[PATCH RFC tip/core/rcu 12/14] rcu: priority boosting for TREE_PREEMPT_RCU

From: Paul E. McKenney
Date: Tue Feb 22 2011 - 20:40:41 EST

From: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>

Add priority boosting for TREE_PREEMPT_RCU, similar to that for
TINY_PREEMPT_RCU. This is enabled by the default-off RCU_BOOST
kernel parameter. The priority to which to boost preempted
RCU readers is controlled by the RCU_BOOST_PRIO kernel parameter
(defaulting to real-time priority 1) and the time to wait before
boosting the readers who are blocking a given grace period is
controlled by the RCU_BOOST_DELAY kernel parameter (defaulting to
500 milliseconds).

Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
init/Kconfig | 2 +-
kernel/rcutree.c | 105 ++++++++++++------
kernel/rcutree.h | 28 +++++-
kernel/rcutree_plugin.h | 279 +++++++++++++++++++++++++++++++++++++++++++++--
4 files changed, 364 insertions(+), 50 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index e11bc79..1855ef0 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -460,7 +460,7 @@ config TREE_RCU_TRACE

config RCU_BOOST
bool "Enable RCU priority boosting"
- depends on RT_MUTEXES && TINY_PREEMPT_RCU
+ depends on RT_MUTEXES && PREEMPT_RCU
default n
help
This option boosts the priority of preempted RCU readers that
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 2241f28..2d317b2 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -81,6 +81,8 @@ DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);
struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);

+static struct rcu_state *rcu_state;
+
int rcu_scheduler_active __read_mostly;
EXPORT_SYMBOL_GPL(rcu_scheduler_active);

@@ -92,7 +94,7 @@ static DEFINE_PER_CPU(char, rcu_cpu_has_work);
static char rcu_kthreads_spawnable;

static void rcu_node_kthread_setaffinity(struct rcu_node *rnp);
-static void invoke_rcu_kthread(void);
+static void invoke_rcu_cpu_kthread(void);

#define RCU_KTHREAD_PRIO 1 /* RT priority for per-CPU kthreads. */

@@ -789,6 +791,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
rnp->completed = rsp->completed;
rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
rcu_start_gp_per_cpu(rsp, rnp, rdp);
+ rcu_preempt_boost_start_gp(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
@@ -824,6 +827,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
rnp->completed = rsp->completed;
if (rnp == rdp->mynode)
rcu_start_gp_per_cpu(rsp, rnp, rdp);
+ rcu_preempt_boost_start_gp(rnp);
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}

@@ -880,7 +884,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
return;
}
rnp->qsmask &= ~mask;
- if (rnp->qsmask != 0 || rcu_preempted_readers(rnp)) {
+ if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {

/* Other bits still set at this level, so done. */
raw_spin_unlock_irqrestore(&rnp->lock, flags);
@@ -1183,7 +1187,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)

/* Re-raise the RCU softirq if there are callbacks remaining. */
if (cpu_has_callbacks_ready_to_invoke(rdp))
- invoke_rcu_kthread();
+ invoke_rcu_cpu_kthread();
}

/*
@@ -1229,7 +1233,7 @@ void rcu_check_callbacks(int cpu, int user)
}
rcu_preempt_check_callbacks(cpu);
if (rcu_pending(cpu))
- invoke_rcu_kthread();
+ invoke_rcu_cpu_kthread();
}

#ifdef CONFIG_SMP
@@ -1237,6 +1241,8 @@ void rcu_check_callbacks(int cpu, int user)
/*
* Scan the leaf rcu_node structures, processing dyntick state for any that
* have not yet encountered a quiescent state, using the function specified.
+ * Also initiate boosting for any threads blocked on the root rcu_node.
+ *
* The caller must have suppressed start of new grace periods.
*/
static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
@@ -1255,6 +1261,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
return;
}
if (rnp->qsmask == 0) {
+ rcu_initiate_boost(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
continue;
}
@@ -1273,6 +1280,11 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
}
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
+ rnp = rcu_get_root(rsp);
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+ if (rnp->qsmask == 0)
+ rcu_initiate_boost(rnp);
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
@@ -1408,7 +1420,7 @@ static void rcu_process_callbacks(void)
* Wake up the current CPU's kthread. This replaces raise_softirq()
* in earlier versions of RCU.
*/
-static void invoke_rcu_kthread(void)
+static void invoke_rcu_cpu_kthread(void)
{
unsigned long flags;
wait_queue_head_t *q;
@@ -1427,24 +1439,33 @@ static void invoke_rcu_kthread(void)
}

