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>

1 files changed, 97 insertions, 66 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 764b5fcc7c56..774f010a4619 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -130,12 +130,12 @@ static void rcu_preempt_qs(int cpu)
  * 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 @@ static void rcu_preempt_note_context_switch(int cpu)
 {
         struct task_struct *t = current;
         unsigned long flags;
-        int phase;
         struct rcu_data *rdp;
         struct rcu_node *rnp;
 
@@ -165,15 +164,26 @@ static void rcu_preempt_note_context_switch(int cpu)
                  * (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 @@ EXPORT_SYMBOL_GPL(__rcu_read_lock);
  */
 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 @@ static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
 }
 
 /*
+ * 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 @@ static void rcu_read_unlock_special(struct task_struct *t)
         int empty;
         int empty_exp;
         unsigned long flags;
+        struct list_head *np;
         struct rcu_node *rnp;
         int special;
 
@@ -305,7 +328,12 @@ static void rcu_read_unlock_special(struct task_struct *t)
                 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 @@ EXPORT_SYMBOL_GPL(__rcu_read_unlock);
 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_detail_task_stall(struct rcu_state *rsp)
  */
 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 @@ static void rcu_preempt_stall_reset(void)
  * 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 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
                                      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 DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
  */
 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 rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
 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 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
 /*
  * 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 @@ void synchronize_rcu_expedited(void)
         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 @@ void synchronize_rcu_expedited(void)
                 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 @@ void synchronize_rcu_expedited(void)
 
         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));