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

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Message-ID: <20141024183226.GW4977@linux.vnet.ibm.com>
Date:	Fri, 24 Oct 2014 11:32:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yanko Kaneti <yaneti@...lera.com>
Cc:	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>,
	jay.vosburgh@...onical.com, mroos@...ux.ee
Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

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.

							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;
> >  	}
> > 
> 

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