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Date:	Tue, 28 Oct 2014 10:12:43 +0200
From:	Yanko Kaneti <yaneti@...lera.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Jay Vosburgh <jay.vosburgh@...onical.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, tj@...nel.org
Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

On Mon-10/27/14-2014 10:45, Paul E. McKenney wrote:
> On Sat, Oct 25, 2014 at 11:18:27AM -0700, Paul E. McKenney wrote:
> > On Sat, Oct 25, 2014 at 09:38:16AM -0700, Jay Vosburgh wrote:
> > > Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > >On Fri, Oct 24, 2014 at 09:33:33PM -0700, Jay Vosburgh wrote:
> > > >> 	Looking at the dmesg, the early boot messages seem to be
> > > >> confused as to how many CPUs there are, e.g.,
> > > >> 
> > > >> [    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1
> > > >> [    0.000000] Hierarchical RCU implementation.
> > > >> [    0.000000]  RCU debugfs-based tracing is enabled.
> > > >> [    0.000000]  RCU dyntick-idle grace-period acceleration is enabled.
> > > >> [    0.000000]  RCU restricting CPUs from NR_CPUS=256 to nr_cpu_ids=4.
> > > >> [    0.000000] RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=4
> > > >> [    0.000000] NR_IRQS:16640 nr_irqs:456 0
> > > >> [    0.000000]  Offload RCU callbacks from all CPUs
> > > >> [    0.000000]  Offload RCU callbacks from CPUs: 0-3.
> > > >> 
> > > >> 	but later shows 2:
> > > >> 
> > > >> [    0.233703] x86: Booting SMP configuration:
> > > >> [    0.236003] .... node  #0, CPUs:      #1
> > > >> [    0.255528] x86: Booted up 1 node, 2 CPUs
> > > >> 
> > > >> 	In any event, the E8400 is a 2 core CPU with no hyperthreading.
> > > >
> > > >Well, this might explain some of the difficulties.  If RCU decides to wait
> > > >on CPUs that don't exist, we will of course get a hang.  And rcu_barrier()
> > > >was definitely expecting four CPUs.
> > > >
> > > >So what happens if you boot with maxcpus=2?  (Or build with
> > > >CONFIG_NR_CPUS=2.) I suspect that this might avoid the hang.  If so,
> > > >I might have some ideas for a real fix.
> > > 
> > > 	Booting with maxcpus=2 makes no difference (the dmesg output is
> > > the same).
> > > 
> > > 	Rebuilding with CONFIG_NR_CPUS=2 makes the problem go away, and
> > > dmesg has different CPU information at boot:
> > > 
> > > [    0.000000] smpboot: 4 Processors exceeds NR_CPUS limit of 2
> > > [    0.000000] smpboot: Allowing 2 CPUs, 0 hotplug CPUs
> > >  [...]
> > > [    0.000000] setup_percpu: NR_CPUS:2 nr_cpumask_bits:2 nr_cpu_ids:2 nr_node_ids:1
> > >  [...]
> > > [    0.000000] Hierarchical RCU implementation.
> > > [    0.000000] 	RCU debugfs-based tracing is enabled.
> > > [    0.000000] 	RCU dyntick-idle grace-period acceleration is enabled.
> > > [    0.000000] NR_IRQS:4352 nr_irqs:440 0
> > > [    0.000000] 	Offload RCU callbacks from all CPUs
> > > [    0.000000] 	Offload RCU callbacks from CPUs: 0-1.
> > 
> > Thank you -- this confirms my suspicions on the fix, though I must admit
> > to being surprised that maxcpus made no difference.
> 
> And here is an alleged fix, lightly tested at this end.  Does this patch
> help?

Tested this on top of rc2 (as found in Fedora, and failing without the patch)
with all my modprobe scenarios and it seems to have fixed it.

