[PATCH RFC tip/core/rcu] Add callback-free CPUs

From: Paul E. McKenney
Date: Wed Sep 05 2012 - 17:39:52 EST

RCU callback execution can add significant OS jitter and also can degrade
scheduling latency. This commit therefore adds the ability for selected
CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded to
kthreads. If the "rcu_nocb_poll" boot parameter is also specified, these
kthreads will do polling, removing the need for the offloaded CPUs to do
wakeups. At least one CPU must be doing normal callback processing:
currently CPU 0 cannot be selected as a no-CBs CPU. In addition, attempts
to offline the last normal-CBs CPU will fail.

This is an experimental patch, so just FYI for the moment. Known
shortcomings include:

o The counters should be atomic_long_t rather than atomic_t.

o No-CBs CPUs can be configured only at boot time.

o Only a modest number of CPUs can be configured as no-CBs CPUs.
Definitely a few tens, perhaps a few hundred, but no way thousands.

o At least one CPU must remain a normal-CBs CPU.

o Not much in the way of energy-efficiency features, though there
are some natural energy savings inherent in the implementation

o The per-no-CBs-CPU kthreads are not subject to RCU priority boosting.

o Care is required when setting the kthreads to RT priority.

Later versions will address some of them, but others are likely to remain.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 5bde94d..d4f559b 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -549,6 +549,7 @@ TRACE_EVENT(rcu_torture_read,
* "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.
* "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
diff --git a/init/Kconfig b/init/Kconfig
index c26b8a1..ccda03a 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -598,6 +598,25 @@ config RCU_BOOST_DELAY

Accept the default if unsure.

+config RCU_NOCB_CPU
+ bool "Offload RCU callback processing from boot-selected CPUs"
+ depends on TREE_RCU || TREE_PREEMPT_RCU
+ default n
+ help
+ Use this option to reduce OS jitter for aggressive HPC or
+ real-time workloads.
+
+ This option offloads callback invocation from the set of CPUs
+ specified at boot time by the rcu_nocbs parameter. For each
+ such CPU, a kthread ("rcuoN") will be created to invoke callbacks.
+ Nothing prevents this kthread from running on of of the specified
+ CPUs, but (1) the kthreads may be preempted between each callback
+ and (2) affinity or cgroups can be used to force the kthreads off
+ of those CPUs if desired.
+
+ Say Y here if you want reduced OS jitter on selected CPUs.
+ Say N here if you are unsure.
+
endmenu # "RCU Subsystem"

config IKCONFIG
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 791aea0..cfc5a91 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -303,7 +303,8 @@ EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);
static int
cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
{
- return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];
+ return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL] &&
+ rdp->nxttail[RCU_DONE_TAIL] != NULL;
}

/*
@@ -312,7 +313,9 @@ cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
static int
cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
{
- return *rdp->nxttail[RCU_DONE_TAIL] && !rcu_gp_in_progress(rsp);
+ return rdp->nxttail[RCU_DONE_TAIL] &&
+ *rdp->nxttail[RCU_DONE_TAIL] &&
+ !rcu_gp_in_progress(rsp);
}

/*
@@ -1071,6 +1074,7 @@ static void init_callback_list(struct rcu_data *rdp)
rdp->nxtlist = NULL;
for (i = 0; i < RCU_NEXT_SIZE; i++)
rdp->nxttail[i] = &rdp->nxtlist;
+ init_nocb_callback_list(rdp);
}

/*
@@ -1560,6 +1564,10 @@ static void
rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
struct rcu_node *rnp, struct rcu_data *rdp)
{
+ /* No-CBs CPUs do not have orphanable callbacks. */
+ if (is_nocb_cpu(rdp->cpu))
+ return;
+
/*
* Orphan the callbacks. First adjust the counts. This is safe
* because ->onofflock excludes _rcu_barrier()'s adoption of
@@ -1611,6 +1619,10 @@ static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
int i;
struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);

+ /* No-CBs CPUs are handled specially. */
+ if (rcu_nocb_adopt_orphan_cbs(rsp, rdp))
+ return;
+
/* Do the accounting first. */
rdp->qlen_lazy += rsp->qlen_lazy;
rdp->qlen += rsp->qlen;
@@ -2087,9 +2099,15 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
}
}

