rcu: Remove TINY_RCU bloat from pointless boot parameters

The rcu_expedited, rcu_normal, and rcu_normal_after_boot kernel boot
parameters are pointless in the case of TINY_RCU because in that case
synchronous grace periods, both expedited and normal, are no-ops.
However, these three symbols contribute several hundred bytes of bloat.
This commit therefore uses CPP directives to avoid compiling this code
in TINY_RCU kernels.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Documentation/kernel-parameters.txt

@@ -3308,20 +3308,23 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			of synchronize_rcu().  This reduces latency,
 			but can increase CPU utilization, degrade
 			real-time latency, and degrade energy efficiency.
+			No effect on CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_normal= [KNL]
 			Use only normal grace-period primitives,
 			for example, synchronize_rcu() instead of
 			synchronize_rcu_expedited().  This improves
-			real-time latency, CPU utilization, and energy
-			efficiency, but can expose users to increased
-			grace-period latency.  This parameter overrides
-			rcupdate.rcu_expedited.
+			real-time latency, CPU utilization, and
+			energy efficiency, but can expose users to
+			increased grace-period latency.  This parameter
+			overrides rcupdate.rcu_expedited.  No effect on
+			CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_normal_after_boot= [KNL]
 			Once boot has completed (that is, after
 			rcu_end_inkernel_boot() has been invoked), use
-			only normal grace-period primitives.
+			only normal grace-period primitives.  No effect
+			on CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning

include/linux/rcupdate.h

@@ -48,8 +48,10 @@
 
 #include <asm/barrier.h>
 
+#ifndef CONFIG_TINY_RCU
 extern int rcu_expedited; /* for sysctl */
 extern int rcu_normal;    /* also for sysctl */
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 #ifdef CONFIG_TINY_RCU
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
@@ -327,7 +329,6 @@ static inline int rcu_preempt_depth(void)
 
 /* Internal to kernel */
 void rcu_init(void);
-void rcu_end_inkernel_boot(void);
 void rcu_sched_qs(void);
 void rcu_bh_qs(void);
 void rcu_check_callbacks(int user);
@@ -335,6 +336,12 @@ struct notifier_block;
 int rcu_cpu_notify(struct notifier_block *self,
 		   unsigned long action, void *hcpu);
 
+#ifndef CONFIG_TINY_RCU
+void rcu_end_inkernel_boot(void);
+#else /* #ifndef CONFIG_TINY_RCU */
+static inline void rcu_end_inkernel_boot(void) { }
+#endif /* #ifndef CONFIG_TINY_RCU */
+
 #ifdef CONFIG_RCU_STALL_COMMON
 void rcu_sysrq_start(void);
 void rcu_sysrq_end(void);

kernel/ksysfs.c

@@ -144,6 +144,7 @@ static ssize_t fscaps_show(struct kobject *kobj,
 }
 KERNEL_ATTR_RO(fscaps);
 
+#ifndef CONFIG_TINY_RCU
 int rcu_expedited;
 static ssize_t rcu_expedited_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
@@ -177,6 +178,7 @@ static ssize_t rcu_normal_store(struct kobject *kobj,
 	return count;
 }
 KERNEL_ATTR_RW(rcu_normal);
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
@@ -219,8 +221,10 @@ static struct attribute * kernel_attrs[] = {
 	&kexec_crash_size_attr.attr,
 	&vmcoreinfo_attr.attr,
 #endif
+#ifndef CONFIG_TINY_RCU
 	&rcu_expedited_attr.attr,
 	&rcu_normal_attr.attr,
+#endif
 	NULL
 };
 

kernel/rcu/update.c

@@ -60,11 +60,12 @@ MODULE_ALIAS("rcupdate");
 #endif
 #define MODULE_PARAM_PREFIX "rcupdate."
 
+#ifndef CONFIG_TINY_RCU
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
-
 static int rcu_normal_after_boot;
 module_param(rcu_normal_after_boot, int, 0);
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
@@ -172,8 +173,6 @@ void rcu_unexpedite_gp(void)
 }
 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
-#endif /* #ifndef CONFIG_TINY_RCU */
-
 /*
  * Inform RCU of the end of the in-kernel boot sequence.
  */
@@ -185,6 +184,8 @@ void rcu_end_inkernel_boot(void)
 		WRITE_ONCE(rcu_normal, 1);
 }
 
+#endif /* #ifndef CONFIG_TINY_RCU */
+
 #ifdef CONFIG_PREEMPT_RCU
 
 /*