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>

1 files changed, 29 insertions, 10 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b33c29a99df3..b4688993e956 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -324,6 +324,28 @@ cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
 }
 
 /*
+ * 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
  * normal callback registry.
@@ -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. */
@@ -350,14 +372,6 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
 }
 
 /*
- * 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
  *
  * If the new value of the ->dynticks_nesting counter now is zero,
@@ -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);
+        }
 }
 
 /*