Thanks
-Yanko

 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> rcu: Make rcu_barrier() understand about missing rcuo kthreads
> 
> Commit 35ce7f29a44a (rcu: Create rcuo kthreads only for onlined CPUs)
> avoids creating rcuo kthreads for CPUs that never come online.  This
> fixes a bug in many instances of firmware: Instead of lying about their
> age, these systems instead lie about the number of CPUs that they have.
> Before commit 35ce7f29a44a, this could result in huge numbers of useless
> rcuo kthreads being created.
> 
> It appears that experience indicates that I should have told the
> people suffering from this problem to fix their broken firmware, but
> I instead produced what turned out to be a partial fix.   The missing
> piece supplied by this commit makes sure that rcu_barrier() knows not to
> post callbacks for no-CBs CPUs that have not yet come online, because
> otherwise rcu_barrier() will hang on systems having firmware that lies
> about the number of CPUs.
> 
> It is tempting to simply have rcu_barrier() refuse to post a callback on
> any no-CBs CPU that does not have an rcuo kthread.  This unfortunately
> does not work because rcu_barrier() is required to wait for all pending
> callbacks.  It is therefore required to wait even for those callbacks
> that cannot possibly be invoked.  Even if doing so hangs the system.
> 
> Given that posting a callback to a no-CBs CPU that does not yet have an
> rcuo kthread can hang rcu_barrier(), It is tempting to report an error
> in this case.  Unfortunately, this will result in false positives at
> boot time, when it is perfectly legal to post callbacks to the boot CPU
> before the scheduler has started, in other words, before it is legal
> to invoke rcu_barrier().
> 
> So this commit instead has rcu_barrier() avoid posting callbacks to
> CPUs having neither rcuo kthread nor pending callbacks, and has it
> complain bitterly if it finds CPUs having no rcuo kthread but some
> pending callbacks.  And when rcu_barrier() does find CPUs having no rcuo
> kthread but pending callbacks, as noted earlier, it has no choice but
> to hang indefinitely.
> 
> Reported-by: Yanko Kaneti <yaneti@...lera.com>
> Reported-by: Jay Vosburgh <jay.vosburgh@...onical.com>
> Reported-by: Meelis Roos <mroos@...ux.ee>
> Reported-by: Eric B Munson <emunson@...mai.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index aa8e5eea3ab4..c78e88ce5ea3 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -660,18 +660,18 @@ TRACE_EVENT(rcu_torture_read,
>  /*
>   * Tracepoint for _rcu_barrier() execution.  The string "s" describes
>   * the _rcu_barrier phase:
> - *	"Begin": rcu_barrier_callback() started.
> - *	"Check": rcu_barrier_callback() checking for piggybacking.
> - *	"EarlyExit": rcu_barrier_callback() piggybacked, thus early exit.
> - *	"Inc1": rcu_barrier_callback() piggyback check counter incremented.
> - *	"Offline": rcu_barrier_callback() found offline CPU
> - *	"OnlineNoCB": rcu_barrier_callback() found online no-CBs CPU.
> - *	"OnlineQ": rcu_barrier_callback() found online CPU with callbacks.
> - *	"OnlineNQ": rcu_barrier_callback() found online CPU, no callbacks.
> + *	"Begin": _rcu_barrier() started.
> + *	"Check": _rcu_barrier() checking for piggybacking.
> + *	"EarlyExit": _rcu_barrier() piggybacked, thus early exit.
> + *	"Inc1": _rcu_barrier() piggyback check counter incremented.
> + *	"OfflineNoCB": _rcu_barrier() found callback on never-online CPU
> + *	"OnlineNoCB": _rcu_barrier() found online no-CBs CPU.
> + *	"OnlineQ": _rcu_barrier() found online CPU with callbacks.
> + *	"OnlineNQ": _rcu_barrier() found online CPU, no callbacks.
>   *	"IRQ": An rcu_barrier_callback() callback posted on remote CPU.
>   *	"CB": An rcu_barrier_callback() invoked a callback, not the last.
>   *	"LastCB": An rcu_barrier_callback() invoked the last callback.
> - *	"Inc2": rcu_barrier_callback() piggyback check counter incremented.
> + *	"Inc2": _rcu_barrier() piggyback check counter incremented.
>   * The "cpu" argument is the CPU or -1 if meaningless, the "cnt" argument
>   * is the count of remaining callbacks, and "done" is the piggybacking count.
>   */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f6880052b917..7680fc275036 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3312,11 +3312,16 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  			continue;
>  		rdp = per_cpu_ptr(rsp->rda, cpu);
>  		if (rcu_is_nocb_cpu(cpu)) {
> -			_rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
> -					   rsp->n_barrier_done);
> -			atomic_inc(&rsp->barrier_cpu_count);
> -			__call_rcu(&rdp->barrier_head, rcu_barrier_callback,
> -				   rsp, cpu, 0);
> +			if (!rcu_nocb_cpu_needs_barrier(rsp, cpu)) {
> +				_rcu_barrier_trace(rsp, "OfflineNoCB", cpu,
> +						   rsp->n_barrier_done);
> +			} else {
> +				_rcu_barrier_trace(rsp, "OnlineNoCB", cpu,
> +						   rsp->n_barrier_done);
> +				atomic_inc(&rsp->barrier_cpu_count);
> +				__call_rcu(&rdp->barrier_head,
> +					   rcu_barrier_callback, rsp, cpu, 0);
> +			}
>  		} else if (ACCESS_ONCE(rdp->qlen)) {
>  			_rcu_barrier_trace(rsp, "OnlineQ", cpu,
>  					   rsp->n_barrier_done);
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 4beab3d2328c..8e7b1843896e 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -587,6 +587,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
>  static void print_cpu_stall_info_end(void);
>  static void zero_cpu_stall_ticks(struct rcu_data *rdp);
>  static void increment_cpu_stall_ticks(void);
> +static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu);
>  static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
>  static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
>  static void rcu_init_one_nocb(struct rcu_node *rnp);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 927c17b081c7..68c5b23b7173 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2050,6 +2050,33 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force)
>  }
>  
>  /*
> + * Does the specified CPU need an RCU callback for the specified flavor
> + * of rcu_barrier()?
> + */
> +static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> +	struct rcu_head *rhp;
> +
> +	/* No-CBs CPUs might have callbacks on any of three lists. */
> +	rhp = ACCESS_ONCE(rdp->nocb_head);
> +	if (!rhp)
> +		rhp = ACCESS_ONCE(rdp->nocb_gp_head);
> +	if (!rhp)
> +		rhp = ACCESS_ONCE(rdp->nocb_follower_head);
> +
> +	/* Having no rcuo kthread but CBs after scheduler starts is bad! */
> +	if (!ACCESS_ONCE(rdp->nocb_kthread) && rhp) {
> +		/* RCU callback enqueued before CPU first came online??? */
> +		pr_err("RCU: Never-onlined no-CBs CPU %d has CB %p\n",
> +		       cpu, rhp->func);
> +		WARN_ON_ONCE(1);
> +	}
> +
> +	return !!rhp;
> +}
> +
> +/*
>   * Enqueue the specified string of rcu_head structures onto the specified
>   * CPU's no-CBs lists.  The CPU is specified by rdp, the head of the
>   * string by rhp, and the tail of the string by rhtp.  The non-lazy/lazy
> @@ -2646,6 +2673,10 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
>  
>  #else /* #ifdef CONFIG_RCU_NOCB_CPU */
>  
> +static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu)
> +{
> +}
> +
>  static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
>  {
>  }
> 
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