/*
+ * Wake up the specified per-rcu_node-structure kthread.
+ * The caller must hold ->lock.
+ */
+static void invoke_rcu_node_kthread(struct rcu_node *rnp)
+{
+ struct task_struct *t;
+
+ t = rnp->node_kthread_task;
+ if (t != NULL)
+ wake_up_process(t);
+}
+
+/*
* Timer handler to initiate the waking up of per-CPU kthreads that
* have yielded the CPU due to excess numbers of RCU callbacks.
+ * We wake up the per-rcu_node kthread, which in turn will wake up
+ * the booster kthread.
*/
static void rcu_cpu_kthread_timer(unsigned long arg)
{
unsigned long flags;
- struct rcu_data *rdp = (struct rcu_data *)arg;
+ struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
struct rcu_node *rnp = rdp->mynode;
- struct task_struct *t;

raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->wakemask |= rdp->grpmask;
- t = rnp->node_kthread_task;
- if (t == NULL) {
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- return;
- }
- wake_up_process(t);
+ invoke_rcu_node_kthread(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

@@ -1454,13 +1475,12 @@ static void rcu_cpu_kthread_timer(unsigned long arg)
* remain preempted. Either way, we restore our real-time priority
* before returning.
*/
-static void rcu_yield(int cpu)
+static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
{
- struct rcu_data *rdp = per_cpu_ptr(rcu_sched_state.rda, cpu);
struct sched_param sp;
struct timer_list yield_timer;

- setup_timer(&yield_timer, rcu_cpu_kthread_timer, (unsigned long)rdp);
+ setup_timer(&yield_timer, f, arg);
mod_timer(&yield_timer, jiffies + 2);
sp.sched_priority = 0;
sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
@@ -1484,12 +1504,13 @@ static void rcu_yield(int cpu)
*/
static int rcu_cpu_kthread_should_stop(int cpu)
{
- while (cpu_is_offline(cpu) || smp_processor_id() != cpu) {
+ while (cpu_is_offline(cpu) ||
+ !cpumask_equal(&current->cpus_allowed, cpumask_of(cpu))) {
if (kthread_should_stop())
return 1;
local_bh_enable();
- schedule_timeout_uninterruptible(1);
- if (smp_processor_id() != cpu)
+ schedule_timeout_interruptible(1);
+ if (!cpumask_equal(&current->cpus_allowed, cpumask_of(cpu)))
set_cpus_allowed_ptr(current, cpumask_of(cpu));
local_bh_disable();
}
@@ -1529,7 +1550,7 @@ static int rcu_cpu_kthread(void *arg)
else
spincnt = 0;
if (spincnt > 10) {
- rcu_yield(cpu);
+ rcu_yield(rcu_cpu_kthread_timer, (unsigned long)cpu);
spincnt = 0;
}
}
@@ -1581,6 +1602,7 @@ static int rcu_node_kthread(void *arg)
mask = rnp->wakemask;
rnp->wakemask = 0;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ rcu_initiate_boost(rnp);
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
if ((mask & 0x1) == 0)
continue;
@@ -1620,6 +1642,7 @@ static void rcu_node_kthread_setaffinity(struct rcu_node *rnp)
if (mask & 01)
cpumask_set_cpu(cpu, cm);
set_cpus_allowed_ptr(rnp->node_kthread_task, cm);
+ rcu_boost_kthread_setaffinity(rnp, cm);
free_cpumask_var(cm);
}

@@ -1634,17 +1657,19 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
struct task_struct *t;

if (!rcu_kthreads_spawnable ||
- rnp->qsmaskinit == 0 ||
- rnp->node_kthread_task != NULL)
+ rnp->qsmaskinit == 0)
return 0;
- t = kthread_create(rcu_node_kthread, (void *)rnp, "rcun%d", rnp_index);
- if (IS_ERR(t))
- return PTR_ERR(t);
- rnp->node_kthread_task = t;
- wake_up_process(t);
- sp.sched_priority = 99;
- sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
- return 0;
+ if (rnp->node_kthread_task == NULL) {
+ t = kthread_create(rcu_node_kthread, (void *)rnp,
+ "rcun%d", rnp_index);
+ if (IS_ERR(t))
+ return PTR_ERR(t);
+ rnp->node_kthread_task = t;
+ wake_up_process(t);
+ sp.sched_priority = 99;
+ sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+ }
+ return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
}

