rcu: Don't offload callbacks unless specifically requested

Enabling NO_HZ_FULL currently has the side effect of enabling callback
offloading on all CPUs.  This results in lots of additional rcuo kthreads,
and can also increase context switching and wakeups, even in cases where
callback offloading is neither needed nor particularly desirable.  This
commit therefore enables callback offloading on a given CPU only if
specifically requested at build time or boot time, or if that CPU has
been specifically designated (again, either at build time or boot time)
as a nohz_full CPU.

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

2 files changed, 5 insertions, 2 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 9d76b99af1b9..9332d33346ac 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -737,7 +737,7 @@ choice
 
 config RCU_NOCB_CPU_NONE
         bool "No build_forced no-CBs CPUs"
-        depends on RCU_NOCB_CPU && !NO_HZ_FULL
+        depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
         help
           This option does not force any of the CPUs to be no-CBs CPUs.
           Only CPUs designated by the rcu_nocbs= boot parameter will be
@@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE
 
 config RCU_NOCB_CPU_ZERO
         bool "CPU 0 is a build_forced no-CBs CPU"
-        depends on RCU_NOCB_CPU && !NO_HZ_FULL
+        depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
         help
           This option forces CPU 0 to be a no-CBs CPU, so that its RCU
           callbacks are invoked by a per-CPU kthread whose name begins
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b27b86c7bbfa..17eed0856b03 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2473,6 +2473,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
 
         if (rcu_nocb_mask == NULL)
                 return;
+#ifdef CONFIG_NO_HZ_FULL
+        cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
+#endif /* #ifdef CONFIG_NO_HZ_FULL */
         if (ls == -1) {
                 ls = int_sqrt(nr_cpu_ids);
                 rcu_nocb_leader_stride = ls;