rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds

Now that RCU-preempt knows about preemption disabling, its implementation
of synchronize_rcu() works for synchronize_sched(), and likewise for the
other RCU-sched update-side API members.  This commit therefore confines
the RCU-sched update-side code to CONFIG_PREEMPT=n builds, and defines
RCU-sched's update-side API members in terms of those of RCU-preempt.

This means that any given build of the Linux kernel has only one
update-side flavor of RCU, namely RCU-preempt for CONFIG_PREEMPT=y builds
and RCU-sched for CONFIG_PREEMPT=n builds.  This in turn means that kernels
built with CONFIG_RCU_NOCB_CPU=y have only one rcuo kthread per CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>

include/linux/rcupdate.h

@@ -49,11 +49,11 @@
 
 /* Exported common interfaces */
 
-#ifdef CONFIG_PREEMPT_RCU
-void call_rcu(struct rcu_head *head, rcu_callback_t func);
-#else /* #ifdef CONFIG_PREEMPT_RCU */
+#ifdef CONFIG_TINY_RCU
 #define	call_rcu	call_rcu_sched
-#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
+#else
+void call_rcu(struct rcu_head *head, rcu_callback_t func);
+#endif
 
 void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
 void synchronize_sched(void);
@@ -92,11 +92,6 @@ static inline void __rcu_read_unlock(void)
 		preempt_enable();
 }
 
-static inline void synchronize_rcu(void)
-{
-	synchronize_sched();
-}
-
 static inline int rcu_preempt_depth(void)
 {
 	return 0;
@@ -107,7 +102,6 @@ static inline int rcu_preempt_depth(void)
 /* Internal to kernel */
 void rcu_init(void);
 extern int rcu_scheduler_active __read_mostly;
-void rcu_sched_qs(void);
 void rcu_check_callbacks(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);

include/linux/rcutiny.h

@@ -36,6 +36,11 @@ static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
 /* Never flag non-existent other CPUs! */
 static inline bool rcu_eqs_special_set(int cpu) { return false; }
 
+static inline void synchronize_rcu(void)
+{
+	synchronize_sched();
+}
+
 static inline unsigned long get_state_synchronize_rcu(void)
 {
 	return 0;
@@ -94,6 +99,8 @@ static inline void kfree_call_rcu(struct rcu_head *head,
 	call_rcu(head, func);
 }
 
+void rcu_sched_qs(void);
+
 static inline void rcu_softirq_qs(void)
 {
 	rcu_sched_qs();

include/linux/rcutree.h

@@ -45,14 +45,19 @@ static inline void rcu_virt_note_context_switch(int cpu)
 	rcu_note_context_switch(false);
 }
 
+void synchronize_rcu(void);
 static inline void synchronize_rcu_bh(void)
 {
 	synchronize_rcu();
 }
 
-void synchronize_sched_expedited(void);
 void synchronize_rcu_expedited(void);
 
+static inline void synchronize_sched_expedited(void)
+{
+	synchronize_rcu_expedited();
+}
+
 void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
 
 /**

kernel/rcu/tree.h

@@ -225,9 +225,6 @@ struct rcu_data {
 
 	/* 5) _rcu_barrier(), OOM callbacks, and expediting. */
 	struct rcu_head barrier_head;
-#ifdef CONFIG_RCU_FAST_NO_HZ
-	struct rcu_head oom_head;
-#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
 	int exp_dynticks_snap;		/* Double-check need for IPI. */
 
 	/* 6) Callback offloading. */
@@ -433,8 +430,7 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
 
 /* Forward declarations for rcutree_plugin.h */
 static void rcu_bootup_announce(void);
-static void rcu_preempt_qs(void);
-static void rcu_preempt_note_context_switch(bool preempt);
+static void rcu_qs(void);
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
 #ifdef CONFIG_HOTPLUG_CPU
 static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
@@ -444,9 +440,8 @@ static int rcu_print_task_stall(struct rcu_node *rnp);
 static int rcu_print_task_exp_stall(struct rcu_node *rnp);
 static void rcu_preempt_check_blocked_tasks(struct rcu_state *rsp,
 					    struct rcu_node *rnp);
-static void rcu_preempt_check_callbacks(void);
+static void rcu_flavor_check_callbacks(int user);
 void call_rcu(struct rcu_head *head, rcu_callback_t func);
-static void __init __rcu_init_preempt(void);
 static void dump_blkd_tasks(struct rcu_state *rsp, struct rcu_node *rnp,
 			    int ncheck);
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);

kernel/rcu/tree_exp.h

@@ -265,7 +265,7 @@ static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp)
 	rcu_report_exp_cpu_mult(rsp, rdp->mynode, rdp->grpmask, true);
 }
 
