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 <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 10 insertions, 64 deletions

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 1c37bdd464f1..c9321d86999b 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -43,16 +43,11 @@
 
 #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);
@@ -102,16 +97,6 @@ static int rcu_qsctr_help(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.
@@ -123,7 +108,7 @@ void rcu_sched_qs(int cpu)
         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);
 }
 
@@ -136,7 +121,7 @@ void rcu_bh_qs(int cpu)
 
         local_irq_save(flags);
         if (rcu_qsctr_help(&rcu_bh_ctrlblk))
-                invoke_rcu_kthread();
+                invoke_rcu_callbacks();
         local_irq_restore(flags);
 }
 
@@ -160,7 +145,7 @@ void rcu_check_callbacks(int cpu, int user)
  * 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;
@@ -200,36 +185,11 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
         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();
 }
 
 /*
@@ -291,17 +251,3 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
         __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);