+/*
+ * Helper function for call_rcu() and friends. The cpu argument will
+ * normally be -1, indicating "currently running CPU". It may specify
+ * a CPU only if that CPU is a no-CBs CPU. Currently, only _rcu_barrier()
+ * is expected to specify a CPU.
+ */
static void
__call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
- struct rcu_state *rsp, bool lazy)
+ struct rcu_state *rsp, int cpu, bool lazy)
{
unsigned long flags;
struct rcu_data *rdp;
@@ -2109,9 +2127,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp = this_cpu_ptr(rsp->rda);

/* Add the callback to our list. */
- if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) {
+ if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL) || cpu != -1) {
+ int offline;
+
+ if (cpu != -1)
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ offline = !__call_rcu_nocb(rdp, head, lazy);
+ WARN_ON_ONCE(offline);
/* _call_rcu() is illegal on offline CPU; leak the callback. */
- WARN_ON_ONCE(1);
local_irq_restore(flags);
return;
}
@@ -2140,7 +2163,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
*/
void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
- __call_rcu(head, func, &rcu_sched_state, 0);
+ __call_rcu(head, func, &rcu_sched_state, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu_sched);

@@ -2149,7 +2172,7 @@ EXPORT_SYMBOL_GPL(call_rcu_sched);
*/
void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
- __call_rcu(head, func, &rcu_bh_state, 0);
+ __call_rcu(head, func, &rcu_bh_state, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

@@ -2538,9 +2561,17 @@ static void _rcu_barrier(struct rcu_state *rsp)
* When that callback is invoked, we will know that all of the
* corresponding CPU's preceding callbacks have been invoked.
*/
- for_each_online_cpu(cpu) {
+ for_each_possible_cpu(cpu) {
+ if (!cpu_online(cpu) && !is_nocb_cpu(cpu))
+ continue;
rdp = per_cpu_ptr(rsp->rda, cpu);
- if (ACCESS_ONCE(rdp->qlen)) {
+ if (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);
+ } else if (ACCESS_ONCE(rdp->qlen)) {
_rcu_barrier_trace(rsp, "OnlineQ", cpu,
rsp->n_barrier_done);
smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
@@ -2614,6 +2645,7 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
#endif
rdp->cpu = cpu;
rdp->rsp = rsp;
+ rcu_boot_init_nocb_percpu_data(rdp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

@@ -2699,6 +2731,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;
struct rcu_state *rsp;
+ int ret = NOTIFY_OK;

trace_rcu_utilization("Start CPU hotplug");
switch (action) {
@@ -2713,8 +2746,12 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
rcu_cpu_kthread_setrt(cpu, 1);
break;
case CPU_DOWN_PREPARE:
- rcu_node_kthread_setaffinity(rnp, cpu);
- rcu_cpu_kthread_setrt(cpu, 0);
+ if (nocb_cpu_expendable(cpu)) {
+ rcu_node_kthread_setaffinity(rnp, cpu);
+ rcu_cpu_kthread_setrt(cpu, 0);
+ } else {
+ ret = NOTIFY_BAD;
+ }
break;
case CPU_DYING:
case CPU_DYING_FROZEN:
@@ -2738,7 +2775,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
break;
}
trace_rcu_utilization("End CPU hotplug");
- return NOTIFY_OK;
+ return ret;
}

/*
@@ -2758,6 +2795,7 @@ static int __init rcu_spawn_gp_kthread(void)
raw_spin_lock_irqsave(&rnp->lock, flags);
rsp->gp_kthread = t;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ rcu_spawn_nocb_kthreads(rsp);
}
return 0;
}
@@ -2952,6 +2990,7 @@ void __init rcu_init(void)
rcu_init_one(&rcu_sched_state, &rcu_sched_data);
rcu_init_one(&rcu_bh_state, &rcu_bh_data);
__rcu_init_preempt();
+ rcu_init_nocb();
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

/*
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 499d661..5e5568d 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -323,6 +323,18 @@ struct rcu_data {
struct rcu_head oom_head;
#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */

