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

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

[Paul E. McKenney]

[ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
	tick_nohz_full_mask when nohz_full= missing) in -tip
	or -rcu.  To make this work on top of rcu/next, move the
	call to rcu_organize_nocb_kthreads(rsp) to the end of the
	for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]

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@linux.vnet.ibm.com>

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/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..095d6e4d2fd7 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 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
 	trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
 }
 
+void 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) && !defined(CONFIG_NO_HZ_FULL_ALL)
+	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 (!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 +2512,6 @@ 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;

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

[Pranith Kumar]

On 07/25/2014 07:36 PM, Paul E. McKenney wrote:
> [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
>     tick_nohz_full_mask when nohz_full= missing) in -tip
>     or -rcu.  To make this work on top of rcu/next, move the
>     call to rcu_organize_nocb_kthreads(rsp) to the end of the
>     for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
>
> 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@linux.vnet.ibm.com>

Please find two points below.

<snip>
>  #ifdef CONFIG_TREE_PREEMPT_RCU
> @@ -2451,6 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
>      trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
>  }
>  
> +void 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);

Please check the return value unless you want to increase my commit count ;)

>
> +        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) && !defined(CONFIG_NO_HZ_FULL_ALL)
> +    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) */

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?

I ask because a user can override CONFIG_NO_HZ_FULL_ALL=y at boot time
using the nohz_full= boot time parameter. In this case even if a user marks
CPU 0 as the only nohz_full cpu, we will offload call backs from all CPUs.
Is this behavior what you have in mind?

--
Pranith

>
> +
> +    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 +2512,6 @@ 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;
>

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

[Paul E. McKenney]

On Fri, Jul 25, 2014 at 08:10:57PM -0400, Pranith Kumar wrote:
> On 07/25/2014 07:36 PM, Paul E. McKenney wrote:
> > [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
> >     tick_nohz_full_mask when nohz_full= missing) in -tip
> >     or -rcu.  To make this work on top of rcu/next, move the
> >     call to rcu_organize_nocb_kthreads(rsp) to the end of the
> >     for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
> >
> > 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@linux.vnet.ibm.com>
> 
> Please find two points below.
> 
> <snip>
> >  #ifdef CONFIG_TREE_PREEMPT_RCU
> > @@ -2451,6 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
> >      trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
> >  }
> >  
> > +void 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);
> 
> Please check the return value unless you want to increase my commit count ;)

I have already queued an adapted version of your patch on top of this
one, see below.  ;-)

> > +        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) && !defined(CONFIG_NO_HZ_FULL_ALL)
> > +    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) */
> 
> 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?
> 
> I ask because a user can override CONFIG_NO_HZ_FULL_ALL=y at boot time
> using the nohz_full= boot time parameter. In this case even if a user marks
> CPU 0 as the only nohz_full cpu, we will offload call backs from all CPUs.
> Is this behavior what you have in mind?

Yep.  The normal setup will be CONFIG_NO_HZ_FULL=y and
CONFIG_NO_HZ_FULL_ALL=n, and that works as you advocate.
If someone builds with CONFIG_NO_HZ_FULL_ALL=y, then they get
all CPUs offloaded.

Longer term, the hope is that offloading is unconditional, so that
CONFIG_NOCB_CPU and friends disappear, but the current code is most
definitely not up to that task yet.

							Thanx, Paul

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

rcu: Check the return value of zalloc_cpumask_var()

This commit checks the return value of the zalloc_cpumask_var() used for
allocating cpumask for rcu_nocb_mask.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 095d6e4d2fd7..5a6398e21bfc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2440,7 +2440,10 @@ void rcu_init_nohz(void)
 #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);
+		if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
+			pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
+			return;
+		}
 		have_rcu_nocb_mask = true;
 	}
 	if (!have_rcu_nocb_mask)

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

[Frederic Weisbecker]

On Fri, Jul 25, 2014 at 08:10:57PM -0400, Pranith Kumar wrote:
> On 07/25/2014 07:36 PM, Paul E. McKenney wrote:
> > [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
> >     tick_nohz_full_mask when nohz_full= missing) in -tip
> >     or -rcu.  To make this work on top of rcu/next, move the
> >     call to rcu_organize_nocb_kthreads(rsp) to the end of the
> >     for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
> >
> > 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@linux.vnet.ibm.com>
> 
> Please find two points below.
> 
> <snip>
> >  #ifdef CONFIG_TREE_PREEMPT_RCU
> > @@ -2451,6 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
> >      trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
> >  }
> >  
> > +void 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);
> 
> Please check the return value unless you want to increase my commit count ;)
> 
> >
> > +        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) && !defined(CONFIG_NO_HZ_FULL_ALL)
> > +    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) */
> 
> 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.

