rcu: Merge preemptable-RCU functionality into hierarchical RCU

Create a kernel/rcutree_plugin.h file that contains definitions
for preemptable RCU (or, under the #else branch of the #ifdef,
empty definitions for the classic non-preemptable semantics).
These definitions fit into plugins defined in kernel/rcutree.c
for this purpose.

This variant of preemptable RCU uses a new algorithm whose
read-side expense is roughly that of classic hierarchical RCU
under CONFIG_PREEMPT. This new algorithm's update-side expense
is similar to that of classic hierarchical RCU, and, in absence
of read-side preemption or blocking, is exactly that of classic
hierarchical RCU.  Perhaps more important, this new algorithm
has a much simpler implementation, saving well over 1,000 lines
of code compared to mainline's implementation of preemptable
RCU, which will hopefully be retired in favor of this new
algorithm.

The simplifications are obtained by maintaining per-task
nesting state for running tasks, and using a simple
lock-protected algorithm to handle accounting when tasks block
within RCU read-side critical sections, making use of lessons
learned while creating numerous user-level RCU implementations
over the past 18 months.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <12509746134003-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/init_task.h

@@ -94,6 +94,20 @@ extern struct group_info init_groups;
 # define CAP_INIT_BSET  CAP_INIT_EFF_SET
 #endif
 
+#ifdef CONFIG_PREEMPT_RCU
+#define INIT_TASK_RCU_PREEMPT(tsk)					\
+	.rcu_read_lock_nesting = 0,					\
+	.rcu_flipctr_idx = 0,
+#elif defined(CONFIG_TREE_PREEMPT_RCU)
+#define INIT_TASK_RCU_PREEMPT(tsk)					\
+	.rcu_read_lock_nesting = 0,					\
+	.rcu_read_unlock_special = 0,					\
+	.rcu_blocked_cpu = -1,						\
+	.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),
+#else
+#define INIT_TASK_RCU_PREEMPT(tsk)
+#endif
+
 extern struct cred init_cred;
 
 #ifdef CONFIG_PERF_COUNTERS
@@ -173,6 +187,7 @@ extern struct cred init_cred;
 	INIT_LOCKDEP							\
 	INIT_FTRACE_GRAPH						\
 	INIT_TRACE_RECURSION						\
+	INIT_TASK_RCU_PREEMPT(tsk)					\
 }
 
 

include/linux/rcupdate.h

@@ -66,7 +66,7 @@ extern void rcu_scheduler_starting(void);
 extern int rcu_needs_cpu(int cpu);
 extern int rcu_scheduler_active;
 
-#if defined(CONFIG_TREE_RCU)
+#if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
 #include <linux/rcutree.h>
 #elif defined(CONFIG_PREEMPT_RCU)
 #include <linux/rcupreempt.h>

include/linux/rcupreempt.h

@@ -98,6 +98,10 @@ static inline long rcu_batches_completed_bh(void)
 	return rcu_batches_completed();
 }
 
+static inline void exit_rcu(void)
+{
+}
+
 #ifdef CONFIG_RCU_TRACE
 struct rcupreempt_trace;
 extern long *rcupreempt_flipctr(int cpu);

include/linux/rcutree.h

@@ -35,14 +35,30 @@ extern void rcu_bh_qs(int cpu);
 
 extern int rcu_needs_cpu(int cpu);
 
+#ifdef CONFIG_TREE_PREEMPT_RCU
+
+extern void __rcu_read_lock(void);
+extern void __rcu_read_unlock(void);
+extern void exit_rcu(void);
+
+#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+
 static inline void __rcu_read_lock(void)
 {
 	preempt_disable();
 }
+
 static inline void __rcu_read_unlock(void)
 {
 	preempt_enable();
 }
+
+static inline void exit_rcu(void)
+{
+}
+
+#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
+
 static inline void __rcu_read_lock_bh(void)
 {
 	local_bh_disable();

include/linux/sched.h

@@ -1210,6 +1210,13 @@ struct task_struct {
 	int rcu_flipctr_idx;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 
+#ifdef CONFIG_TREE_PREEMPT_RCU
+	int rcu_read_lock_nesting;
+	char rcu_read_unlock_special;
+	int rcu_blocked_cpu;
+	struct list_head rcu_node_entry;
+#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
+
 #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
 	struct sched_info sched_info;
 #endif
@@ -1723,6 +1730,36 @@ extern cputime_t task_gtime(struct task_struct *p);
 #define tsk_used_math(p) ((p)->flags & PF_USED_MATH)
 #define used_math() tsk_used_math(current)
 
