rcu: Add a TINY_PREEMPT_RCU
Implement a small-memory-footprint uniprocessor-only implementation of
preemptible RCU. This implementation uses but a single blocked-tasks
list rather than the combinatorial number used per leaf rcu_node by
TREE_PREEMPT_RCU, which reduces memory consumption and greatly simplifies
processing. This version also takes advantage of uniprocessor execution
to accelerate grace periods in the case where there are no readers.
The general design is otherwise broadly similar to that of TREE_PREEMPT_RCU.
This implementation is a step towards having RCU implementation driven
off of the SMP and PREEMPT kernel configuration variables, which can
happen once this implementation has accumulated sufficient experience.
Removed ACCESS_ONCE() from __rcu_read_unlock() and added barrier() as
suggested by Steve Rostedt in order to avoid the compiler-reordering
issue noted by Mathieu Desnoyers (http://lkml.org/lkml/2010/8/16/183).
As can be seen below, CONFIG_TINY_PREEMPT_RCU represents almost 5Kbyte
savings compared to CONFIG_TREE_PREEMPT_RCU. Of course, for non-real-time
workloads, CONFIG_TINY_RCU is even better.
CONFIG_TREE_PREEMPT_RCU
text data bss dec filename
13 0 0 13 kernel/rcupdate.o
6170 825 28 7023 kernel/rcutree.o
----
7026 Total
CONFIG_TINY_PREEMPT_RCU
text data bss dec filename
13 0 0 13 kernel/rcupdate.o
2081 81 8 2170 kernel/rcutiny.o
----
2183 Total
CONFIG_TINY_RCU (non-preemptible)
text data bss dec filename
13 0 0 13 kernel/rcupdate.o
719 25 0 744 kernel/rcutiny.o
---
757 Total
Requested-by: Loïc Minier <loic.minier@canonical.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index d5b3876..1f4517d 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -139,7 +139,7 @@
#endif
#if defined(CONFIG_NO_HZ)
-#if defined(CONFIG_TINY_RCU)
+#if defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
extern void rcu_enter_nohz(void);
extern void rcu_exit_nohz(void);
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 6460fc6..2fea6c8 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -82,11 +82,17 @@
# define CAP_INIT_BSET CAP_FULL_SET
#ifdef CONFIG_TREE_PREEMPT_RCU
+#define INIT_TASK_RCU_TREE_PREEMPT() \
+ .rcu_blocked_node = NULL,
+#else
+#define INIT_TASK_RCU_TREE_PREEMPT(tsk)
+#endif
+#ifdef CONFIG_PREEMPT_RCU
#define INIT_TASK_RCU_PREEMPT(tsk) \
.rcu_read_lock_nesting = 0, \
.rcu_read_unlock_special = 0, \
- .rcu_blocked_node = NULL, \
- .rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),
+ .rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry), \
+ INIT_TASK_RCU_TREE_PREEMPT()
#else
#define INIT_TASK_RCU_PREEMPT(tsk)
#endif
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 27b44b3..24b8966 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -58,7 +58,6 @@
};
/* Exported common interfaces */
-extern void rcu_barrier(void);
extern void rcu_barrier_bh(void);
extern void rcu_barrier_sched(void);
extern void synchronize_sched_expedited(void);
@@ -69,7 +68,7 @@
#if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
#include <linux/rcutree.h>
-#elif defined(CONFIG_TINY_RCU)
+#elif defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
#include <linux/rcutiny.h>
#else
#error "Unknown RCU implementation specified to kernel configuration"
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e2e8931..4cc5eba 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -29,66 +29,51 @@
void rcu_sched_qs(int cpu);
void rcu_bh_qs(int cpu);
-static inline void rcu_note_context_switch(int cpu)
-{
- rcu_sched_qs(cpu);
-}
+#ifdef CONFIG_TINY_RCU
#define __rcu_read_lock() preempt_disable()
#define __rcu_read_unlock() preempt_enable()
+#else /* #ifdef CONFIG_TINY_RCU */
+void __rcu_read_lock(void);
+void __rcu_read_unlock(void);
+#endif /* #else #ifdef CONFIG_TINY_RCU */
#define __rcu_read_lock_bh() local_bh_disable()
#define __rcu_read_unlock_bh() local_bh_enable()
-#define call_rcu_sched call_rcu
+extern void call_rcu_sched(struct rcu_head *head,
+ void (*func)(struct rcu_head *rcu));
#define rcu_init_sched() do { } while (0)
-extern void rcu_check_callbacks(int cpu, int user);
-
-static inline int rcu_needs_cpu(int cpu)
-{
- return 0;
-}
-
-/*
- * Return the number of grace periods.
