rcu: Update cpu_needs_another_gp() for futures from non-NOCB CPUs

In the old days, the only source of requests for future grace periods
was NOCB CPUs.  This has changed: CPUs routinely post requests for
future grace periods in order to promote power efficiency and reduce
OS jitter with minimal impact on grace-period latency.  This commit
therefore updates cpu_needs_another_gp() to invoke rcu_future_needs_gp()
instead of rcu_nocb_needs_gp().  The latter is no longer used, so is
now removed.  This commit also adds tracing for the irq_work_queue()
wakeup case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -323,6 +323,28 @@ cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
 	       rdp->nxttail[RCU_DONE_TAIL] != NULL;
 }
 
+/*
+ * Return the root node of the specified rcu_state structure.
+ */
+static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+{
+	return &rsp->node[0];
+}
+
+/*
+ * Is there any need for future grace periods?
+ * 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_future_needs_gp(struct rcu_state *rsp)
+{
+	struct rcu_node *rnp = rcu_get_root(rsp);
+	int idx = (ACCESS_ONCE(rnp->completed) + 1) & 0x1;
+	int *fp = &rnp->need_future_gp[idx];
+
+	return ACCESS_ONCE(*fp);
+}
+
 /*
  * Does the current CPU require a not-yet-started grace period?
  * The caller must have disabled interrupts to prevent races with
@@ -335,7 +357,7 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
 
 	if (rcu_gp_in_progress(rsp))
 		return 0;  /* No, a grace period is already in progress. */
-	if (rcu_nocb_needs_gp(rsp))
+	if (rcu_future_needs_gp(rsp))
 		return 1;  /* Yes, a no-CBs CPU needs one. */
 	if (!rdp->nxttail[RCU_NEXT_TAIL])
 		return 0;  /* No, this is a no-CBs (or offline) CPU. */
@@ -349,14 +371,6 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
 	return 0; /* No grace period needed. */
 }
 
-/*
- * Return the root node of the specified rcu_state structure.
- */
-static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
-{
-	return &rsp->node[0];
-}
-
 /*
  * rcu_eqs_enter_common - current CPU is moving towards extended quiescent state
  *
@@ -1672,6 +1686,8 @@ static void rsp_wakeup(struct irq_work *work)
 
 	/* Wake up rcu_gp_kthread() to start the grace period. */
 	wake_up(&rsp->gp_wq);
+	trace_rcu_grace_period(rsp->name, ACCESS_ONCE(rsp->gpnum),
+			       "Workqueuewoken");
 }
 
 /*
@@ -1706,8 +1722,11 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 	 * the wakeup to interrupt context.  And don't bother waking
 	 * up the running kthread.
 	 */
-	if (current != rsp->gp_kthread)
+	if (current != rsp->gp_kthread) {
+		trace_rcu_grace_period(rsp->name, ACCESS_ONCE(rsp->gpnum),
+				       "Workqueuewake");
 		irq_work_queue(&rsp->wakeup_work);
+	}
 }
 
 /*

kernel/rcu/tree.h

@@ -547,7 +547,6 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
 static void print_cpu_stall_info_end(void);
 static void zero_cpu_stall_ticks(struct rcu_data *rdp);
 static void increment_cpu_stall_ticks(void);
-static int rcu_nocb_needs_gp(struct rcu_state *rsp);
 static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
 static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
 static void rcu_init_one_nocb(struct rcu_node *rnp);

kernel/rcu/tree_plugin.h

@@ -2067,19 +2067,6 @@ static int __init parse_rcu_nocb_poll(char *arg)
 }
 early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
 
-/*
- * 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->need_future_gp[(ACCESS_ONCE(rnp->completed) + 1) & 0x1];
-}
-
 /*
  * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
  * grace period.
@@ -2402,11 +2389,6 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
 
 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
 
-static int rcu_nocb_needs_gp(struct rcu_state *rsp)
-{
-	return 0;
-}
-
 static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
 }