+ /* 7) Callback offloading. */
+#ifdef CONFIG_RCU_NOCB_CPU
+ struct rcu_head *nocb_head; /* CBs waiting for kthread. */
+ struct rcu_head **nocb_tail;
+ atomic_t nocb_q_count; /* # CBs waiting for kthread */
+ atomic_t nocb_q_count_lazy; /* (approximate). */
+ int nocb_p_count; /* # CBs being invoked by kthread */
+ int nocb_p_count_lazy; /* (approximate). */
+ wait_queue_head_t nocb_wq; /* For nocb kthreads to sleep on. */
+ struct task_struct *nocb_kthread;
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
+
int cpu;
struct rcu_state *rsp;
};
@@ -375,6 +387,12 @@ struct rcu_state {
struct rcu_data __percpu *rda; /* pointer of percu rcu_data. */
void (*call)(struct rcu_head *head, /* call_rcu() flavor. */
void (*func)(struct rcu_head *head));
+#ifdef CONFIG_RCU_NOCB_CPU
+ void (*call_remote)(struct rcu_head *head,
+ void (*func)(struct rcu_head *head));
+ /* call_rcu() flavor, but for */
+ /* placing on remote CPU. */
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU */

/* The following fields are guarded by the root rcu_node's lock. */

@@ -428,6 +446,8 @@ struct rcu_state {
#define RCU_GP_FLAG_FQS 0x2 /* Need grace-period quiescent-state forcing. */

extern struct list_head rcu_struct_flavors;
+
+/* Sequence through rcu_state structures for each RCU flavor. */
#define for_each_rcu_flavor(rsp) \
list_for_each_entry((rsp), &rcu_struct_flavors, flavors)

@@ -511,5 +531,32 @@ 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 is_nocb_cpu(int cpu);
+static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
+ bool lazy);
+static bool rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
+ struct rcu_data *rdp);
+static bool nocb_cpu_expendable(int cpu);
+static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
+static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
+static void init_nocb_callback_list(struct rcu_data *rdp);
+static void __init rcu_init_nocb(void);

#endif /* #ifndef RCU_TREE_NONCORE */
+
+#ifdef CONFIG_RCU_TRACE
+#ifdef CONFIG_RCU_NOCB_CPU
+/* Sum up queue lengths for tracing. */
+static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
+{
+ *ql = atomic_read(&rdp->nocb_q_count) + rdp->nocb_p_count;
+ *qll = atomic_read(&rdp->nocb_q_count_lazy) + rdp->nocb_p_count_lazy;
+}
+#else /* #ifdef CONFIG_RCU_NOCB_CPU */
+static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
+{
+ *ql = 0;
+ *qll = 0;
+}
+#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
+#endif /* #ifdef CONFIG_RCU_TRACE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 5384eda..2341ddd 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -35,6 +35,13 @@
#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
#endif

+#ifdef CONFIG_RCU_NOCB_CPU
+static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
+static bool have_rcu_nocb_mask; /* Was rcu_nocb_mask allocated? */
+static bool rcu_nocb_poll; /* Offload kthread are to poll. */
+module_param(rcu_nocb_poll, bool, 0444);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
+
/*
* Check the RCU kernel configuration parameters and print informative
* messages about anything out of the ordinary. If you like #ifdef, you
@@ -75,6 +82,20 @@ static void __init rcu_bootup_announce_oddness(void)
printk(KERN_INFO "\tExperimental boot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
if (nr_cpu_ids != NR_CPUS)
printk(KERN_INFO "\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
+#ifdef CONFIG_RCU_NOCB_CPU
+ if (have_rcu_nocb_mask) {
+ char buf[NR_CPUS * 5];
+
+ if (cpumask_test_cpu(0, rcu_nocb_mask)) {
+ cpumask_clear_cpu(0, rcu_nocb_mask);
+ pr_info("\tCPU 0: illegal no-CBs CPU (cleared).\n");
+ }
+ cpulist_scnprintf(buf, sizeof(buf), rcu_nocb_mask);
+ pr_info("\tExperimental no-CBs CPUs: %s.\n", buf);
+ if (rcu_nocb_poll)
+ pr_info("\tExperimental polled no-CBs CPUs.\n");
+ }
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
}

