CONFIG_RCU_BOOST -- one of PREEMPT_RCU.
CONFIG_RCU_FANOUT -- Cover hierarchy, but overlap with others.
CONFIG_RCU_FANOUT_LEAF -- Do one non-default.
-CONFIG_RCU_FAST_NO_HZ -- Do one, but not with CONFIG_RCU_NOCB_CPU_ALL.
-CONFIG_RCU_NOCB_CPU -- Do three, see below.
-CONFIG_RCU_NOCB_CPU_ALL -- Do one.
-CONFIG_RCU_NOCB_CPU_NONE -- Do one.
-CONFIG_RCU_NOCB_CPU_ZERO -- Do one.
+CONFIG_RCU_FAST_NO_HZ -- Do one, but not with all nohz_full CPUs.
+CONFIG_RCU_NOCB_CPU -- Do three, one with no rcu_nocbs CPUs, one with
+ rcu_nocbs=0, and one with all rcu_nocbs CPUs.
CONFIG_RCU_TRACE -- Do half.
CONFIG_SMP -- Need one !SMP for PREEMPT_RCU.
CONFIG_RCU_EXPERT=n -- Do a few, but these have to be vanilla configurations.