/*
@@ -1662,10 +1687,16 @@ static int __init rcu_spawn_kthreads(void)
if (cpu_online(cpu))
(void)rcu_spawn_one_cpu_kthread(cpu);
}
- rcu_for_each_leaf_node(&rcu_sched_state, rnp) {
- init_waitqueue_head(&rnp->node_wq);
- (void)rcu_spawn_one_node_kthread(&rcu_sched_state, rnp);
- }
+ rnp = rcu_get_root(rcu_state);
+ init_waitqueue_head(&rnp->node_wq);
+ rcu_init_boost_waitqueue(rnp);
+ (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
+ if (NUM_RCU_NODES > 1)
+ rcu_for_each_leaf_node(rcu_state, rnp) {
+ init_waitqueue_head(&rnp->node_wq);
+ rcu_init_boost_waitqueue(rnp);
+ (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
+ }
return 0;
}
early_initcall(rcu_spawn_kthreads);
@@ -2071,14 +2102,14 @@ static void __cpuinit rcu_online_cpu(int cpu)

static void __cpuinit rcu_online_kthreads(int cpu)
{
- struct rcu_data *rdp = per_cpu_ptr(rcu_sched_state.rda, cpu);
+ struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;

/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
if (rcu_kthreads_spawnable) {
(void)rcu_spawn_one_cpu_kthread(cpu);
if (rnp->node_kthread_task == NULL)
- (void)rcu_spawn_one_node_kthread(&rcu_sched_state, rnp);
+ (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
}
}

@@ -2089,7 +2120,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
unsigned long action, void *hcpu)
{
long cpu = (long)hcpu;
- struct rcu_data *rdp = per_cpu_ptr(rcu_sched_state.rda, cpu);
+ struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;

switch (action) {
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index c021380..4c820a8 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -135,6 +135,24 @@ struct rcu_node {
/* if there is no such task. If there */
/* is no current expedited grace period, */
/* then there can cannot be any such task. */
+#ifdef CONFIG_RCU_BOOST
+ struct list_head *boost_tasks;
+ /* Pointer to first task that needs to be */
+ /* priority boosted, or NULL if no priority */
+ /* boosting is needed for this rcu_node */
+ /* structure. If there are no tasks */
+ /* queued on this rcu_node structure that */
+ /* are blocking the current grace period, */
+ /* there can be no such task. */
+ unsigned long boost_time;
+ /* When to start boosting (jiffies). */
+ struct task_struct *boost_kthread_task;
+ /* kthread that takes care of priority */
+ /* boosting for this rcu_node structure. */
+ wait_queue_head_t boost_wq;
+ /* Wait queue on which to park the boost */
+ /* kthread. */
+#endif /* #ifdef CONFIG_RCU_BOOST */
struct task_struct *node_kthread_task;
/* kthread that takes care of this rcu_node */
/* structure, for example, awakening the */
@@ -365,7 +383,7 @@ DECLARE_PER_CPU(struct rcu_data, rcu_preempt_data);
static void rcu_bootup_announce(void);
long rcu_batches_completed(void);
static void rcu_preempt_note_context_switch(int cpu);
-static int rcu_preempted_readers(struct rcu_node *rnp);
+static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
#ifdef CONFIG_HOTPLUG_CPU
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp,
unsigned long flags);
@@ -392,5 +410,13 @@ static void __cpuinit rcu_preempt_init_percpu_data(int cpu);
static void rcu_preempt_send_cbs_to_online(void);
static void __init __rcu_init_preempt(void);
static void rcu_needs_cpu_flush(void);
+static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp);
+static void rcu_initiate_boost(struct rcu_node *rnp);
+static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
+ cpumask_var_t cm);
+static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
+static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
+ struct rcu_node *rnp,
+ int rnp_index);

#endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index b9bd69a..1242949 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -66,6 +66,7 @@ static void __init rcu_bootup_announce_oddness(void)

struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
+static struct rcu_state *rcu_state = &rcu_preempt_state;

static int rcu_preempted_readers_exp(struct rcu_node *rnp);

