rcu: merge TREE_PREEPT_RCU blocked_tasks[] lists
Combine the current TREE_PREEMPT_RCU ->blocked_tasks[] lists in the
rcu_node structure into a single ->blkd_tasks list with ->gp_tasks
and ->exp_tasks tail pointers. This is in preparation for RCU priority
boosting, which will add a third dimension to the combinatorial explosion
in the ->blocked_tasks[] case, but simply a third pointer in the new
->blkd_tasks case.
Also update documentation to reflect blocked_tasks[] merge
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 764b5fc..774f010 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -130,12 +130,12 @@
* We have entered the scheduler, and the current task might soon be
* context-switched away from. If this task is in an RCU read-side
* critical section, we will no longer be able to rely on the CPU to
- * record that fact, so we enqueue the task on the appropriate entry
- * of the blocked_tasks[] array. The task will dequeue itself when
- * it exits the outermost enclosing RCU read-side critical section.
- * Therefore, the current grace period cannot be permitted to complete
- * until the blocked_tasks[] entry indexed by the low-order bit of
- * rnp->gpnum empties.
+ * record that fact, so we enqueue the task on the blkd_tasks list.
+ * The task will dequeue itself when it exits the outermost enclosing
+ * RCU read-side critical section. Therefore, the current grace period
+ * cannot be permitted to complete until the blkd_tasks list entries
+ * predating the current grace period drain, in other words, until
+ * rnp->gp_tasks becomes NULL.
*
* Caller must disable preemption.
*/
@@ -143,7 +143,6 @@
{
struct task_struct *t = current;
unsigned long flags;
- int phase;
struct rcu_data *rdp;
struct rcu_node *rnp;
@@ -165,15 +164,26 @@
* (i.e., this CPU has not yet passed through a quiescent
* state for the current grace period), then as long
* as that task remains queued, the current grace period
- * cannot end.
+ * cannot end. Note that there is some uncertainty as
+ * to exactly when the current grace period started.
+ * We take a conservative approach, which can result
+ * in unnecessarily waiting on tasks that started very
+ * slightly after the current grace period began. C'est
+ * la vie!!!
*
* But first, note that the current CPU must still be
* on line!
*/
WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
- phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
- list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
+ 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;
+ } else {
+ list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
+ if (rnp->qsmask & rdp->grpmask)
+ rnp->gp_tasks = &t->rcu_node_entry;
+ }
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
@@ -210,10 +220,7 @@
*/
static int rcu_preempted_readers(struct rcu_node *rnp)
{
- int phase = rnp->gpnum & 0x1;
-
- return !list_empty(&rnp->blocked_tasks[phase]) ||
- !list_empty(&rnp->blocked_tasks[phase + 2]);
+ return rnp->gp_tasks != NULL;
}
/*
@@ -253,6 +260,21 @@
}
/*
+ * Advance a ->blkd_tasks-list pointer to the next entry, instead
+ * returning NULL if at the end of the list.
+ */
+static struct list_head *rcu_next_node_entry(struct task_struct *t,
+ struct rcu_node *rnp)
+{
+ struct list_head *np;
+
+ np = t->rcu_node_entry.next;
+ if (np == &rnp->blkd_tasks)
+ np = NULL;
+ return np;
+}
+
+/*
* Handle special cases during rcu_read_unlock(), such as needing to
* notify RCU core processing or task having blocked during the RCU
* read-side critical section.
@@ -262,6 +284,7 @@
int empty;
int empty_exp;
unsigned long flags;
+ struct list_head *np;
struct rcu_node *rnp;
int special;
@@ -305,7 +328,12 @@
empty = !rcu_preempted_readers(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);
list_del_init(&t->rcu_node_entry);
+ if (&t->rcu_node_entry == rnp->gp_tasks)
+ rnp->gp_tasks = np;
+ if (&t->rcu_node_entry == rnp->exp_tasks)
+ rnp->exp_tasks = np;
t->rcu_blocked_node = NULL;
/*
@@ -361,18 +389,16 @@
static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
unsigned long flags;
- struct list_head *lp;
- int phase;
struct task_struct *t;
- if (rcu_preempted_readers(rnp)) {
- raw_spin_lock_irqsave(&rnp->lock, flags);
- phase = rnp->gpnum & 0x1;
- lp = &rnp->blocked_tasks[phase];
- list_for_each_entry(t, lp, rcu_node_entry)
- sched_show_task(t);
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- }
+ if (!rcu_preempted_readers(rnp))
+ return;
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+ t = list_entry(rnp->gp_tasks,
+ struct task_struct, rcu_node_entry);
+ list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
+ sched_show_task(t);
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
/*
@@ -402,16 +428,14 @@
*/
static void rcu_print_task_stall(struct rcu_node *rnp)
{
- struct list_head *lp;
- int phase;
struct task_struct *t;
- if (rcu_preempted_readers(rnp)) {
- phase = rnp->gpnum & 0x1;
- lp = &rnp->blocked_tasks[phase];
- list_for_each_entry(t, lp, rcu_node_entry)
- printk(" P%d", t->pid);
- }
+ if (!rcu_preempted_readers(rnp))
+ return;
+ t = list_entry(rnp->gp_tasks,
+ struct task_struct, rcu_node_entry);
+ list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
+ printk(" P%d", t->pid);
}
/*
@@ -430,10 +454,15 @@
* period that still has RCU readers blocked! This function must be
* invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
* must be held by the caller.
