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>

1 files changed, 89 insertions, 37 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e2e893144a84..4cc5eba41616 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;
-}
+extern void synchronize_sched(void);
 
-/*
- * Return the number of grace periods.
- */
-static inline long rcu_batches_completed(void)
-{
-        return 0;
-}
+#ifdef CONFIG_TINY_RCU
 
-/*
- * Return the number of bottom-half grace periods.
- */
-static inline long rcu_batches_completed_bh(void)
-{
-        return 0;
-}
+#define call_rcu                call_rcu_sched
 
-static inline void rcu_force_quiescent_state(void)
+static inline void synchronize_rcu(void)
 {
+        synchronize_sched();
 }
 
-static inline void rcu_bh_force_quiescent_state(void)
+static inline void synchronize_rcu_expedited(void)
 {
+        synchronize_sched();    /* Only one CPU, so pretty fast anyway!!! */
 }
 
-static inline void rcu_sched_force_quiescent_state(void)
+static inline void rcu_barrier(void)
 {
+        rcu_barrier_sched();  /* Only one CPU, so only one list of callbacks! */
 }
 
-extern void synchronize_sched(void);
+#else /* #ifdef CONFIG_TINY_RCU */
 
-static inline void synchronize_rcu(void)
-{
-        synchronize_sched();
-}
+void synchronize_rcu(void);
+void rcu_barrier(void);
+void synchronize_rcu_expedited(void);
 
-static inline void synchronize_rcu_bh(void)
-{
-        synchronize_sched();
-}
+#endif /* #else #ifdef CONFIG_TINY_RCU */
 
-static inline void synchronize_rcu_expedited(void)
+static inline void synchronize_rcu_bh(void)
 {
         synchronize_sched();
 }
@@ -117,15 +102,82 @@ static inline void rcu_exit_nohz(void)
 
 #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;