rcu: Introduce proper blocking to no-CBs kthreads GP waits

Currently, the no-CBs kthreads do repeated timed waits for grace periods
to elapse.  This is crude and energy inefficient, so this commit allows
no-CBs kthreads to specify exactly which grace period they are waiting
for and also allows them to block for the entire duration until the
desired grace period completes.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 112 insertions, 17 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 3e33aefce0ea..90a191452550 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2176,11 +2176,51 @@ static int __init parse_rcu_nocb_poll(char *arg)
 early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
 
 /*
- * Does this CPU needs a grace period due to offloaded callbacks?
+ * Do any no-CBs CPUs need another grace period?
+ *
+ * Interrupts must be disabled.  If the caller does not hold the root
+ * rnp_node structure's ->lock, the results are advisory only.
+ */
+static int rcu_nocb_needs_gp(struct rcu_state *rsp)
+{
+        struct rcu_node *rnp = rcu_get_root(rsp);
+
+        return rnp->n_nocb_gp_requests[(ACCESS_ONCE(rnp->completed) + 1) & 0x1];
+}
+
+/*
+ * Clean up this rcu_node structure's no-CBs state at the end of
+ * a grace period, and also return whether any no-CBs CPU associated
+ * with this rcu_node structure needs another grace period.
+ */
+static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+{
+        int c = rnp->completed;
+        int needmore;
+
+        wake_up_all(&rnp->nocb_gp_wq[c & 0x1]);
+        rnp->n_nocb_gp_requests[c & 0x1] = 0;
+        needmore = rnp->n_nocb_gp_requests[(c + 1) & 0x1];
+        return needmore;
+}
+
+/*
+ * Set the root rcu_node structure's ->n_nocb_gp_requests field
+ * based on the sum of those of all rcu_node structures.  This does
+ * double-count the root rcu_node structure's requests, but this
+ * is necessary to handle the possibility of a rcu_nocb_kthread()
+ * having awakened during the time that the rcu_node structures
+ * were being updated for the end of the previous grace period.
  */
-static int rcu_nocb_needs_gp(struct rcu_data *rdp)
+static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
+{
+        rnp->n_nocb_gp_requests[(rnp->completed + 1) & 0x1] += nrq;
+}
+
+static void rcu_init_one_nocb(struct rcu_node *rnp)
 {
-        return rdp->nocb_needs_gp;
+        init_waitqueue_head(&rnp->nocb_gp_wq[0]);
+        init_waitqueue_head(&rnp->nocb_gp_wq[1]);
 }
 
 /* Is the specified CPU a no-CPUs CPU? */
@@ -2289,31 +2329,73 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
 static void rcu_nocb_wait_gp(struct rcu_data *rdp)
 {
         unsigned long c;
+        bool d;
         unsigned long flags;
-        unsigned long j;
+        unsigned long flags1;
         struct rcu_node *rnp = rdp->mynode;
+        struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
 
         raw_spin_lock_irqsave(&rnp->lock, flags);
         c = rnp->completed + 2;
-        rdp->nocb_needs_gp = true;
-        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+
+        /* Count our request for a grace period. */
+        rnp->n_nocb_gp_requests[c & 0x1]++;
+
+        if (rnp->gpnum != rnp->completed) {
+
+                /*
+                 * This rcu_node structure believes that a grace period
+                 * is in progress, so we are done.  When this grace
+                 * period ends, our request will be acted upon.
+                 */
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
+
+        } else {
+
+                /*
+                 * Might not be a grace period, check root rcu_node
+                 * structure to see if we must start one.
+                 */
+                if (rnp != rnp_root)
+                        raw_spin_lock(&rnp_root->lock); /* irqs disabled. */
+                if (rnp_root->gpnum != rnp_root->completed) {
+                        raw_spin_unlock(&rnp_root->lock); /* irqs disabled. */
+                } else {
+
+                        /*
+                         * No grace period, so we need to start one.
+                         * The good news is that we can wait for exactly
+                         * one grace period instead of part of the current
+                         * grace period and all of the next grace period.
+                         * Adjust counters accordingly and start the
+                         * needed grace period.
+                         */
+                        rnp->n_nocb_gp_requests[c & 0x1]--;
+                        c = rnp_root->completed + 1;
+                        rnp->n_nocb_gp_requests[c & 0x1]++;
+                        rnp_root->n_nocb_gp_requests[c & 0x1]++;
+                        local_save_flags(flags1);
+                        rcu_start_gp(rdp->rsp, flags1); /* Rlses ->lock. */
+                }
+
+                /* Clean up locking and irq state. */
+                if (rnp != rnp_root)
+                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+                else
+                        local_irq_restore(flags);
+        }
 
         /*
          * Wait for the grace period.  Do so interruptibly to avoid messing
          * up the load average.
          */
         for (;;) {
-                j = jiffies;
-                schedule_timeout_interruptible(2);
-                raw_spin_lock_irqsave(&rnp->lock, flags);
-                if (ULONG_CMP_GE(rnp->completed, c)) {
-                        rdp->nocb_needs_gp = false;
-                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+                wait_event_interruptible(
+                        rnp->nocb_gp_wq[c & 0x1],
+                        (d = ULONG_CMP_GE(ACCESS_ONCE(rnp->completed), c)));
+                if (likely(d))
                         break;
-                }
-                if (j == jiffies)
-                        flush_signals(current);
-                raw_spin_unlock_irqrestore(&rnp->lock, flags);
+                flush_signals(current);
         }
         smp_mb(); /* Ensure that CB invocation happens after GP end. */
 }
@@ -2416,11 +2498,24 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 
 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
 
-static int rcu_nocb_needs_gp(struct rcu_data *rdp)
+static int rcu_nocb_needs_gp(struct rcu_state *rsp)
+{
+        return 0;
+}
+
+static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
         return 0;
 }
 
+static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
+{
+}
+
+static void rcu_init_one_nocb(struct rcu_node *rnp)
+{
+}
+
 static bool is_nocb_cpu(int cpu)
 {
         return false;