+ *
+ * Also, if there are blocked tasks on the list, they automatically
+ * block the newly created grace period, so set up ->gp_tasks accordingly.
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
WARN_ON_ONCE(rcu_preempted_readers(rnp));
+ if (!list_empty(&rnp->blkd_tasks))
+ rnp->gp_tasks = rnp->blkd_tasks.next;
WARN_ON_ONCE(rnp->qsmask);
}
@@ -457,45 +486,49 @@
struct rcu_node *rnp,
struct rcu_data *rdp)
{
- int i;
struct list_head *lp;
struct list_head *lp_root;
int retval = 0;
struct rcu_node *rnp_root = rcu_get_root(rsp);
- struct task_struct *tp;
+ struct task_struct *t;
if (rnp == rnp_root) {
WARN_ONCE(1, "Last CPU thought to be offlined?");
return 0; /* Shouldn't happen: at least one CPU online. */
}
- WARN_ON_ONCE(rnp != rdp->mynode &&
- (!list_empty(&rnp->blocked_tasks[0]) ||
- !list_empty(&rnp->blocked_tasks[1]) ||
- !list_empty(&rnp->blocked_tasks[2]) ||
- !list_empty(&rnp->blocked_tasks[3])));
+
+ /* If we are on an internal node, complain bitterly. */
+ WARN_ON_ONCE(rnp != rdp->mynode);
/*
- * 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.
+ * Move tasks up to root rcu_node. Don't try to get fancy for
+ * this corner-case operation -- just put this node's tasks
+ * at the head of the root node's list, and update the root node's
+ * ->gp_tasks and ->exp_tasks pointers to those of this node's,
+ * if non-NULL. This might result in waiting for more tasks than
+ * absolutely necessary, but this is a good performance/complexity
+ * tradeoff.
*/
if (rcu_preempted_readers(rnp))
retval |= RCU_OFL_TASKS_NORM_GP;
if (rcu_preempted_readers_exp(rnp))
retval |= RCU_OFL_TASKS_EXP_GP;
- for (i = 0; i < 4; 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);
- raw_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);
- raw_spin_unlock(&rnp_root->lock); /* irqs remain disabled */
- }
+ lp = &rnp->blkd_tasks;
+ lp_root = &rnp_root->blkd_tasks;
+ while (!list_empty(lp)) {
+ t = list_entry(lp->next, typeof(*t), rcu_node_entry);
+ raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
+ list_del(&t->rcu_node_entry);
+ t->rcu_blocked_node = rnp_root;
+ list_add(&t->rcu_node_entry, lp_root);
+ if (&t->rcu_node_entry == rnp->gp_tasks)
+ rnp_root->gp_tasks = rnp->gp_tasks;
+ if (&t->rcu_node_entry == rnp->exp_tasks)
+ rnp_root->exp_tasks = rnp->exp_tasks;
+ raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
}
+ rnp->gp_tasks = NULL;
+ rnp->exp_tasks = NULL;
return retval;
}
@@ -586,8 +619,7 @@
*/
static int rcu_preempted_readers_exp(struct rcu_node *rnp)
{
- return !list_empty(&rnp->blocked_tasks[2]) ||
- !list_empty(&rnp->blocked_tasks[3]);
+ return rnp->exp_tasks != NULL;
}
/*
@@ -647,12 +679,13 @@
static void
sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
{
- int must_wait;
+ int must_wait = 0;
raw_spin_lock(&rnp->lock); /* irqs already disabled */
- list_splice_init(&rnp->blocked_tasks[0], &rnp->blocked_tasks[2]);
- list_splice_init(&rnp->blocked_tasks[1], &rnp->blocked_tasks[3]);
- must_wait = rcu_preempted_readers_exp(rnp);
+ if (!list_empty(&rnp->blkd_tasks)) {
+ rnp->exp_tasks = rnp->blkd_tasks.next;
+ must_wait = 1;
+ }
raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
if (!must_wait)
rcu_report_exp_rnp(rsp, rnp);
@@ -661,9 +694,7 @@
/*
* Wait for an rcu-preempt grace period, but expedite it. The basic idea
* is to invoke synchronize_sched_expedited() to push all the tasks to
- * the ->blocked_tasks[] lists, move all entries from the first set of
- * ->blocked_tasks[] lists to the second set, and finally wait for this
- * second set to drain.
+ * the ->blkd_tasks lists and wait for this list to drain.
*/
void synchronize_rcu_expedited(void)
{
@@ -695,7 +726,7 @@
if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
goto unlock_mb_ret; /* Others did our work for us. */
- /* force all RCU readers onto blocked_tasks[]. */
+ /* force all RCU readers onto ->blkd_tasks lists. */
synchronize_sched_expedited();
raw_spin_lock_irqsave(&rsp->onofflock, flags);
@@ -707,7 +738,7 @@
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
- /* Snapshot current state of ->blocked_tasks[] lists. */
+ /* Snapshot current state of ->blkd_tasks lists. */
rcu_for_each_leaf_node(rsp, rnp)
sync_rcu_preempt_exp_init(rsp, rnp);
if (NUM_RCU_NODES > 1)
@@ -715,7 +746,7 @@
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
- /* Wait for snapshotted ->blocked_tasks[] lists to drain. */
+ /* Wait for snapshotted ->blkd_tasks lists to drain. */
rnp = rcu_get_root(rsp);
wait_event(sync_rcu_preempt_exp_wq,
sync_rcu_preempt_exp_done(rnp));
