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>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 3e33aef..90a1914 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2176,11 +2176,51 @@
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_data *rdp)
+static int rcu_nocb_needs_gp(struct rcu_state *rsp)
{
- return rdp->nocb_needs_gp;
+ 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 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)
+{
+ 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 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 @@
#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;