Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752510AbaJ0Uni (ORCPT ); Mon, 27 Oct 2014 16:43:38 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:53323 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752247AbaJ0Ung (ORCPT ); Mon, 27 Oct 2014 16:43:36 -0400 From: Jay Vosburgh To: paulmck@linux.vnet.ibm.com cc: Yanko Kaneti , Josh Boyer , "Eric W. Biederman" , Cong Wang , Kevin Fenzi , netdev , "Linux-Kernel@Vger. Kernel. Org" , mroos@linux.ee, tj@kernel.org Subject: Re: localed stuck in recent 3.18 git in copy_net_ns? In-reply-to: <20141027174539.GC27568@linux.vnet.ibm.com> References: <20141024214927.GA4977@linux.vnet.ibm.com> <8915.1414190047@famine> <20141024225931.GC4977@linux.vnet.ibm.com> <20141024230524.GA16023@linux.vnet.ibm.com> <10136.1414196448@famine> <20141025020324.GA28247@linux.vnet.ibm.com> <11813.1414211613@famine> <20141025051602.GB28247@linux.vnet.ibm.com> <15891.1414255096@famine> <20141025181827.GE28247@linux.vnet.ibm.com> <20141027174539.GC27568@linux.vnet.ibm.com> Comments: In-reply-to "Paul E. McKenney" message dated "Mon, 27 Oct 2014 10:45:39 -0700." X-Mailer: MH-E 8.5+bzr; nmh 1.5; GNU Emacs 24.4.50 Date: Mon, 27 Oct 2014 13:43:21 -0700 Message-ID: <25166.1414442601@famine> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 -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 >Reported-by: Jay Vosburgh >Reported-by: Meelis Roos >Reported-by: Eric B Munson >Signed-off-by: Paul E. McKenney > >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@canonical.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/