@@ -179,6 +180,10 @@ static void rcu_preempt_note_context_switch(int cpu)
if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
rnp->gp_tasks = &t->rcu_node_entry;
+#ifdef CONFIG_RCU_BOOST
+ if (rnp->boost_tasks != NULL)
+ rnp->boost_tasks = rnp->gp_tasks;
+#endif /* #ifdef CONFIG_RCU_BOOST */
} else {
list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
if (rnp->qsmask & rdp->grpmask)
@@ -218,7 +223,7 @@ EXPORT_SYMBOL_GPL(__rcu_read_lock);
* for the specified rcu_node structure. If the caller needs a reliable
* answer, it must hold the rcu_node's ->lock.
*/
-static int rcu_preempted_readers(struct rcu_node *rnp)
+static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return rnp->gp_tasks != NULL;
}
@@ -236,7 +241,7 @@ static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
unsigned long mask;
struct rcu_node *rnp_p;

- if (rnp->qsmask != 0 || rcu_preempted_readers(rnp)) {
+ if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return; /* Still need more quiescent states! */
}
@@ -325,7 +330,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
break;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
- empty = !rcu_preempted_readers(rnp);
+ empty = !rcu_preempt_blocked_readers_cgp(rnp);
empty_exp = !rcu_preempted_readers_exp(rnp);
smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
np = rcu_next_node_entry(t, rnp);
@@ -334,6 +339,10 @@ static void rcu_read_unlock_special(struct task_struct *t)
rnp->gp_tasks = np;
if (&t->rcu_node_entry == rnp->exp_tasks)
rnp->exp_tasks = np;
+#ifdef CONFIG_RCU_BOOST
+ if (&t->rcu_node_entry == rnp->boost_tasks)
+ rnp->boost_tasks = np;
+#endif /* #ifdef CONFIG_RCU_BOOST */
t->rcu_blocked_node = NULL;

/*
@@ -346,6 +355,15 @@ static void rcu_read_unlock_special(struct task_struct *t)
else
rcu_report_unblock_qs_rnp(rnp, flags);

+#ifdef CONFIG_RCU_BOOST
+ /* Unboost if we were boosted. */
+ if (special & RCU_READ_UNLOCK_BOOSTED) {
+ t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BOOSTED;
+ rt_mutex_unlock(t->rcu_boost_mutex);
+ t->rcu_boost_mutex = NULL;
+ }
+#endif /* #ifdef CONFIG_RCU_BOOST */
+
/*
* If this was the last task on the expedited lists,
* then we need to report up the rcu_node hierarchy.
@@ -391,7 +409,7 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
unsigned long flags;
struct task_struct *t;

- if (!rcu_preempted_readers(rnp))
+ if (!rcu_preempt_blocked_readers_cgp(rnp))
return;
raw_spin_lock_irqsave(&rnp->lock, flags);
t = list_entry(rnp->gp_tasks,
@@ -430,7 +448,7 @@ static void rcu_print_task_stall(struct rcu_node *rnp)
{
struct task_struct *t;

- if (!rcu_preempted_readers(rnp))
+ if (!rcu_preempt_blocked_readers_cgp(rnp))
return;
t = list_entry(rnp->gp_tasks,
struct task_struct, rcu_node_entry);
@@ -460,7 +478,7 @@ static void rcu_preempt_stall_reset(void)
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
- WARN_ON_ONCE(rcu_preempted_readers(rnp));
+ WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
if (!list_empty(&rnp->blkd_tasks))
rnp->gp_tasks = rnp->blkd_tasks.next;
WARN_ON_ONCE(rnp->qsmask);
@@ -509,7 +527,7 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
* absolutely necessary, but this is a good performance/complexity
* tradeoff.
*/
- if (rcu_preempted_readers(rnp))
+ if (rcu_preempt_blocked_readers_cgp(rnp))
retval |= RCU_OFL_TASKS_NORM_GP;
if (rcu_preempted_readers_exp(rnp))
retval |= RCU_OFL_TASKS_EXP_GP;
@@ -525,8 +543,20 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
rnp_root->gp_tasks = rnp->gp_tasks;
if (&t->rcu_node_entry == rnp->exp_tasks)
rnp_root->exp_tasks = rnp->exp_tasks;
+#ifdef CONFIG_RCU_BOOST
+ if (&t->rcu_node_entry == rnp->boost_tasks)
+ rnp_root->boost_tasks = rnp->boost_tasks;
+#endif /* #ifdef CONFIG_RCU_BOOST */
raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
}
+
+#ifdef CONFIG_RCU_BOOST
+ /* In case root is being boosted and leaf is not. */
+ if (rnp_root->boost_tasks != NULL &&
+ rnp_root->boost_tasks != rnp_root->gp_tasks)
+ rnp_root->boost_tasks = rnp_root->gp_tasks;
+#endif /* #ifdef CONFIG_RCU_BOOST */
+
rnp->gp_tasks = NULL;
rnp->exp_tasks = NULL;
return retval;
@@ -684,6 +714,7 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
raw_spin_lock(&rnp->lock); /* irqs already disabled */
if (!list_empty(&rnp->blkd_tasks)) {
rnp->exp_tasks = rnp->blkd_tasks.next;
+ rcu_initiate_boost(rnp);
must_wait = 1;
}
raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
@@ -830,6 +861,8 @@ void exit_rcu(void)