#ifdef CONFIG_TREE_PREEMPT_RCU
@@ -641,7 +662,7 @@ static void rcu_preempt_do_callbacks(void)
*/
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
- __call_rcu(head, func, &rcu_preempt_state, 0);
+ __call_rcu(head, func, &rcu_preempt_state, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu);

@@ -655,7 +676,7 @@ EXPORT_SYMBOL_GPL(call_rcu);
void kfree_call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *rcu))
{
- __call_rcu(head, func, &rcu_preempt_state, 1);
+ __call_rcu(head, func, &rcu_preempt_state, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

@@ -1012,7 +1033,7 @@ static void rcu_preempt_check_callbacks(int cpu)
void kfree_call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *rcu))
{
- __call_rcu(head, func, &rcu_sched_state, 1);
+ __call_rcu(head, func, &rcu_sched_state, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);

@@ -2347,3 +2368,373 @@ static void increment_cpu_stall_ticks(void)
}

#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
+
+#ifdef CONFIG_RCU_NOCB_CPU
+
+/*
+ * Offload callback processing from the boot-time-specified set of CPUs
+ * specified by rcu_nocb_mask. For each CPU in the set, there is a
+ * kthread created that pulls the callbacks from the corresponding CPU,
+ * waits for a grace period to elapse, and invokes the callbacks.
+ * The no-CBs CPUs do a wake_up() on their kthread when they insert
+ * a callback into any empty list, unless the rcu_nocb_poll boot parameter
+ * has been specified, in which case each kthread actively polls its
+ * CPU. (Which isn't so great for energy efficiency, but which does
+ * reduce RCU's overhead on that CPU.)
+ *
+ * This is intended to be used in conjunction with Frederic Weisbecker's
+ * adaptive-idle work, which would seriously reduce OS jitter on CPUs
+ * running CPU-bound user-mode computations.
+ *
+ * Offloading of callback processing could also in theory be used as
+ * an energy-efficiency measure because CPUs with no RCU callbacks
+ * queued are more aggressive about entering dyntick-idle mode.
+ */
+
+
+/* Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters. */
+static int __init rcu_nocb_setup(char *str)
+{
+ alloc_bootmem_cpumask_var(&rcu_nocb_mask);
+ have_rcu_nocb_mask = true;
+ cpulist_parse(str, rcu_nocb_mask);
+ return 1;
+}
+__setup("rcu_nocbs=", rcu_nocb_setup);
+
+/* Is the specified CPU a no-CPUs CPU? */
+static bool is_nocb_cpu(int cpu)
+{
+ if (have_rcu_nocb_mask)
+ return cpumask_test_cpu(cpu, rcu_nocb_mask);
+ return false;
+}
+
+/*
+ * 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
+ * counts are supplied by rhcount and rhcount_lazy.
+ *
+ * If warranted, also wake up the kthread servicing this CPUs queues.
+ */
+static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
+ struct rcu_head *rhp,
+ struct rcu_head **rhtp,
+ int rhcount, int rhcount_lazy)
+{
+ int len;
+ struct rcu_head **old_rhpp;
+ struct task_struct *t;
+
+ /* Enqueue the callback on the nocb list and update counts. */
+ old_rhpp = xchg(&rdp->nocb_tail, rhtp);
+ ACCESS_ONCE(*old_rhpp) = rhp;
+ atomic_add(rhcount, &rdp->nocb_q_count);
+ atomic_add(rhcount_lazy, &rdp->nocb_q_count_lazy);
+
+ /* If we are not being polled and there is a kthread, awaken it ... */
+ t = ACCESS_ONCE(rdp->nocb_kthread);
+ if (rcu_nocb_poll | !t)
+ return;
+ len = atomic_read(&rdp->nocb_q_count);
+ if (old_rhpp == &rdp->nocb_head) {
+ wake_up(&rdp->nocb_wq); /* ... only if queue was empty ... */
+ rdp->qlen_last_fqs_check = 0;
+ } else if (len > rdp->qlen_last_fqs_check + qhimark) {
+ wake_up_process(t); /* ... or if many callbacks queued. */
+ rdp->qlen_last_fqs_check = LONG_MAX / 2;
+ }
+ return;
+}
+
+/*
+ * This is a helper for __call_rcu(), which invokes this when the normal
+ * callback queue is inoperable. If this is not a no-CBs CPU, this
+ * function returns failure back to __call_rcu(), which can complain
+ * appropriately.
