linux-kernel - Re: localed stuck in recent 3.18 git in copy_net

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Date:	Fri, 24 Oct 2014 11:57:53 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jay Vosburgh <jay.vosburgh@...onical.com>
Cc:	Yanko Kaneti <yaneti@...lera.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Cong Wang <cwang@...pensource.com>,
	Kevin Fenzi <kevin@...ye.com>, netdev <netdev@...r.kernel.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	mroos@...ux.ee
Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

On Fri, Oct 24, 2014 at 11:49:48AM -0700, Jay Vosburgh wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> >On Fri, Oct 24, 2014 at 08:35:26PM +0300, Yanko Kaneti wrote:
> >> On Fri-10/24/14-2014 10:20, Paul E. McKenney wrote:
> >> > On Fri, Oct 24, 2014 at 08:09:31PM +0300, Yanko Kaneti wrote:
> >> > > On Fri-10/24/14-2014 09:54, Paul E. McKenney wrote:
> >> > > > On Fri, Oct 24, 2014 at 07:29:43PM +0300, Yanko Kaneti wrote:
> >> > > > > On Fri-10/24/14-2014 08:40, Paul E. McKenney wrote:
> >> > > > > > On Fri, Oct 24, 2014 at 12:08:57PM +0300, Yanko Kaneti wrote:
> >> > > > > > > On Thu-10/23/14-2014 15:04, Paul E. McKenney wrote:
> >> > > > > > > > On Fri, Oct 24, 2014 at 12:45:40AM +0300, Yanko Kaneti wrote:
> >> > > > > > > > > 
> >> > > > > > > > > On Thu, 2014-10-23 at 13:05 -0700, Paul E. McKenney wrote:
> >> > > > > > > > > > On Thu, Oct 23, 2014 at 10:51:59PM +0300, Yanko Kaneti wrote:
> >> > > > 
> >> > > > [ . . . ]
> >> > > > 
> >> > > > > > > Ok, unless I've messsed up something major, bisecting points to:
> >> > > > > > > 
> >> > > > > > > 35ce7f29a44a rcu: Create rcuo kthreads only for onlined CPUs
> >> > > > > > > 
> >> > > > > > > Makes any sense ?
> >> > > > > > 
> >> > > > > > Good question.  ;-)
> >> > > > > > 
> >> > > > > > Are any of your online CPUs missing rcuo kthreads?  There should be
> >> > > > > > kthreads named rcuos/0, rcuos/1, rcuos/2, and so on for each online CPU.
> >> > > > > 
> >> > > > > Its a Phenom II X6. With 3.17 and linux-tip with 35ce7f29a44a reverted, the rcuos are 8
> >> > > > > and the modprobe ppp_generic testcase reliably works, libvirt also manages
> >> > > > > to setup its bridge.
> >> > > > > 
> >> > > > > Just with linux-tip , the rcuos are 6 but the failure is as reliable as
> >> > > > > before.
> >> > > 
> >> > > > Thank you, very interesting.  Which 6 of the rcuos are present?
> >> > > 
> >> > > Well, the rcuos are 0 to 5. Which sounds right for a 6 core CPU like this   
> >> > > Phenom II.
> >> > 
> >> > Ah, you get 8 without the patch because it creates them for potential
> >> > CPUs as well as real ones.  OK, got it.
> >> > 
> >> > > > > Awating instructions: :)
> >> > > > 
> >> > > > Well, I thought I understood the problem until you found that only 6 of
> >> > > > the expected 8 rcuos are present with linux-tip without the revert.  ;-)
> >> > > > 
> >> > > > I am putting together a patch for the part of the problem that I think
> >> > > > I understand, of course, but it would help a lot to know which two of
> >> > > > the rcuos are missing.  ;-)
> >> > > 
> >> > > Ready to test
> >> > 
> >> > Well, if you are feeling aggressive, give the following patch a spin.
> >> > I am doing sanity tests on it in the meantime.
> >> 
> >> Doesn't seem to make a difference here
> >
> >OK, inspection isn't cutting it, so time for tracing.  Does the system
> >respond to user input?  If so, please enable rcu:rcu_barrier ftrace before
> >the problem occurs, then dump the trace buffer after the problem occurs.
> 
> 	My system is up and responsive when the problem occurs, so this
> shouldn't be a problem.

Nice!  ;-)

> 	Do you want the ftrace with your patch below, or unmodified tip
> of tree?

Let's please start with the patch.

							Thanx, Paul

> 	-J
> 
> 
> >							Thanx, Paul
> >
> >> > ------------------------------------------------------------------------
> >> > 
> >> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> >> > index 29fb23f33c18..927c17b081c7 100644
> >> > --- a/kernel/rcu/tree_plugin.h
> >> > +++ b/kernel/rcu/tree_plugin.h
> >> > @@ -2546,9 +2546,13 @@ static void rcu_spawn_one_nocb_kthread(struct rcu_state *rsp, int cpu)
> >> >  			rdp->nocb_leader = rdp_spawn;
> >> >  			if (rdp_last && rdp != rdp_spawn)
> >> >  				rdp_last->nocb_next_follower = rdp;
> >> > -			rdp_last = rdp;
> >> > -			rdp = rdp->nocb_next_follower;
> >> > -			rdp_last->nocb_next_follower = NULL;
> >> > +			if (rdp == rdp_spawn) {
> >> > +				rdp = rdp->nocb_next_follower;
> >> > +			} else {
> >> > +				rdp_last = rdp;
> >> > +				rdp = rdp->nocb_next_follower;
> >> > +				rdp_last->nocb_next_follower = NULL;
> >> > +			}
> >> >  		} while (rdp);
> >> >  		rdp_spawn->nocb_next_follower = rdp_old_leader;
> >> >  	}
> >> > 
> 
> ---
> 	-Jay Vosburgh, jay.vosburgh@...onical.com
> 

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