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>

2 files changed, 224 insertions, 50 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);
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 95f9239df512..24f43165f222 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -24,6 +24,29 @@
 
 #include <linux/kthread.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 */
+
 #ifdef CONFIG_TINY_PREEMPT_RCU
 
 #include <linux/delay.h>
@@ -48,17 +71,27 @@ struct rcu_preempt_ctrlblk {
         struct list_head *gp_tasks;
                                 /* Pointer to the first task blocking the */
                                 /*  current grace period, or NULL if there */
-                                /*  is not such task. */
+                                /*  is no such task. */
         struct list_head *exp_tasks;
                                 /* Pointer to first task blocking the */
                                 /*  current expedited grace period, or NULL */
                                 /*  if there is no such task.  If there */
                                 /*  is no current expedited grace period, */
                                 /*  then there cannot be any such task. */
+#ifdef CONFIG_RCU_BOOST
+        struct list_head *boost_tasks;
+                                /* Pointer to first task that needs to be */
+                                /*  priority-boosted, or NULL if no priority */
+                                /*  boosting is needed.  If there is no */
+                                /*  current or expedited grace period, there */
+                                /*  can be no such task. */
+#endif /* #ifdef CONFIG_RCU_BOOST */
         u8 gpnum;               /* Current grace period. */
         u8 gpcpu;               /* Last grace period blocked by the CPU. */
         u8 completed;           /* Last grace period completed. */
                                 /*  If all three are equal, RCU is idle. */
+        s8 boosted_this_gp;     /* Has boosting already happened? */
+        unsigned long boost_time; /* When to start boosting (jiffies) */
 };
 
 static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
@@ -124,6 +157,130 @@ static int rcu_preempt_gp_in_progress(void)
 }
 
 /*
+ * Advance a ->blkd_tasks-list pointer to the next entry, instead
+ * returning NULL if at the end of the list.
+ */
+static struct list_head *rcu_next_node_entry(struct task_struct *t)
+{
+        struct list_head *np;
+
+        np = t->rcu_node_entry.next;
+        if (np == &rcu_preempt_ctrlblk.blkd_tasks)
+                np = NULL;
+        return np;
+}
+
+#ifdef CONFIG_RCU_BOOST
+
+#include "rtmutex_common.h"
+
+/*
+ * 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.
+ */
+static int rcu_boost(void)
+{
+        unsigned long flags;
+        struct rt_mutex mtx;
+        struct list_head *np;
+        struct task_struct *t;
+
+        if (rcu_preempt_ctrlblk.boost_tasks == NULL)
+                return 0;  /* Nothing to boost. */
+        raw_local_irq_save(flags);
+        rcu_preempt_ctrlblk.boosted_this_gp++;
+        t = container_of(rcu_preempt_ctrlblk.boost_tasks, struct task_struct,
+                         rcu_node_entry);
+        np = rcu_next_node_entry(t);
+        rt_mutex_init_proxy_locked(&mtx, t);
+        t->rcu_boost_mutex = &mtx;
+        t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
+        raw_local_irq_restore(flags);
+        rt_mutex_lock(&mtx);
+        rt_mutex_unlock(&mtx);
+        return rcu_preempt_ctrlblk.boost_tasks != NULL;
+}
+
+/*
+ * Check to see if it is now time to start boosting RCU readers blocking
+ * the current grace period, and, if so, tell the rcu_kthread_task to
+ * start boosting them.  If there is an expedited boost in progress,
+ * we wait for it to complete.
+ */
+static void rcu_initiate_boost(void)
+{
+        if (rcu_preempt_ctrlblk.gp_tasks != NULL &&
+            rcu_preempt_ctrlblk.boost_tasks == NULL &&
+            rcu_preempt_ctrlblk.boosted_this_gp == 0 &&
+            ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time)) {
+                rcu_preempt_ctrlblk.boost_tasks = rcu_preempt_ctrlblk.gp_tasks;
+                invoke_rcu_kthread();
+        }
+}
+
+/*
+ * Initiate boosting for an expedited grace period.
+ */
+static void rcu_initiate_expedited_boost(void)
+{
+        unsigned long flags;
+
+        raw_local_irq_save(flags);
+        if (!list_empty(&rcu_preempt_ctrlblk.blkd_tasks)) {
+                rcu_preempt_ctrlblk.boost_tasks =
+                        rcu_preempt_ctrlblk.blkd_tasks.next;
+                rcu_preempt_ctrlblk.boosted_this_gp = -1;
+                invoke_rcu_kthread();
+        }
+        raw_local_irq_restore(flags);
+}
+
+#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000);
+
+/*
+ * Do priority-boost accounting for the start of a new grace period.
+ */
+static void rcu_preempt_boost_start_gp(void)
+{
+        rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
+        if (rcu_preempt_ctrlblk.boosted_this_gp > 0)
+                rcu_preempt_ctrlblk.boosted_this_gp = 0;
+}
+
+#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.
+ */
+static void rcu_initiate_boost(void)
+{
+}
+
+/*
+ * If there is no RCU priority boosting, we don't initiate expedited boosting.
+ */
+static void rcu_initiate_expedited_boost(void)
+{
+}
+
+/*
+ * If there is no RCU priority boosting, nothing to do at grace-period start.
+ */
+static void rcu_preempt_boost_start_gp(void)
+{
+}
+
+#endif /* else #ifdef CONFIG_RCU_BOOST */
+
+/*
  * Record a preemptible-RCU quiescent state for the specified CPU.  Note
  * that this just means that the task currently running on the CPU is
  * in a quiescent state.  There might be any number of tasks blocked
@@ -150,12 +307,14 @@ static void rcu_preempt_cpu_qs(void)
         rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
         current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 
-        /*
-         * If there is no GP, or if blocked readers are still blocking GP,
-         * then there is nothing more to do.
-         */
+        /* If there is no GP then there is nothing more to do.  */
         if (!rcu_preempt_gp_in_progress() || rcu_preempt_blocked_readers_cgp())
                 return;
+        /* If there are blocked readers, go check up on boosting. */
+        if (rcu_preempt_blocked_readers_cgp()) {
+                rcu_initiate_boost();
+                return;
+        }
 