+ *
+ * Otherwise, this function queues the callback where the corresponding
+ * "rcuo" kthread can find it.
+ */
+static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
+ bool lazy)
+{
+
+ if (!is_nocb_cpu(rdp->cpu))
+ return 0;
+ __call_rcu_nocb_enqueue(rdp, rhp, &rhp->next, 1, lazy);
+ return 1;
+}
+
+/*
+ * Adopt orphaned callbacks on a no-CBs CPU, or return 0 if this is
+ * not a no-CBs CPU.
+ */
+static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
+ struct rcu_data *rdp)
+{
+ long ql = rsp->qlen;
+ long qll = rsp->qlen_lazy;
+
+ /* If this is not a no-CBs CPU, tell the caller to do it the old way. */
+ if (!is_nocb_cpu(smp_processor_id()))
+ return 0;
+ rsp->qlen = 0;
+ rsp->qlen_lazy = 0;
+
+ /* First, enqueue the donelist, if any. This preserves CB ordering. */
+ if (rsp->orphan_donelist != NULL) {
+ __call_rcu_nocb_enqueue(rdp, rsp->orphan_donelist,
+ rsp->orphan_donetail, ql, qll);
+ ql = qll = 0;
+ rsp->orphan_donelist = NULL;
+ rsp->orphan_donetail = &rsp->orphan_donelist;
+ }
+ if (rsp->orphan_nxtlist != NULL) {
+ __call_rcu_nocb_enqueue(rdp, rsp->orphan_nxtlist,
+ rsp->orphan_nxttail, ql, qll);
+ ql = qll = 0;
+ rsp->orphan_nxtlist = NULL;
+ rsp->orphan_nxttail = &rsp->orphan_nxtlist;
+ }
+ return 1;
+}
+
+/*
+ * There must be at least one non-no-CBs CPU in operation at any given
+ * time, because no-CBs CPUs are not capable of initiating grace periods
+ * independently. This function therefore complains if the specified
+ * CPU is the last non-no-CBs CPU, allowing the CPU-hotplug system to
+ * avoid offlining the last such CPU. (Recursion is a wonderful thing,
+ * but you have to have a base case!)
+ */
+static bool nocb_cpu_expendable(int cpu)
+{
+ cpumask_var_t non_nocb_cpus;
+ int ret;
+
+ /*
+ * If there are no no-CB CPUs or if this CPU is not a no-CB CPU,
+ * then offlining this CPU is harmless. Let it happen.
+ */
+ if (!have_rcu_nocb_mask || is_nocb_cpu(cpu))
+ return 1;
+
+ /* If no memory, play it safe and keep the CPU around. */
+ if (!alloc_cpumask_var(&non_nocb_cpus, GFP_NOIO))
+ return 0;
+ cpumask_andnot(non_nocb_cpus, cpu_online_mask, rcu_nocb_mask);
+ cpumask_clear_cpu(cpu, non_nocb_cpus);
+ ret = !cpumask_empty(non_nocb_cpus);
+ free_cpumask_var(non_nocb_cpus);
+ return ret;
+}
+
+/*
+ * Helper structure for remote registry of RCU callbacks.
+ * This is needed for when a no-CBs CPU needs to start a grace period.
+ * If it just invokes call_rcu(), the resulting callback will be queued,
+ * which can result in deadlock.
+ */
+struct rcu_head_remote {
+ struct rcu_head *rhp;
+ call_rcu_func_t *crf;
+ void (*func)(struct rcu_head *rhp);
+};
+
+/*
+ * Register a callback as specified by the rcu_head_remote struct.
+ * This function is intended to be invoked via smp_call_function_single().
+ */
+static void call_rcu_local(void *arg)
+{
+ struct rcu_head_remote *rhrp =
+ container_of(arg, struct rcu_head_remote, rhp);
+
+ rhrp->crf(rhrp->rhp, rhrp->func);
+}
+
+/*
+ * Set up an rcu_head_remote structure and the invoke call_rcu_local()
+ * on CPU 0 (which is guaranteed to be a non-no-CBs CPU) via
+ * smp_call_function_single().
+ */
+static void invoke_crf_remote(struct rcu_head *rhp,
+ void (*func)(struct rcu_head *rhp),
+ call_rcu_func_t crf)
+{
+ struct rcu_head_remote rhr;
+
+ rhr.rhp = rhp;
+ rhr.crf = crf;
+ rhr.func = func;
+ smp_call_function_single(0, call_rcu_local, &rhr, 1);
+}
+
+/*
+ * Helper functions to be passed to wait_rcu_gp(), each of which
+ * invokes invoke_crf_remote() to register a callback appropriately.
+ */
+static void __maybe_unused
+call_rcu_preempt_remote(struct rcu_head *rhp,
+ void (*func)(struct rcu_head *rhp))
+{
+ invoke_crf_remote(rhp, func, call_rcu);
+}
+static void call_rcu_bh_remote(struct rcu_head *rhp,
+ void (*func)(struct rcu_head *rhp))
+{
+ invoke_crf_remote(rhp, func, call_rcu_bh);
+}
+static void call_rcu_sched_remote(struct rcu_head *rhp,
