rcutorture: Allow rcutorture without RCU Tasks

Currently, a CONFIG_PREEMPT_NONE=y kernel substitutes normal RCU for
RCU Tasks.  Unless that kernel builds rcutorture, whether built-in or as
a module, in which case RCU Tasks is (unnecessarily) used.  This both
increases kernel size and increases the complexity of certain tracing
operations.  This commit therefore decouples the presence of rcutorture
from the presence of RCU Tasks.

However, there is a need to select CONFIG_TASKS_RCU for testing purposes.
Except that casual users must not be bothered with questions -- for them,
this needs to be fully automated.  There is thus a CONFIG_FORCE_TASKS_RCU
that selects CONFIG_TASKS_RCU, is user-selectable, but which depends
on CONFIG_RCU_EXPERT.

[ paulmck: Apply kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/Kconfig

@@ -77,13 +77,21 @@ config TASKS_RCU_GENERIC
 	  This option enables generic infrastructure code supporting
 	  task-based RCU implementations.  Not for manual selection.
 
+config FORCE_TASKS_RCU
+	bool "Force selection of TASKS_RCU"
+	depends on RCU_EXPERT
+	select TASKS_RCU
+	default n
+	help
+	  This option force-enables a task-based RCU implementation
+	  that uses only voluntary context switch (not preemption!),
+	  idle, and user-mode execution as quiescent states.  Not for
+	  manual selection in most cases.
+
 config TASKS_RCU
-	def_bool 0
+	bool
+	default n
 	select IRQ_WORK
-	help
-	  This option enables a task-based RCU implementation that uses
-	  only voluntary context switch (not preemption!), idle, and
-	  user-mode execution as quiescent states.  Not for manual selection.
 
 config TASKS_RUDE_RCU
 	def_bool 0

kernel/rcu/Kconfig.debug

@@ -47,7 +47,6 @@ config RCU_TORTURE_TEST
 	depends on DEBUG_KERNEL
 	select TORTURE_TEST
 	select SRCU
-	select TASKS_RCU
 	select TASKS_RUDE_RCU
 	default n
 	help

kernel/rcu/rcutorture.c

@@ -779,6 +779,8 @@ static struct rcu_torture_ops trivial_ops = {
 	.name		= "trivial"
 };
 
+#ifdef CONFIG_TASKS_RCU
+
 /*
  * Definitions for RCU-tasks torture testing.
  */
@@ -822,6 +824,15 @@ static struct rcu_torture_ops tasks_ops = {
 	.name		= "tasks"
 };
 
+#define TASKS_OPS &tasks_ops,
+
+#else // #ifdef CONFIG_TASKS_RCU
+
+#define TASKS_OPS
+
+#endif // #else #ifdef CONFIG_TASKS_RCU
+
+
 /*
  * Definitions for rude RCU-tasks torture testing.
  */
@@ -3108,7 +3119,7 @@ rcu_torture_init(void)
 	unsigned long gp_seq = 0;
 	static struct rcu_torture_ops *torture_ops[] = {
 		&rcu_ops, &rcu_busted_ops, &srcu_ops, &srcud_ops, &busted_srcud_ops,
-		&tasks_ops, &tasks_rude_ops, TASKS_TRACING_OPS
+		TASKS_OPS &tasks_rude_ops, TASKS_TRACING_OPS
 		&trivial_ops,
 	};
 

tools/testing/selftests/rcutorture/configs/rcu/TASKS01

@@ -7,4 +7,5 @@ CONFIG_PREEMPT=y
 CONFIG_DEBUG_LOCK_ALLOC=y
 CONFIG_PROVE_LOCKING=y
 #CHECK#CONFIG_PROVE_RCU=y
+CONFIG_TASKS_RCU=y
 CONFIG_RCU_EXPERT=y

tools/testing/selftests/rcutorture/configs/rcu/TASKS02

@@ -2,3 +2,6 @@ CONFIG_SMP=n
 CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
+#CHECK#CONFIG_TASKS_RCU=y
+CONFIG_FORCE_TASKS_RCU=y
+CONFIG_RCU_EXPERT=y

tools/testing/selftests/rcutorture/configs/rcu/TASKS03

@@ -7,3 +7,5 @@ CONFIG_HZ_PERIODIC=n
 CONFIG_NO_HZ_IDLE=n
 CONFIG_NO_HZ_FULL=y
 #CHECK#CONFIG_RCU_EXPERT=n
+CONFIG_TASKS_RCU=y
+CONFIG_RCU_EXPERT=y