Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested

From: Paul E. McKenney
Date: Mon Jul 28 2014 - 19:17:48 EST

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On Mon, Jul 28, 2014 at 11:03:06PM +0200, Frederic Weisbecker wrote:
> On Sat, Jul 26, 2014 at 09:51:52PM -0400, Pranith Kumar wrote:
> > On Fri, Jul 25, 2014 at 10:30 PM, Frederic Weisbecker
> > <fweisbec@xxxxxxxxx> wrote:
> >
> > >> I understand that if CONFIG_NO_HZ_FULL_ALL is set then CONFIG_NOCB_CPU_ALL
> > >> will also be set and there is no need for this cpumask_or().
> > >>
> > >> Is there any reason for the coupling between CONFIG_NO_HZ_FULL_ALL
> > >> and CONFIG_NOCB_CPU_ALL?
> > >
> > > Yeah, for any nohz full CPU, we need the corresponding CPU to be rcu_nocb.
> > > So if all CPUs are full dynticks, all CPUs must be rcunocb.
> > >
> > > That said with this patch, the dependency is perhaps not needed anymore.
> > >
> > >>
> > >> I ask because a user can override CONFIG_NO_HZ_FULL_ALL=y at boot time
> > >> using the nohz_full= boot time parameter.
> > >
> > > No, the content of nohz_full= is ignored with CONFIG_NO_HZ_FULL_ALL=y.
> > >
> >
> > Please correct me if I am wrong but that does not seem to be the case.
> > If a boot parameter is passed, we are setting up tick_nohz_full_mask
> > from tick_nohz_full_setup() and marking tick_nohz_full_running as true.
> > Later on we check this flag and skip the CONFIG_NO_HZ_FULL_ALL
> > initialization.
>
> You're right, I missed the tick_nohz_full_running check :)
>
> So if nohz_full is passed, we ignore CONFIG_NO_HZ_FULL_ALL. That looks like
> the right behaviour though.
>
> Paul what do you think? If we keep that behaviour, Maybe you could blindly do
> rcu_nocb_mask |= tick_nohz_full and remove the CONFIG_NO_HZ_FULL_ALL dependency
> on RCU_NOCB_ALL?

You mean something like the following updated patch?

Thanx, Paul

------------------------------------------------------------------------

rcu: Fix attempt to avoid unsolicited offloading of callbacks

Commit b58cc46c5f6b (rcu: Don't offload callbacks unless specifically
requested) failed to adjust the callback lists of the CPUs that are
known to be no-CBs CPUs only because they are also nohz_full= CPUs.
This failure can result in callbacks that are posted during early boot
getting stranded on nxtlist for CPUs whose no-CBs property becomes
apparent late, and there can also be spurious warnings about offline
CPUs posting callbacks.

This commit fixes these problems by adding an early-boot rcu_init_nohz()
that properly initializes the no-CBs CPUs.

Note that kernels built with CONFIG_RCU_NOCB_CPU_ALL=y or with
CONFIG_RCU_NOCB_CPU=n do not exhibit this bug. Neither do kernels
booted without the nohz_full= boot parameter.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Pranith Kumar <bobby.prani@xxxxxxxxx>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d231aa17b1d7..cc7bed1c90dc 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -269,6 +269,14 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
struct task_struct *next) { }
#endif /* CONFIG_RCU_USER_QS */

+#ifdef CONFIG_RCU_NOCB_CPU
+void rcu_init_nohz(void);
+#else /* #ifdef CONFIG_RCU_NOCB_CPU */
+static inline void rcu_init_nohz(void)
+{
+}
+#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
+
/**
* RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
* @a: Code that RCU needs to pay attention to.
diff --git a/init/Kconfig b/init/Kconfig
index 41066e49e880..6944acd00dff 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -737,7 +737,7 @@ choice

config RCU_NOCB_CPU_NONE
bool "No build_forced no-CBs CPUs"
- depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
+ depends on RCU_NOCB_CPU
help
This option does not force any of the CPUs to be no-CBs CPUs.
Only CPUs designated by the rcu_nocbs= boot parameter will be
@@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE

