[PATCH v3] rcutorture: Create nocb tasks only for CONFIG_RCU_NOCB_CPU=y kernels

[Zqiang]

When setting nocbs_nthreads to start rcutorture test with a non-zero value,
the nocb tasks will be created and invoke rcu_nocb_cpu_offload/deoffload()
to toggle CPU's callback-offload state, but for CONFIG_RCU_NOCB_CPU=n
kernel, the rcu_nocb_cpu_offload/deoffload() is a no-op and this is also
meaningless for torture_type is non-rcu.

This commit therefore add member can_nocbs_toggle to rcu_torture_ops
structure to avoid unnecessary nocb tasks creation.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/rcutorture.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 297da28ce92d..d01127e0e8cd 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -3708,6 +3708,12 @@ rcu_torture_init(void)
 		pr_alert("rcu-torture: ->fqs NULL and non-zero fqs_duration, fqs disabled.\n");
 		fqs_duration = 0;
 	}
+	if (nocbs_nthreads != 0 && (cur_ops != &rcu_ops ||
+					!IS_ENABLED(CONFIG_RCU_NOCB_CPU))) {
+		pr_alert("rcu-torture types: %s and CONFIG_RCU_NOCB_CPU=%d, nocb toggle disabled.\n",
+				cur_ops->name, IS_ENABLED(CONFIG_RCU_NOCB_CPU));
+		nocbs_nthreads = 0;
+	}
 	if (cur_ops->init)
 		cur_ops->init();
 
-- 
2.25.1