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Subject[PATCH tip/core/rcu] Create rcutree plugins to handle hotplug CPU for multi-level trees.
When offlining CPUs from a multi-level tree, there is the possibility
of offlining the last CPU from a given node when there are preempted
RCU read-side critical sections that started life on one of the CPUs on
that node. In this case, the corresponding tasks will be enqueued via
the task_struct's rcu_node_entry list_head onto one of the rcu_node's
blocked_tasks[] lists. These tasks need to be moved somewhere else
so that they will prevent the current grace period from ending.
That somewhere is the root rcu_node.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/init_task.h | 2 -
include/linux/sched.h | 4 +-
kernel/rcutree.c | 2 +
kernel/rcutree_plugin.h | 69 ++++++++++++++++++++++++++++++++++++++++++----
4 files changed, 69 insertions(+), 8 deletions(-)
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 79d4bae..9e7f2e8 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -98,7 +98,7 @@ extern struct group_info init_groups;
#define INIT_TASK_RCU_PREEMPT(tsk) \
.rcu_read_lock_nesting = 0, \
.rcu_read_unlock_special = 0, \
- .rcu_blocked_cpu = -1, \
+ .rcu_blocked_node = NULL, \
.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),
#else
#define INIT_TASK_RCU_PREEMPT(tsk)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bfca26d..3fe0315 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1208,7 +1208,7 @@ struct task_struct {
#ifdef CONFIG_TREE_PREEMPT_RCU
int rcu_read_lock_nesting;
char rcu_read_unlock_special;
- int rcu_blocked_cpu;
+ void *rcu_blocked_node;
struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */

@@ -1735,7 +1735,7 @@ static inline void rcu_copy_process(struct task_struct *p)
{
p->rcu_read_lock_nesting = 0;
p->rcu_read_unlock_special = 0;
- p->rcu_blocked_cpu = -1;
+ p->rcu_blocked_node = NULL;
INIT_LIST_HEAD(&p->rcu_node_entry);
}

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index fee6316..d903e2f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -81,6 +81,7 @@ struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);

extern long rcu_batches_completed_sched(void);
+static struct rcu_node *rcu_get_root(struct rcu_state *rsp);
static void cpu_quiet_msk(unsigned long mask, struct rcu_state *rsp,
struct rcu_node *rnp, unsigned long flags);
static void cpu_quiet_msk_finish(struct rcu_state *rsp, unsigned long flags);
@@ -876,6 +877,7 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
spin_unlock(&rnp->lock); /* irqs remain disabled. */
break;
}
+ rcu_preempt_offline_tasks(rsp, rnp);
mask = rnp->grpmask;
spin_unlock(&rnp->lock); /* irqs remain disabled. */
rnp = rnp->parent;
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 201334c..04343be 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -92,7 +92,7 @@ static void rcu_preempt_qs(int cpu)
rnp = rdp->mynode;
spin_lock(&rnp->lock);
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
- t->rcu_blocked_cpu = cpu;
+ t->rcu_blocked_node = (void *)rnp;

/*
* If this CPU has already checked in, then this task
@@ -170,12 +170,21 @@ static void rcu_read_unlock_special(struct task_struct *t)
if (special & RCU_READ_UNLOCK_BLOCKED) {
t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;

- /* Remove this task from the list it blocked on. */
- rnp = rcu_preempt_state.rda[t->rcu_blocked_cpu]->mynode;
- spin_lock(&rnp->lock);
+ /*
+ * Remove this task from the list it blocked on. The
+ * task can migrate while we acquire the lock, but at
+ * most one time. So at most two passes through loop.
+ */
+ for (;;) {
+ rnp = (struct rcu_node *)t->rcu_blocked_node;
+ spin_lock(&rnp->lock);
+ if (rnp == (struct rcu_node *)t->rcu_blocked_node)
+ break;
+ spin_unlock(&rnp->lock);
+ }
empty = list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
list_del_init(&t->rcu_node_entry);
- t->rcu_blocked_cpu = -1;
+ t->rcu_blocked_node = NULL;

/*
* If this was the last task on the current list, and if
@@ -262,6 +271,47 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
#ifdef CONFIG_HOTPLUG_CPU

/*
+ * Handle tasklist migration for case in which all CPUs covered by the
+ * specified rcu_node have gone offline. Move them up to the root
+ * rcu_node. The reason for not just moving them to the immediate
+ * parent is to remove the need for rcu_read_unlock_special() to
+ * make more than two attempts to acquire the target rcu_node's lock.
+ *
+ * The caller must hold rnp->lock with irqs disabled.
+ */
+static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
+ struct rcu_node *rnp)
+{
+ int i;
+ struct list_head *lp;
+ struct list_head *lp_root;
+ struct rcu_node *rnp_root = rcu_get_root(rsp);
+ struct task_struct *tp;
+
+ if (rnp == rnp_root)
+ return; /* Shouldn't happen: at least one CPU online. */
+
+ /*
+ * Move tasks up to root rcu_node. Rely on the fact that the
+ * root rcu_node can be at most one ahead of the rest of the
+ * rcu_nodes in terms of gp_num value. This fact allows us to
+ * move the blocked_tasks[] array directly, element by element.
+ */
+ for (i = 0; i < 2; i++) {
+ lp = &rnp->blocked_tasks[i];
+ lp_root = &rnp_root->blocked_tasks[i];
+ while (!list_empty(lp)) {
+ tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
+ spin_lock(&rnp_root->lock); /* irqs already disabled */
+ list_del(&tp->rcu_node_entry);
+ tp->rcu_blocked_node = rnp_root;
+ list_add(&tp->rcu_node_entry, lp_root);
+ spin_unlock(&rnp_root->lock); /* irqs remain disabled */
+ }
+ }
+}
+
+/*
* Do CPU-offline processing for preemptable RCU.
*/
static void rcu_preempt_offline_cpu(int cpu)
@@ -410,6 +460,15 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
#ifdef CONFIG_HOTPLUG_CPU

/*
+ * Because preemptable RCU does not exist, it never needs to migrate
+ * tasks that were blocked within RCU read-side critical sections.
+ */
+static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
+ struct rcu_node *rnp)
+{
+}
+
+/*
* Because preemptable RCU does not exist, it never needs CPU-offline
* processing.
*/

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