config RCU_NOCB_CPU_ZERO
bool "CPU 0 is a build_forced no-CBs CPU"
- depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
+ depends on RCU_NOCB_CPU
help
This option forces CPU 0 to be a no-CBs CPU, so that its RCU
callbacks are invoked by a per-CPU kthread whose name begins
diff --git a/init/main.c b/init/main.c
index e8ae1fef0908..5d8c83ae6c55 100644
--- a/init/main.c
+++ b/init/main.c
@@ -577,6 +577,7 @@ asmlinkage __visible void __init start_kernel(void)
idr_init_cache();
rcu_init();
tick_nohz_init();
+ rcu_init_nohz();
context_tracking_init();
radix_tree_init();
/* init some links before init_ISA_irqs() */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 00dc411e9676..91208497e407 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -85,33 +85,6 @@ static void __init rcu_bootup_announce_oddness(void)
pr_info("\tBoot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
if (nr_cpu_ids != NR_CPUS)
pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
-#ifdef CONFIG_RCU_NOCB_CPU
-#ifndef CONFIG_RCU_NOCB_CPU_NONE
- if (!have_rcu_nocb_mask) {
- zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
- have_rcu_nocb_mask = true;
- }
-#ifdef CONFIG_RCU_NOCB_CPU_ZERO
- pr_info("\tOffload RCU callbacks from CPU 0\n");
- cpumask_set_cpu(0, rcu_nocb_mask);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
-#ifdef CONFIG_RCU_NOCB_CPU_ALL
- pr_info("\tOffload RCU callbacks from all CPUs\n");
- cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
-#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
- if (have_rcu_nocb_mask) {
- if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
- pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
- cpumask_and(rcu_nocb_mask, cpu_possible_mask,
- rcu_nocb_mask);
- }
- cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
- pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
- if (rcu_nocb_poll)
- pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
- }
-#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
}

#ifdef CONFIG_TREE_PREEMPT_RCU
@@ -2451,6 +2424,67 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
}

+void __init rcu_init_nohz(void)
+{
+ int cpu;
+ bool need_rcu_nocb_mask = true;
+ struct rcu_state *rsp;
+
+#ifdef CONFIG_RCU_NOCB_CPU_NONE
+ need_rcu_nocb_mask = false;
+#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
+
+#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
+ if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
+ need_rcu_nocb_mask = true;
+#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
+
+ if (!have_rcu_nocb_mask && need_rcu_nocb_mask) {
+ zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
+ have_rcu_nocb_mask = true;
+ }
+ if (!have_rcu_nocb_mask)
+ return;
+
+#ifdef CONFIG_RCU_NOCB_CPU_ZERO
+ pr_info("\tOffload RCU callbacks from CPU 0\n");
+ cpumask_set_cpu(0, rcu_nocb_mask);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
+#ifdef CONFIG_RCU_NOCB_CPU_ALL
+ pr_info("\tOffload RCU callbacks from all CPUs\n");
+ cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
+#if defined(CONFIG_NO_HZ_FULL)
+ if (tick_nohz_full_running)
+ cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
+#endif /* #if defined(CONFIG_NO_HZ_FULL)
+
+ if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
+ pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
+ cpumask_and(rcu_nocb_mask, cpu_possible_mask,
+ rcu_nocb_mask);
+ }
+ cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
+ pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
+ if (rcu_nocb_poll)
+ pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
+
+ for_each_rcu_flavor(rsp) {
+ for_each_cpu(cpu, rcu_nocb_mask) {
+ struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
+
+ /*
+ * If there are early callbacks, they will need
+ * to be moved to the nocb lists.
+ */
+ WARN_ON_ONCE(rdp->nxttail[RCU_NEXT_TAIL] !=
+ &rdp->nxtlist &&
+ rdp->nxttail[RCU_NEXT_TAIL] != NULL);
+ init_nocb_callback_list(rdp);
+ }
+ }
+}
+
/* Initialize per-rcu_data variables for no-CBs CPUs. */
static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
@@ -2479,10 +2513,6 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)

if (rcu_nocb_mask == NULL)
return;
-#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
- if (tick_nohz_full_running)
- cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
-#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
if (ls == -1) {
ls = int_sqrt(nr_cpu_ids);
rcu_nocb_leader_stride = ls;

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Next message: Dave Hansen: "Re: [PATCH] memory hotplug: update the variables after memory removed"
Previous message: David Rientjes: "Re: [PATCH] memory hotplug: update the variables after memory removed"
In reply to: Frederic Weisbecker: "Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested"
Next in thread: Pranith Kumar: "Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested"
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