#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */

+static struct rcu_state *rcu_state = &rcu_sched_state;
+
/*
* Tell them what RCU they are running.
*/
@@ -870,7 +903,7 @@ static void rcu_preempt_note_context_switch(int cpu)
* Because preemptable RCU does not exist, there are never any preempted
* RCU readers.
*/
-static int rcu_preempted_readers(struct rcu_node *rnp)
+static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return 0;
}
@@ -1034,6 +1067,230 @@ static void __init __rcu_init_preempt(void)

#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */

+#ifdef CONFIG_RCU_BOOST
+
+#include "rtmutex_common.h"
+
+/*
+ * Carry out RCU priority boosting on the task indicated by ->exp_tasks
+ * or ->boost_tasks, advancing the pointer to the next task in the
+ * ->blkd_tasks list.
+ *
+ * Note that irqs must be enabled: boosting the task can block.
+ * Returns 1 if there are more tasks needing to be boosted.
+ */
+static int rcu_boost(struct rcu_node *rnp)
+{
+ unsigned long flags;
+ struct rt_mutex mtx;
+ struct task_struct *t;
+ struct list_head *tb;
+
+ if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
+ return 0; /* Nothing left to boost. */
+
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+
+ /*
+ * Recheck under the lock: all tasks in need of boosting
+ * might exit their RCU read-side critical sections on their own.
+ */
+ if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ return 0;
+ }
+
+ /*
+ * Preferentially boost tasks blocking expedited grace periods.
+ * This cannot starve the normal grace periods because a second
+ * expedited grace period must boost all blocked tasks, including
+ * those blocking the pre-existing normal grace period.
+ */
+ if (rnp->exp_tasks != NULL)
+ tb = rnp->exp_tasks;
+ else
+ tb = rnp->boost_tasks;
+
+ /*
+ * We boost task t by manufacturing an rt_mutex that appears to
+ * be held by task t. We leave a pointer to that rt_mutex where
+ * task t can find it, and task t will release the mutex when it
+ * exits its outermost RCU read-side critical section. Then
+ * simply acquiring this artificial rt_mutex will boost task
+ * t's priority. (Thanks to tglx for suggesting this approach!)
+ *
+ * Note that task t must acquire rnp->lock to remove itself from
+ * the ->blkd_tasks list, which it will do from exit() if from
+ * nowhere else. We therefore are guaranteed that task t will
+ * stay around at least until we drop rnp->lock. Note that
+ * rnp->lock also resolves races between our priority boosting
+ * and task t's exiting its outermost RCU read-side critical
+ * section.
+ */
+ t = container_of(tb, struct task_struct, rcu_node_entry);
+ rt_mutex_init_proxy_locked(&mtx, t);
+ t->rcu_boost_mutex = &mtx;
+ t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
+ rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
+
+ return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
+}
+
+/*
+ * Timer handler to initiate waking up of boost kthreads that
+ * have yielded the CPU due to excessive numbers of tasks to
+ * boost. We wake up the per-rcu_node kthread, which in turn
+ * will wake up the booster kthread.
+ */
+static void rcu_boost_kthread_timer(unsigned long arg)
+{
+ unsigned long flags;
+ struct rcu_node *rnp = (struct rcu_node *)arg;
+
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+ invoke_rcu_node_kthread(rnp);
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+}
+
+/*
+ * Priority-boosting kthread. One per leaf rcu_node and one for the
+ * root rcu_node.
+ */
+static int rcu_boost_kthread(void *arg)
+{
+ struct rcu_node *rnp = (struct rcu_node *)arg;
+ int spincnt = 0;
+ int more2boost;
+
+ for (;;) {
+ wait_event_interruptible(rnp->boost_wq, rnp->boost_tasks ||
+ rnp->exp_tasks ||
