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Date:	Mon, 27 Oct 2014 13:43:21 -0700
From:	Jay Vosburgh <jay.vosburgh@...onical.com>
To:	paulmck@...ux.vnet.ibm.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?

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.

	FWIW, dmesg is unchanged, and still shows messages like:

[    0.000000]  Offload RCU callbacks from CPUs: 0-3.

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

	-J

>							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)
> {
> }
>

---
	-Jay Vosburgh, jay.vosburgh@...onical.com
--
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