it works. Just survived a git clone a complete kernel build while cpu
hotplug stress was running. Thanks!

Tested-by: Heiko Carstens <heiko.carstens@...ibm.com>

> Can I have your hotplug script (cpukill2.sh?) to test, a simply while(1)
> loop offlining and onlining cpu1 doesn't seem to impress my machine
> (with the below folded in)..

That's one of the scripts our testers are using:

#! /bin/bash
RANDOM=`date '+%s'`
let "NUMBER = $RANDOM % 2 + 0"
CPUS=`ls /sys/devices/system/cpu/ | grep cpu`
count=`echo $CPUS | wc -w`
echo You have $count no. of CPUs
echo You have the following CPUs: $CPUS
while [ 1 ] ; do
   for ((i=0 ; i<=$count ; i++)) ; do
      rnd=$[($RANDOM % $count) ]     # RND between 0..max_no_of_cpu
      let "NUMBER = $RANDOM % 2 + 0"
      echo "echo $NUMBER > /sys/devices/system/cpu/cpu$rnd/online"
      echo $NUMBER > /sys/devices/system/cpu/cpu$rnd/online
   done
done
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