+ kthread_should_stop());
+ if (kthread_should_stop())
+ break;
+ more2boost = rcu_boost(rnp);
+ if (more2boost)
+ spincnt++;
+ else
+ spincnt = 0;
+ if (spincnt > 10) {
+ rcu_yield(rcu_boost_kthread_timer, (unsigned long)rnp);
+ spincnt = 0;
+ }
+ }
+ return 0;
+}
+
+/*
+ * Check to see if it is time to start boosting RCU readers that are
+ * blocking the current grace period, and, if so, tell the per-rcu_node
+ * kthread to start boosting them. If there is an expedited grace
+ * period in progress, it is always time to boost.
+ *
+ * The caller must hold rnp->lock.
+ */
+static void rcu_initiate_boost(struct rcu_node *rnp)
+{
+ struct task_struct *t;
+
+ if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
+ rnp->n_balk_exp_gp_tasks++;
+ return;
+ }
+ if (rnp->exp_tasks != NULL ||
+ (rnp->gp_tasks != NULL &&
+ rnp->boost_tasks == NULL &&
+ rnp->qsmask == 0 &&
+ ULONG_CMP_GE(jiffies, rnp->boost_time))) {
+ if (rnp->exp_tasks == NULL)
+ rnp->boost_tasks = rnp->gp_tasks;
+ t = rnp->boost_kthread_task;
+ if (t != NULL)
+ wake_up_process(t);
+ }
+}
+
+static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
+ cpumask_var_t cm)
+{
+ set_cpus_allowed_ptr(rnp->boost_kthread_task, cm);
+}
+
+#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
+
+/*
+ * Do priority-boost accounting for the start of a new grace period.
+ */
+static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
+{
+ rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
+}
+
+/*
+ * Initialize the RCU-boost waitqueue.
+ */
+static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp)
+{
+ init_waitqueue_head(&rnp->boost_wq);
+}
+
+/*
+ * Create an RCU-boost kthread for the specified node if one does not
+ * already exist. We only create this kthread for preemptible RCU.
+ * Returns zero if all is well, a negated errno otherwise.
+ */
+static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
+ struct rcu_node *rnp,
+ int rnp_index)
+{
+ struct sched_param sp;
+ struct task_struct *t;
+
+ if (&rcu_preempt_state != rsp)
+ return 0;
+ if (rnp->boost_kthread_task != NULL)
+ return 0;
+ t = kthread_create(rcu_boost_kthread, (void *)rnp,
+ "rcub%d", rnp_index);
+ if (IS_ERR(t))
+ return PTR_ERR(t);
+ rnp->boost_kthread_task = t;
+ wake_up_process(t);
+ sp.sched_priority = RCU_KTHREAD_PRIO;
+ sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+ return 0;
+}
+
+#else /* #ifdef CONFIG_RCU_BOOST */
+
+static void rcu_initiate_boost(struct rcu_node *rnp)
+{
+}
+
+static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
+ cpumask_var_t cm)
+{
+}
+
+static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
+{
+}
+
+static void __init rcu_init_boost_waitqueue(struct rcu_node *rnp)
+{
+}
+
+static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
+ struct rcu_node *rnp,
+ int rnp_index)
+{
+ return 0;
+}
+
+#endif /* #else #ifdef CONFIG_RCU_BOOST */
+
#ifndef CONFIG_SMP

void synchronize_sched_expedited(void)
@@ -1206,8 +1463,8 @@ static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
*
* Because it is not legal to invoke rcu_process_callbacks() with irqs
* disabled, we do one pass of force_quiescent_state(), then do a
- * invoke_rcu_kthread() to cause rcu_process_callbacks() to be invoked later.
- * The per-cpu rcu_dyntick_drain variable controls the sequencing.
+ * invoke_rcu_cpu_kthread() to cause rcu_process_callbacks() to be invoked
+ * later. The per-cpu rcu_dyntick_drain variable controls the sequencing.
*/
int rcu_needs_cpu(int cpu)
{
@@ -1257,7 +1514,7 @@ int rcu_needs_cpu(int cpu)

/* If RCU callbacks are still pending, RCU still needs this CPU. */
if (c)
- invoke_rcu_kthread();
+ invoke_rcu_cpu_kthread();
return c;
}

--
1.7.3.2

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