rcu: Don't track sysidle state if no nohz_full= CPUs

If there are no nohz_full= CPUs, then there is currently no reason to
track sysidle state.  This commit therefore short-circuits this state
tracking if !tick_nohz_full_enabled().

Note that these checks will need to be revisited if nohz_full= state
can ever be changed at runtime.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3ddad4fb11a8..d5aec549558d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2707,6 +2707,10 @@ static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
 {
         unsigned long j;
 
+        /* If there are no nohz_full= CPUs, no need to track this. */
+        if (!tick_nohz_full_enabled())
+                return;
+
         /* Adjust nesting, check for fully idle. */
         if (irq) {
                 rdtp->dynticks_idle_nesting--;
@@ -2772,6 +2776,10 @@ void rcu_sysidle_force_exit(void)
  */
 static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
 {
+        /* If there are no nohz_full= CPUs, no need to track this. */
+        if (!tick_nohz_full_enabled())
+                return;
+
         /* Adjust nesting, check for already non-idle. */
         if (irq) {
                 rdtp->dynticks_idle_nesting++;
@@ -2826,6 +2834,10 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
         unsigned long j;
         struct rcu_dynticks *rdtp = rdp->dynticks;
 
+        /* If there are no nohz_full= CPUs, don't check system-wide idleness. */
+        if (!tick_nohz_full_enabled())
+                return;
+
         /*
          * If some other CPU has already reported non-idle, if this is
          * not the flavor of RCU that tracks sysidle state, or if this
@@ -2952,6 +2964,10 @@ static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
 static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
                                   unsigned long maxj)
 {
+        /* If there are no nohz_full= CPUs, no need to track this. */
+        if (!tick_nohz_full_enabled())
+                return;
+
         rcu_sysidle_report(rsp, isidle, maxj, true);
 }
 
@@ -2978,7 +2994,8 @@ static void rcu_sysidle_cb(struct rcu_head *rhp)
 
 /*
  * Check to see if the system is fully idle, other than the timekeeping CPU.
- * The caller must have disabled interrupts.
+ * The caller must have disabled interrupts.  This is not intended to be
+ * called unless tick_nohz_full_enabled().
  */
 bool rcu_sys_is_idle(void)
 {