+#ifdef CONFIG_TREE_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_GOT_QS  (1 << 2) /* CPU has responded to RCU core. */
+
+static inline void rcu_copy_process(struct task_struct *p)
+{
+	p->rcu_read_lock_nesting = 0;
+	p->rcu_read_unlock_special = 0;
+	p->rcu_blocked_cpu = -1;
+	INIT_LIST_HEAD(&p->rcu_node_entry);
+}
+
+#elif defined(CONFIG_PREEMPT_RCU)
+
+static inline void rcu_copy_process(struct task_struct *p)
+{
+	p->rcu_read_lock_nesting = 0;
+	p->rcu_flipctr_idx = 0;
+}
+
+#else
+
+static inline void rcu_copy_process(struct task_struct *p)
+{
+}
+
+#endif
+
 #ifdef CONFIG_SMP
 extern int set_cpus_allowed_ptr(struct task_struct *p,
 				const struct cpumask *new_mask);

init/Kconfig

@@ -335,11 +335,20 @@ config PREEMPT_RCU
 	  now-naive assumptions about each RCU read-side critical section
 	  remaining on a given CPU through its execution.
 
+config TREE_PREEMPT_RCU
+	bool "Preemptable tree-based hierarchical RCU"
+	depends on PREEMPT
+	help
+	  This option selects the RCU implementation that is
+	  designed for very large SMP systems with hundreds or
+	  thousands of CPUs, but for which real-time response
+	  is also required.
+
 endchoice
 
 config RCU_TRACE
 	bool "Enable tracing for RCU"
-	depends on TREE_RCU || PREEMPT_RCU
+	depends on TREE_RCU || PREEMPT_RCU || TREE_PREEMPT_RCU
 	help
 	  This option provides tracing in RCU which presents stats
 	  in debugfs for debugging RCU implementation.
@@ -351,7 +360,7 @@ config RCU_FANOUT
 	int "Tree-based hierarchical RCU fanout value"
 	range 2 64 if 64BIT
 	range 2 32 if !64BIT
-	depends on TREE_RCU
+	depends on TREE_RCU || TREE_PREEMPT_RCU
 	default 64 if 64BIT
 	default 32 if !64BIT
 	help
@@ -366,7 +375,7 @@ config RCU_FANOUT
 
 config RCU_FANOUT_EXACT
 	bool "Disable tree-based hierarchical RCU auto-balancing"
-	depends on TREE_RCU
+	depends on TREE_RCU || TREE_PREEMPT_RCU
 	default n
 	help
 	  This option forces use of the exact RCU_FANOUT value specified,
@@ -379,11 +388,12 @@ config RCU_FANOUT_EXACT
 	  Say N if unsure.
 
 config TREE_RCU_TRACE
-	def_bool RCU_TRACE && TREE_RCU
+	def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU )
 	select DEBUG_FS
 	help
-	  This option provides tracing for the TREE_RCU implementation,
-	  permitting Makefile to trivially select kernel/rcutree_trace.c.
+	  This option provides tracing for the TREE_RCU and
+	  TREE_PREEMPT_RCU implementations, permitting Makefile to
+	  trivially select kernel/rcutree_trace.c.
 
 config PREEMPT_RCU_TRACE
 	def_bool RCU_TRACE && PREEMPT_RCU

kernel/Makefile

@@ -81,6 +81,7 @@ obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
 obj-$(CONFIG_SECCOMP) += seccomp.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
 obj-$(CONFIG_TREE_RCU) += rcutree.o
+obj-$(CONFIG_TREE_PREEMPT_RCU) += rcutree.o
 obj-$(CONFIG_PREEMPT_RCU) += rcupreempt.o
 obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
 obj-$(CONFIG_PREEMPT_RCU_TRACE) += rcupreempt_trace.o

kernel/exit.c

@@ -1010,6 +1010,7 @@ NORET_TYPE void do_exit(long code)
 		__free_pipe_info(tsk->splice_pipe);
 
 	preempt_disable();
+	exit_rcu();
 	/* causes final put_task_struct in finish_task_switch(). */
 	tsk->state = TASK_DEAD;
 	schedule();

kernel/fork.c

@@ -1022,10 +1022,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	copy_flags(clone_flags, p);
 	INIT_LIST_HEAD(&p->children);
 	INIT_LIST_HEAD(&p->sibling);
-#ifdef CONFIG_PREEMPT_RCU
-	p->rcu_read_lock_nesting = 0;
-	p->rcu_flipctr_idx = 0;
-#endif /* #ifdef CONFIG_PREEMPT_RCU */
+	rcu_copy_process(p);
 	p->vfork_done = NULL;
 	spin_lock_init(&p->alloc_lock);