- */
-static inline long rcu_batches_completed(void)
-{
- return 0;
-}
-
-/*
- * Return the number of bottom-half grace periods.
- */
-static inline long rcu_batches_completed_bh(void)
-{
- return 0;
-}
-
-static inline void rcu_force_quiescent_state(void)
-{
-}
-
-static inline void rcu_bh_force_quiescent_state(void)
-{
-}
-
-static inline void rcu_sched_force_quiescent_state(void)
-{
-}
extern void synchronize_sched(void);
+#ifdef CONFIG_TINY_RCU
+
+#define call_rcu call_rcu_sched
+
static inline void synchronize_rcu(void)
{
synchronize_sched();
}
-static inline void synchronize_rcu_bh(void)
+static inline void synchronize_rcu_expedited(void)
{
- synchronize_sched();
+ synchronize_sched(); /* Only one CPU, so pretty fast anyway!!! */
}
-static inline void synchronize_rcu_expedited(void)
+static inline void rcu_barrier(void)
+{
+ rcu_barrier_sched(); /* Only one CPU, so only one list of callbacks! */
+}
+
+#else /* #ifdef CONFIG_TINY_RCU */
+
+void synchronize_rcu(void);
+void rcu_barrier(void);
+void synchronize_rcu_expedited(void);
+
+#endif /* #else #ifdef CONFIG_TINY_RCU */
+
+static inline void synchronize_rcu_bh(void)
{
synchronize_sched();
}
@@ -117,15 +102,82 @@
#endif /* #else #ifdef CONFIG_NO_HZ */
+#ifdef CONFIG_TINY_RCU
+
+static inline void rcu_preempt_note_context_switch(void)
+{
+}
+
static inline void exit_rcu(void)
{
}
+static inline int rcu_needs_cpu(int cpu)
+{
+ return 0;
+}
+
static inline int rcu_preempt_depth(void)
{
return 0;
}
+#else /* #ifdef CONFIG_TINY_RCU */
+
+void rcu_preempt_note_context_switch(void);
+extern void exit_rcu(void);
+int rcu_preempt_needs_cpu(void);
+
+static inline int rcu_needs_cpu(int cpu)
+{
+ return rcu_preempt_needs_cpu();
+}
+
+/*
+ * Defined as macro as it is a very low level header
+ * included from areas that don't even know about current
+ * FIXME: combine with include/linux/rcutree.h into rcupdate.h.
+ */
+#define rcu_preempt_depth() (current->rcu_read_lock_nesting)
+
+#endif /* #else #ifdef CONFIG_TINY_RCU */
+
+static inline void rcu_note_context_switch(int cpu)
+{
+ rcu_sched_qs(cpu);
+ rcu_preempt_note_context_switch();
+}
+
+extern void rcu_check_callbacks(int cpu, int user);
+
+/*
+ * Return the number of grace periods.
+ */
+static inline long rcu_batches_completed(void)
+{
+ return 0;
+}
+
+/*
+ * Return the number of bottom-half grace periods.
+ */
+static inline long rcu_batches_completed_bh(void)
+{
+ return 0;
+}
+
+static inline void rcu_force_quiescent_state(void)
+{
+}
+
+static inline void rcu_bh_force_quiescent_state(void)
+{
+}
+
+static inline void rcu_sched_force_quiescent_state(void)
+{
+}
+
#ifdef CONFIG_DEBUG_LOCK_ALLOC
extern int rcu_scheduler_active __read_mostly;
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index c0ed1c0..c13b85d 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -95,6 +95,8 @@
synchronize_sched_expedited();
}
+extern void rcu_barrier(void);
+
extern void rcu_check_callbacks(int cpu, int user);
extern long rcu_batches_completed(void);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2c75666..e18473f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1202,11 +1202,13 @@
unsigned int policy;
cpumask_t cpus_allowed;
-#ifdef CONFIG_TREE_PREEMPT_RCU
+#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
char rcu_read_unlock_special;
- struct rcu_node *rcu_blocked_node;
struct list_head rcu_node_entry;
+#endif /* #ifdef CONFIG_PREEMPT_RCU */
+#ifdef CONFIG_TREE_PREEMPT_RCU
+ struct rcu_node *rcu_blocked_node;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
@@ -1740,7 +1742,7 @@
#define tsk_used_math(p) ((p)->flags & PF_USED_MATH)
#define used_math() tsk_used_math(current)
-#ifdef CONFIG_TREE_PREEMPT_RCU
+#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. */
@@ -1749,7 +1751,9 @@
{
p->rcu_read_lock_nesting = 0;
p->rcu_read_unlock_special = 0;
+#ifdef CONFIG_TREE_PREEMPT_RCU
p->rcu_blocked_node = NULL;
+#endif
INIT_LIST_HEAD(&p->rcu_node_entry);
}