+ void (*func)(struct rcu_head *rhp))
+{
+ invoke_crf_remote(rhp, func, call_rcu_sched);
+}
+
+/*
+ * Per-rcu_data kthread, but only for no-CBs CPUs. Each kthread invokes
+ * callbacks queued by the corresponding no-CBs CPU.
+ */
+static int rcu_nocb_kthread(void *arg)
+{
+ int c, cl;
+ struct rcu_head *list;
+ struct rcu_head *next;
+ struct rcu_head **tail;
+ struct rcu_data *rdp = arg;
+
+ /* Each pass through this loop invokes one batch of callbacks */
+ for (;;) {
+ /* If not polling, wait for next batch of callbacks. */
+ if (!rcu_nocb_poll)
+ wait_event(rdp->nocb_wq, rdp->nocb_head);
+ list = ACCESS_ONCE(rdp->nocb_head);
+ if (!list) {
+ schedule_timeout_interruptible(1);
+ continue;
+ }
+
+ /*
+ * Extract queued callbacks, update counts, and wait
+ * for a grace period to elapse.
+ */
+ ACCESS_ONCE(rdp->nocb_head) = NULL;
+ tail = xchg(&rdp->nocb_tail, &rdp->nocb_head);
+ c = atomic_xchg(&rdp->nocb_q_count, 0);
+ cl = atomic_xchg(&rdp->nocb_q_count_lazy, 0);
+ ACCESS_ONCE(rdp->nocb_p_count) += c;
+ ACCESS_ONCE(rdp->nocb_p_count_lazy) += cl;
+ wait_rcu_gp(rdp->rsp->call_remote);
+
+ /* Each pass through the following loop invokes a callback. */
+ trace_rcu_batch_start(rdp->rsp->name, cl, c, -1);
+ c = cl = 0;
+ while (list) {
+ next = list->next;
+ /* Wait for enqueuing to complete, if needed. */
+ while (next == NULL && &list->next != tail) {
+ schedule_timeout_interruptible(1);
+ next = list->next;
+ }
+ debug_rcu_head_unqueue(list);
+ local_bh_disable();
+ if (__rcu_reclaim(rdp->rsp->name, list))
+ cl++;
+ c++;
+ local_bh_enable();
+ list = next;
+ }
+ trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
+ ACCESS_ONCE(rdp->nocb_p_count) -= c;
+ ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
+ rdp->n_cbs_invoked += c;
+ }
+ return 0;
+}
+
+/* Initialize per-rcu_data variables for no-CBs CPUs. */
+static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
+{
+ rdp->nocb_tail = &rdp->nocb_head;
+ init_waitqueue_head(&rdp->nocb_wq);
+}
+
+/* Create a kthread for each RCU flavor for each no-CBs CPU. */
+static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
+{
+ int cpu;
+ struct rcu_data *rdp;
+ struct task_struct *t;
+
+ if (rcu_nocb_mask == NULL)
+ return;
+ for_each_cpu(cpu, rcu_nocb_mask) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ t = kthread_run(rcu_nocb_kthread, rdp, "rcuo%d", cpu);
+ BUG_ON(IS_ERR(t));
+ ACCESS_ONCE(rdp->nocb_kthread) = t;
+ }
+}
+
+/* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
+static void init_nocb_callback_list(struct rcu_data *rdp)
+{
+ if (rcu_nocb_mask == NULL ||
+ !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
+ return;
+ rdp->nxttail[RCU_NEXT_TAIL] = NULL;
+}
+
+/* Initialize the ->call_remote fields in the rcu_state structures. */
+static void __init rcu_init_nocb(void)
+{
+#ifdef CONFIG_PREEMPT_RCU
+ rcu_preempt_state.call_remote = call_rcu_preempt_remote;
+#endif /* #ifdef CONFIG_PREEMPT_RCU */
+ rcu_bh_state.call_remote = call_rcu_bh_remote;
+ rcu_sched_state.call_remote = call_rcu_sched_remote;
+}
+
+#else /* #ifdef CONFIG_RCU_NOCB_CPU */
+
+static bool is_nocb_cpu(int cpu)
+{
+ return false;
+}
+
+static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
+ bool lazy)
+{
+ return 0;
+}
+
+static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
+ struct rcu_data *rdp)
+{
+ return 0;
+}
+
+static bool nocb_cpu_expendable(int cpu)
+{
+ return 1;
+}
+
+static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
+{
+}
+
+static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
+{
+}
+
+static void init_nocb_callback_list(struct rcu_data *rdp)
+{
+}
+
+static void __init rcu_init_nocb(void)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index 7340efd..4ce0f91 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -84,6 +84,8 @@ static char convert_kthread_status(unsigned int kthread_status)

