rcu: Make TINY_RCU also use softirq for RCU_BOOST=n

This patch #ifdefs TINY_RCU kthreads out of the kernel unless RCU_BOOST=y,
thus eliminating context-switch overhead if RCU priority boosting has
not been configured.

Signed-off-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

include/linux/rcutiny.h

···
 
 #include <linux/cache.h>
 
+#ifdef CONFIG_RCU_BOOST
 static inline void rcu_init(void)
 {
 }
+#else /* #ifdef CONFIG_RCU_BOOST */
+void rcu_init(void);
+#endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 static inline void rcu_barrier_bh(void)
 {

kernel/rcutiny.c

···
 
 #include "rcu.h"
 
-/* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
-static struct task_struct *rcu_kthread_task;
-static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
-static unsigned long have_rcu_kthread_work;
-
 /* Forward declarations for rcutiny_plugin.h. */
 struct rcu_ctrlblk;
-static void invoke_rcu_kthread(void);
-static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
-static int rcu_kthread(void *arg);
+static void invoke_rcu_callbacks(void);
+static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp);
+static void rcu_process_callbacks(struct softirq_action *unused);
 static void __call_rcu(struct rcu_head *head,
 		       void (*func)(struct rcu_head *rcu),
 		       struct rcu_ctrlblk *rcp);
···
 }
 
 /*
- * Wake up rcu_kthread() to process callbacks now eligible for invocation
- * or to boost readers.
- */
-static void invoke_rcu_kthread(void)
-{
-	have_rcu_kthread_work = 1;
-	wake_up(&rcu_kthread_wq);
-}
-
-/*
  * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
  * are at it, given that any rcu quiescent state is also an rcu_bh
  * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
···
 	local_irq_save(flags);
 	if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
 	    rcu_qsctr_help(&rcu_bh_ctrlblk))
-		invoke_rcu_kthread();
+		invoke_rcu_callbacks();
 	local_irq_restore(flags);
 }
 
···
 
 	local_irq_save(flags);
 	if (rcu_qsctr_help(&rcu_bh_ctrlblk))
-		invoke_rcu_kthread();
+		invoke_rcu_callbacks();
 	local_irq_restore(flags);
 }
 
···
  * Invoke the RCU callbacks on the specified rcu_ctrlkblk structure
  * whose grace period has elapsed.
  */
-static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
+static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 {
 	struct rcu_head *next, *list;
 	unsigned long flags;
···
 	RCU_TRACE(trace_rcu_batch_end(rcp->name, cb_count));
 }
 
-/*
- * This kthread invokes RCU callbacks whose grace periods have
- * elapsed.  It is awakened as needed, and takes the place of the
- * RCU_SOFTIRQ that was used previously for this purpose.
- * This is a kthread, but it is never stopped, at least not until
- * the system goes down.
- */
-static int rcu_kthread(void *arg)
+static void rcu_process_callbacks(struct softirq_action *unused)
 {
-	unsigned long work;
-	unsigned long morework;
-	unsigned long flags;
-
-	for (;;) {
-		wait_event_interruptible(rcu_kthread_wq,
-					 have_rcu_kthread_work != 0);
-		morework = rcu_boost();
-		local_irq_save(flags);
-		work = have_rcu_kthread_work;
-		have_rcu_kthread_work = morework;
-		local_irq_restore(flags);
-		if (work) {
-			rcu_process_callbacks(&rcu_sched_ctrlblk);
-			rcu_process_callbacks(&rcu_bh_ctrlblk);
-			rcu_preempt_process_callbacks();
-		}
-		schedule_timeout_interruptible(1); /* Leave CPU for others. */
-	}
-
-	return 0;  /* Not reached, but needed to shut gcc up. */
+	__rcu_process_callbacks(&rcu_sched_ctrlblk);
+	__rcu_process_callbacks(&rcu_bh_ctrlblk);
+	rcu_preempt_process_callbacks();
 }
 
 /*
···
 	__call_rcu(head, func, &rcu_bh_ctrlblk);
 }
 EXPORT_SYMBOL_GPL(call_rcu_bh);
-
-/*
- * Spawn the kthread that invokes RCU callbacks.
- */
-static int __init rcu_spawn_kthreads(void)
-{
-	struct sched_param sp;
-
-	rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
-	sp.sched_priority = RCU_BOOST_PRIO;
-	sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
-	return 0;
-}
-early_initcall(rcu_spawn_kthreads);

kernel/rcutiny_plugin.h

···
 
 #include "rtmutex_common.h"
 