That said you made me check and I realize that when that happens, we alloc
the mask two times and we leak the first. I need to fix that.

Thanks.

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

[Pranith Kumar]

On Fri, Jul 25, 2014 at 7:36 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
>         tick_nohz_full_mask when nohz_full= missing) in -tip
>         or -rcu.  To make this work on top of rcu/next, move the
>         call to rcu_organize_nocb_kthreads(rsp) to the end of the
>         for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
>
> 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@linux.vnet.ibm.com>


Please note one change below, with that

Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>


>
> 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/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..095d6e4d2fd7 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 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
>         trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
>  }
>
> +void 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) && !defined(CONFIG_NO_HZ_FULL_ALL)
> +       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) */

I think we can still come here when there is garbage in
tick_nohz_full_mask. So checking again for tick_nohz_full_running is
necessary before cpumask_or().

> +
> +       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 +2512,6 @@ 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;
>



-- 
Pranith

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

[Pranith Kumar]

On Fri, Jul 25, 2014 at 10:30 PM, Frederic Weisbecker
<fweisbec@gmail.com> 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.

> That said you made me check and I realize that when that happens, we alloc
> the mask two times and we leak the first. I need to fix that.

This does not actually happen as we do the initialization only once.
Am I missing something?

>
> Thanks.


-- 
Pranith

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

[Paul E. McKenney]

On Sat, Jul 26, 2014 at 09:34:11PM -0400, Pranith Kumar wrote:
> On Fri, Jul 25, 2014 at 7:36 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > [ Note: This applies on top of commit 187497fa5e9e (rcu: Allow for NULL
> >         tick_nohz_full_mask when nohz_full= missing) in -tip
> >         or -rcu.  To make this work on top of rcu/next, move the
> >         call to rcu_organize_nocb_kthreads(rsp) to the end of the
> >         for_each_rcu_flavor(rsp) loop in rcu_init_nohz(). ]
> >
> > 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@linux.vnet.ibm.com>
> 
> Please note one change below, with that
> 
> Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

Good catch, fixed and applied your Reviewed-by.

							Thanx, Paul

> > 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/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..095d6e4d2fd7 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 +2424,66 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
> >         trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("DeferredWakeEmpty"));
> >  }
> >
> > +void 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) && !defined(CONFIG_NO_HZ_FULL_ALL)
> > +       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) */
> 
> I think we can still come here when there is garbage in
> tick_nohz_full_mask. So checking again for tick_nohz_full_running is
> necessary before cpumask_or().
> 
> > +
> > +       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 +2512,6 @@ 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;
> >
> 
> 
> 
> -- 
> Pranith
> 

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

[Frederic Weisbecker]

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@gmail.com> 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?

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

[Paul E. McKenney]

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@gmail.com> 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@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

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;

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

[Pranith Kumar]

On Mon, Jul 28, 2014 at 7:17 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Mon, Jul 28, 2014 at 11:03:06PM +0200, Frederic Weisbecker wrote:
>>
>> 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@linux.vnet.ibm.com>
> Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
>
> 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 you are removing the dependency on NO_HZ_FULL_ALL, I think this can just be

#ifdef CONFIG_NO_HZ_FULL

Otherwise, a kernel config with CONFIG_RCU_NOCB_NONE=y,
CONFIG_NO_HZ_FULL_ALL=y with nohz_full= boot parameter will not have a
rcu_nocb_mask.

--
Pranith

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

[Paul E. McKenney]

On Mon, Jul 28, 2014 at 07:35:55PM -0400, Pranith Kumar wrote:
> On Mon, Jul 28, 2014 at 7:17 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Mon, Jul 28, 2014 at 11:03:06PM +0200, Frederic Weisbecker wrote:
> >>
> >> 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@linux.vnet.ibm.com>
> > Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
> >
> > 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 you are removing the dependency on NO_HZ_FULL_ALL, I think this can just be
> 
> #ifdef CONFIG_NO_HZ_FULL
> 
> Otherwise, a kernel config with CONFIG_RCU_NOCB_NONE=y,
> CONFIG_NO_HZ_FULL_ALL=y with nohz_full= boot parameter will not have a
> rcu_nocb_mask.

Good point, fixed.

							Thanx, Paul