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Date:	Mon, 27 Oct 2014 14:07:36 -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, tj@...nel.org
Subject: Re: localed stuck in recent 3.18 git in copy_net_ns?

On Mon, Oct 27, 2014 at 01:43:21PM -0700, Jay Vosburgh wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> 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?
> 
> 	This patch appears to make the problem go away; I've run about
> 10 iterations.  I applied this patch to the same -net tree I was using
> previously (-net as of Oct 22), with all other test patches removed.

So I finally produced a patch that helps!  It was bound to happen sooner
or later, I guess.  ;-)

> 	FWIW, dmesg is unchanged, and still shows messages like:
> 
> [    0.000000]  Offload RCU callbacks from CPUs: 0-3.

Yep, at that point in boot, RCU has no way of knowing that the firmware
is lying to it about the number of CPUs.  ;-)

> Tested-by: Jay Vosburgh <jay.vosburgh@...onical.com>

Thank you for your testing efforts!!!

							Thanx, Paul

> 	-J
> >
> >------------------------------------------------------------------------
> >
> >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)
> > {
> > }
> >
> 
> ---
> 	-Jay Vosburgh, jay.vosburgh@...onical.com
> 

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