+#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
+
+/* Controls for rcu_kthread() kthread. */
+static struct task_struct *rcu_kthread_task;
+static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
+static unsigned long have_rcu_kthread_work;
+
 /*
  * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
  * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
···
 		if (rcu_preempt_ctrlblk.exp_tasks == NULL)
 			rcu_preempt_ctrlblk.boost_tasks =
 				rcu_preempt_ctrlblk.gp_tasks;
-		invoke_rcu_kthread();
+		invoke_rcu_callbacks();
 	} else
 		RCU_TRACE(rcu_initiate_boost_trace());
 	return 1;
···
 }
 
 #else /* #ifdef CONFIG_RCU_BOOST */
-
-/*
- * If there is no RCU priority boosting, we don't boost.
- */
-static int rcu_boost(void)
-{
-	return 0;
-}
 
 /*
  * If there is no RCU priority boosting, we don't initiate boosting,
···
 
 	/* If there are done callbacks, cause them to be invoked. */
 	if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
-		invoke_rcu_kthread();
+		invoke_rcu_callbacks();
 }
 
 /*
···
 		rcu_preempt_cpu_qs();
 	if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
 	    rcu_preempt_ctrlblk.rcb.donetail)
-		invoke_rcu_kthread();
+		invoke_rcu_callbacks();
 	if (rcu_preempt_gp_in_progress() &&
 	    rcu_cpu_blocking_cur_gp() &&
 	    rcu_preempt_running_reader())
···
  */
 static void rcu_preempt_process_callbacks(void)
 {
-	rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
+	__rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
 }
 
 /*
···
 #endif /* #ifdef CONFIG_RCU_TRACE */
 
 /*
- * Because preemptible RCU does not exist, it is never necessary to
- * boost preempted RCU readers.
- */
-static int rcu_boost(void)
-{
-	return 0;
-}
-
-/*
  * Because preemptible RCU does not exist, it never has any callbacks
  * to check.
  */
···
 
 #endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
 
+#ifdef CONFIG_RCU_BOOST
+
+/*
+ * Wake up rcu_kthread() to process callbacks now eligible for invocation
+ * or to boost readers.
+ */
+static void invoke_rcu_callbacks(void)
+{
+	have_rcu_kthread_work = 1;
+	wake_up(&rcu_kthread_wq);
+}
+
+/*
+ * This kthread invokes RCU callbacks whose grace periods have
+ * elapsed.  It is awakened as needed, and takes the place of the
+ * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
+ * This is a kthread, but it is never stopped, at least not until
+ * the system goes down.
+ */
+static int rcu_kthread(void *arg)
+{
+	unsigned long work;
+	unsigned long morework;
+	unsigned long flags;
+
+	for (;;) {
+		wait_event_interruptible(rcu_kthread_wq,
+					 have_rcu_kthread_work != 0);
+		morework = rcu_boost();
+		local_irq_save(flags);
+		work = have_rcu_kthread_work;
+		have_rcu_kthread_work = morework;
+		local_irq_restore(flags);
+		if (work)
+			rcu_process_callbacks(NULL);
+		schedule_timeout_interruptible(1); /* Leave CPU for others. */
+	}
+
+	return 0;  /* Not reached, but needed to shut gcc up. */
+}
+
+/*
+ * Spawn the kthread that invokes RCU callbacks.
+ */
+static int __init rcu_spawn_kthreads(void)
+{
+	struct sched_param sp;
+
+	rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
+	sp.sched_priority = RCU_BOOST_PRIO;
+	sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
+	return 0;
+}
+early_initcall(rcu_spawn_kthreads);
+
+#else /* #ifdef CONFIG_RCU_BOOST */
+
+/*
+ * Start up softirq processing of callbacks.
+ */
+void invoke_rcu_callbacks(void)
+{
+	raise_softirq(RCU_SOFTIRQ);
+}
+
+void rcu_init(void)
+{
+	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
+}
+
+#endif /* #else #ifdef CONFIG_RCU_BOOST */
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 #include <linux/kernel_stat.h>
 
···
 }
 
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
-
-#ifdef CONFIG_RCU_BOOST
-#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
-#else /* #ifdef CONFIG_RCU_BOOST */
-#define RCU_BOOST_PRIO 1
-#endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 #ifdef CONFIG_RCU_TRACE