Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

From: Yanko Kaneti <[email-masked]>
Date: Sat, 25 Oct 2014 00:25:57 +0300

On Fri-10/24/14-2014 11:32, Paul E. McKenney 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.

Sorry for being unresposive here, but I know next to nothing about tracing
or most things about the kernel, so I have some cathing up to do.

In the meantime some layman observations while I tried to find what exactly
triggers the problem.
- Even in runlevel 1 I can reliably trigger the problem by starting libvirtd
- libvirtd seems to be very active in using all sorts of kernel facilities
  that are modules on fedora so it seems to cause many simultaneous kworker 
  calls to modprobe
- there are 8 kworker/u16 from 0 to 7
- one of these kworkers always deadlocks, while there appear to be two
  kworker/u16:6 - the seventh

  6 vs 8 as in 6 rcuos where before they were always 8

Just observations from someone who still doesn't know what the u16
kworkers are..

-- Yanko

> 							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;
> > >  	}
> > > 
> > 
>