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>

include/linux/init_task.h

@@ -81,6 +81,12 @@ extern struct group_info init_groups;
  */
 # define CAP_INIT_BSET  CAP_FULL_SET
 
+#ifdef CONFIG_RCU_BOOST
+#define INIT_TASK_RCU_BOOST()						\
+	.rcu_boost_mutex = NULL,
+#else
+#define INIT_TASK_RCU_BOOST()
+#endif
 #ifdef CONFIG_TREE_PREEMPT_RCU
 #define INIT_TASK_RCU_TREE_PREEMPT()					\
 	.rcu_blocked_node = NULL,
@@ -92,7 +98,8 @@ extern struct group_info init_groups;
 	.rcu_read_lock_nesting = 0,					\
 	.rcu_read_unlock_special = 0,					\
 	.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),		\
-	INIT_TASK_RCU_TREE_PREEMPT()
+	INIT_TASK_RCU_TREE_PREEMPT()					\
+	INIT_TASK_RCU_BOOST()
 #else
 #define INIT_TASK_RCU_PREEMPT(tsk)
 #endif

include/linux/sched.h

@@ -1210,6 +1210,9 @@ struct task_struct {
 #ifdef CONFIG_TREE_PREEMPT_RCU
 	struct rcu_node *rcu_blocked_node;
 #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+#ifdef CONFIG_RCU_BOOST
+	struct rt_mutex *rcu_boost_mutex;
+#endif /* #ifdef CONFIG_RCU_BOOST */
 
 #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
 	struct sched_info sched_info;
@@ -1745,15 +1748,19 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #ifdef CONFIG_PREEMPT_RCU
 
 #define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
-#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
+#define RCU_READ_UNLOCK_BOOSTED (1 << 1) /* boosted while in RCU read-side. */
+#define RCU_READ_UNLOCK_NEED_QS (1 << 2) /* RCU core needs CPU response. */
 
 static inline void rcu_copy_process(struct task_struct *p)
 {
 	p->rcu_read_lock_nesting = 0;
 	p->rcu_read_unlock_special = 0;
 #ifdef CONFIG_TREE_PREEMPT_RCU
 	p->rcu_blocked_node = NULL;
-#endif
+#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+#ifdef CONFIG_RCU_BOOST
+	p->rcu_boost_mutex = NULL;
+#endif /* #ifdef CONFIG_RCU_BOOST */
 	INIT_LIST_HEAD(&p->rcu_node_entry);
 }
 

init/Kconfig

@@ -450,6 +450,45 @@ config TREE_RCU_TRACE
 	  TREE_PREEMPT_RCU implementations, permitting Makefile to
 	  trivially select kernel/rcutree_trace.c.
 
+config RCU_BOOST
+	bool "Enable RCU priority boosting"
+	depends on RT_MUTEXES && TINY_PREEMPT_RCU
+	default n
+	help
+	  This option boosts the priority of preempted RCU readers that
+	  block the current preemptible RCU grace period for too long.
+	  This option also prevents heavy loads from blocking RCU
+	  callback invocation for all flavors of RCU.
+
+	  Say Y here if you are working with real-time apps or heavy loads
+	  Say N here if you are unsure.
+
+config RCU_BOOST_PRIO
+	int "Real-time priority to boost RCU readers to"
+	range 1 99
+	depends on RCU_BOOST
+	default 1
+	help
+	  This option specifies the real-time priority to which preempted
+	  RCU readers are to be boosted.  If you are working with CPU-bound
+	  real-time applications, you should specify a priority higher then
+	  the highest-priority CPU-bound application.
+
+	  Specify the real-time priority, or take the default if unsure.
+
+config RCU_BOOST_DELAY
+	int "Milliseconds to delay boosting after RCU grace-period start"
+	range 0 3000
+	depends on RCU_BOOST
+	default 500
+	help
+	  This option specifies the time to wait after the beginning of
+	  a given grace period before priority-boosting preempted RCU
+	  readers blocking that grace period.  Note that any RCU reader
+	  blocking an expedited RCU grace period is boosted immediately.
+
+	  Accept the default if unsure.
+
 endmenu # "RCU Subsystem"
 
 config IKCONFIG

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);