rcu: priority boosting for TINY_PREEMPT_RCU

Add priority boosting, but only for TINY_PREEMPT_RCU.  This is enabled
by the default-off RCU_BOOST kernel parameter.  The priority to which to
boost preempted RCU readers is controlled by the RCU_BOOST_PRIO kernel
parameter (defaulting to real-time priority 1) and the time to wait
before boosting the readers blocking a given grace period is controlled
by the RCU_BOOST_DELAY kernel parameter (defaulting to 500 milliseconds).

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

1 files changed, 26 insertions, 40 deletions

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 86eef29cdfb2..93d166582cbb 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -36,38 +36,16 @@
 #include <linux/time.h>
 #include <linux/cpu.h>
 
-/* Global control variables for rcupdate callback mechanism. */
-struct rcu_ctrlblk {
-        struct rcu_head *rcucblist;     /* List of pending callbacks (CBs). */
-        struct rcu_head **donetail;     /* ->next pointer of last "done" CB. */
-        struct rcu_head **curtail;      /* ->next pointer of last CB. */
-};
-
-/* Definition for rcupdate control block. */
-static struct rcu_ctrlblk rcu_sched_ctrlblk = {
-        .donetail       = &rcu_sched_ctrlblk.rcucblist,
-        .curtail        = &rcu_sched_ctrlblk.rcucblist,
-};
-
-static struct rcu_ctrlblk rcu_bh_ctrlblk = {
-        .donetail       = &rcu_bh_ctrlblk.rcucblist,
-        .curtail        = &rcu_bh_ctrlblk.rcucblist,
-};
-
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-int rcu_scheduler_active __read_mostly;
-EXPORT_SYMBOL_GPL(rcu_scheduler_active);
-#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
-
-/* Controls for rcu_cbs() kthread, replacing RCU_SOFTIRQ used previously. */
-static struct task_struct *rcu_cbs_task;
-static DECLARE_WAIT_QUEUE_HEAD(rcu_cbs_wq);
-static unsigned long have_rcu_cbs;
-static void invoke_rcu_cbs(void);
+/* 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;
+static void invoke_rcu_kthread(void);
 
 /* Forward declarations for rcutiny_plugin.h. */
+struct rcu_ctrlblk;
 static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
-static int rcu_cbs(void *arg);
+static int rcu_kthread(void *arg);
 static void __call_rcu(struct rcu_head *head,
                        void (*func)(struct rcu_head *rcu),
                        struct rcu_ctrlblk *rcp);
@@ -130,7 +108,7 @@ void rcu_sched_qs(int cpu)
 {
         if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
             rcu_qsctr_help(&rcu_bh_ctrlblk))
-                invoke_rcu_cbs();
+                invoke_rcu_kthread();
 }
 
 /*
@@ -139,7 +117,7 @@ void rcu_sched_qs(int cpu)
 void rcu_bh_qs(int cpu)
 {
         if (rcu_qsctr_help(&rcu_bh_ctrlblk))
-                invoke_rcu_cbs();
+                invoke_rcu_kthread();
 }
 
 /*
@@ -201,37 +179,41 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
  * This is a kthread, but it is never stopped, at least not until
  * the system goes down.
  */
-static int rcu_cbs(void *arg)
+static int rcu_kthread(void *arg)
 {
         unsigned long work;
+        unsigned long morework;
         unsigned long flags;
 
         for (;;) {
-                wait_event(rcu_cbs_wq, have_rcu_cbs != 0);
+                wait_event(rcu_kthread_wq, have_rcu_kthread_work != 0);
+                morework = rcu_boost();
                 local_irq_save(flags);
-                work = have_rcu_cbs;
-                have_rcu_cbs = 0;
+                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. */
 }
 
 /*
- * Wake up rcu_cbs() to process callbacks now eligible for invocation.
+ * Wake up rcu_kthread() to process callbacks now eligible for invocation
+ * or to boost readers.
  */
-static void invoke_rcu_cbs(void)
+static void invoke_rcu_kthread(void)
 {
         unsigned long flags;
 
         local_irq_save(flags);
-        have_rcu_cbs = 1;
-        wake_up(&rcu_cbs_wq);
+        have_rcu_kthread_work = 1;
+        wake_up(&rcu_kthread_wq);
         local_irq_restore(flags);
 }
 
@@ -327,7 +309,11 @@ EXPORT_SYMBOL_GPL(rcu_barrier_sched);
  */
 static int __init rcu_spawn_kthreads(void)
 {
-        rcu_cbs_task = kthread_run(rcu_cbs, NULL, "rcu_cbs");
+        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);