static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
{
+ long ql, qll;
+
if (!rdp->beenonline)
return;
seq_printf(m, "%3d%cc=%lu g=%lu pq=%d qp=%d",
@@ -97,8 +99,11 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
rdp->dynticks->dynticks_nmi_nesting,
rdp->dynticks_fqs);
seq_printf(m, " of=%lu", rdp->offline_fqs);
+ rcu_nocb_q_lengths(rdp, &ql, &qll);
+ qll += rdp->qlen_lazy;
+ ql += rdp->qlen;
seq_printf(m, " ql=%ld/%ld qs=%c%c%c%c",
- rdp->qlen_lazy, rdp->qlen,
+ qll, ql,
".N"[rdp->nxttail[RCU_NEXT_READY_TAIL] !=
rdp->nxttail[RCU_NEXT_TAIL]],
".R"[rdp->nxttail[RCU_WAIT_TAIL] !=
@@ -147,6 +152,8 @@ static const struct file_operations rcudata_fops = {

static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
{
+ long ql, qll;
+
if (!rdp->beenonline)
return;
seq_printf(m, "%d,%s,%lu,%lu,%d,%d",
@@ -160,7 +167,10 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
rdp->dynticks->dynticks_nmi_nesting,
rdp->dynticks_fqs);
seq_printf(m, ",%lu", rdp->offline_fqs);
- seq_printf(m, ",%ld,%ld,\"%c%c%c%c\"", rdp->qlen_lazy, rdp->qlen,
+ rcu_nocb_q_lengths(rdp, &ql, &qll);
+ qll += rdp->qlen_lazy;
+ ql += rdp->qlen;
+ seq_printf(m, ",%ld,%ld,\"%c%c%c%c\"", qll, ql,
".N"[rdp->nxttail[RCU_NEXT_READY_TAIL] !=
rdp->nxttail[RCU_NEXT_TAIL]],
".R"[rdp->nxttail[RCU_WAIT_TAIL] !=

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