         /* Advance callbacks. */
         rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
@@ -168,7 +327,7 @@ static void rcu_preempt_cpu_qs(void)
 
         /* If there are done callbacks, cause them to be invoked. */
         if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
-                invoke_rcu_cbs();
+                invoke_rcu_kthread();
 }
 
 /*
@@ -186,6 +345,9 @@ static void rcu_preempt_start_gp(void)
                         rcu_preempt_ctrlblk.gp_tasks =
                                 rcu_preempt_ctrlblk.blkd_tasks.next;
 
+                /* Set up for RCU priority boosting. */
+                rcu_preempt_boost_start_gp();
+
                 /* If there is no running reader, CPU is done with GP. */
                 if (!rcu_preempt_running_reader())
                         rcu_preempt_cpu_qs();
@@ -306,14 +468,16 @@ static void rcu_read_unlock_special(struct task_struct *t)
                  */
                 empty = !rcu_preempt_blocked_readers_cgp();
                 empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
-                np = t->rcu_node_entry.next;
-                if (np == &rcu_preempt_ctrlblk.blkd_tasks)
-                        np = NULL;
+                np = rcu_next_node_entry(t);
                 list_del(&t->rcu_node_entry);
                 if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
                         rcu_preempt_ctrlblk.gp_tasks = np;
                 if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
                         rcu_preempt_ctrlblk.exp_tasks = np;
+#ifdef CONFIG_RCU_BOOST
+                if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
+                        rcu_preempt_ctrlblk.boost_tasks = np;
+#endif /* #ifdef CONFIG_RCU_BOOST */
                 INIT_LIST_HEAD(&t->rcu_node_entry);
 
                 /*
@@ -333,6 +497,14 @@ static void rcu_read_unlock_special(struct task_struct *t)
                 if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
                         rcu_report_exp_done();
         }
+#ifdef CONFIG_RCU_BOOST
+        /* Unboost self if was boosted. */
+        if (special & RCU_READ_UNLOCK_BOOSTED) {
+                t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BOOSTED;
+                rt_mutex_unlock(t->rcu_boost_mutex);
+                t->rcu_boost_mutex = NULL;
+        }
+#endif /* #ifdef CONFIG_RCU_BOOST */
         local_irq_restore(flags);
 }
 
@@ -376,7 +548,7 @@ static void rcu_preempt_check_callbacks(void)
                 rcu_preempt_cpu_qs();
         if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
             rcu_preempt_ctrlblk.rcb.donetail)
-                invoke_rcu_cbs();
+                invoke_rcu_kthread();
         if (rcu_preempt_gp_in_progress() &&
             rcu_cpu_blocking_cur_gp() &&
             rcu_preempt_running_reader())
@@ -534,6 +706,7 @@ void synchronize_rcu_expedited(void)
 
         /* Wait for tail of ->blkd_tasks list to drain. */
         if (rcu_preempted_readers_exp())
+                rcu_initiate_expedited_boost();
                 wait_event(sync_rcu_preempt_exp_wq,
                            !rcu_preempted_readers_exp());
 
@@ -575,6 +748,15 @@ void exit_rcu(void)
 #else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
 
 /*
+ * 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.
  */
@@ -614,3 +796,9 @@ void __init rcu_scheduler_starting(void)
 }
 
 #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 */