-/* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
+/* Common code for work-done checking. */
 static bool sync_exp_work_done(struct rcu_state *rsp, unsigned long s)
 {
 	if (rcu_exp_gp_seq_done(rsp, s)) {
@@ -337,45 +337,6 @@ static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 	return false;
 }
 
-/* Invoked on each online non-idle CPU for expedited quiescent state. */
-static void sync_sched_exp_handler(void *data)
-{
-	struct rcu_data *rdp;
-	struct rcu_node *rnp;
-	struct rcu_state *rsp = data;
-
-	rdp = this_cpu_ptr(rsp->rda);
-	rnp = rdp->mynode;
-	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
-	    __this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
-		return;
-	if (rcu_is_cpu_rrupt_from_idle()) {
-		rcu_report_exp_rdp(&rcu_sched_state,
-				   this_cpu_ptr(&rcu_sched_data));
-		return;
-	}
-	__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
-	/* Store .exp before .rcu_urgent_qs. */
-	smp_store_release(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs), true);
-	resched_cpu(smp_processor_id());
-}
-
-/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
-static void sync_sched_exp_online_cleanup(int cpu)
-{
-	struct rcu_data *rdp;
-	int ret;
-	struct rcu_node *rnp;
-	struct rcu_state *rsp = &rcu_sched_state;
-
-	rdp = per_cpu_ptr(rsp->rda, cpu);
-	rnp = rdp->mynode;
-	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
-		return;
-	ret = smp_call_function_single(cpu, sync_sched_exp_handler, rsp, 0);
-	WARN_ON_ONCE(ret);
-}
-
 /*
  * Select the CPUs within the specified rcu_node that the upcoming
  * expedited grace period needs to wait for.
@@ -691,39 +652,6 @@ static void _synchronize_rcu_expedited(struct rcu_state *rsp,
 	mutex_unlock(&rsp->exp_mutex);
 }
 
-/**
- * synchronize_sched_expedited - Brute-force RCU-sched grace period
- *
- * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
- * approach to force the grace period to end quickly.  This consumes
- * significant time on all CPUs and is unfriendly to real-time workloads,
- * so is thus not recommended for any sort of common-case code.  In fact,
- * if you are using synchronize_sched_expedited() in a loop, please
- * restructure your code to batch your updates, and then use a single
- * synchronize_sched() instead.
- *
- * This implementation can be thought of as an application of sequence
- * locking to expedited grace periods, but using the sequence counter to
- * determine when someone else has already done the work instead of for
- * retrying readers.
- */
-void synchronize_sched_expedited(void)
-{
-	struct rcu_state *rsp = &rcu_sched_state;
-
-	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
-			 lock_is_held(&rcu_lock_map) ||
-			 lock_is_held(&rcu_sched_lock_map),
-			 "Illegal synchronize_sched_expedited() in RCU read-side critical section");
-
-	/* If only one CPU, this is automatically a grace period. */
-	if (rcu_blocking_is_gp())
-		return;
-
-	_synchronize_rcu_expedited(rsp, sync_sched_exp_handler);
-}
-EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
-
 #ifdef CONFIG_PREEMPT_RCU
 
 /*
@@ -801,6 +729,11 @@ static void sync_rcu_exp_handler(void *info)
 		resched_cpu(rdp->cpu);
 }
 
+/* PREEMPT=y, so no RCU-sched to clean up after. */
+static void sync_sched_exp_online_cleanup(int cpu)
+{
+}
+
 /**
  * synchronize_rcu_expedited - Brute-force RCU grace period
  *
@@ -818,6 +751,8 @@ static void sync_rcu_exp_handler(void *info)
  * you are using synchronize_rcu_expedited() in a loop, please restructure
  * your code to batch your updates, and then Use a single synchronize_rcu()
  * instead.
+ *
+ * This has the same semantics as (but is more brutal than) synchronize_rcu().
  */
 void synchronize_rcu_expedited(void)
 {
@@ -836,13 +771,79 @@ EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 
 #else /* #ifdef CONFIG_PREEMPT_RCU */
 
+/* Invoked on each online non-idle CPU for expedited quiescent state. */
+static void sync_sched_exp_handler(void *data)
+{
+	struct rcu_data *rdp;
+	struct rcu_node *rnp;
+	struct rcu_state *rsp = data;
+
+	rdp = this_cpu_ptr(rsp->rda);
+	rnp = rdp->mynode;
+	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
+	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
+		return;
+	if (rcu_is_cpu_rrupt_from_idle()) {
+		rcu_report_exp_rdp(&rcu_state, this_cpu_ptr(&rcu_data));
+		return;
+	}
+	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
+	/* Store .exp before .rcu_urgent_qs. */
+	smp_store_release(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs), true);
+	resched_cpu(smp_processor_id());
+}
+
+/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
+static void sync_sched_exp_online_cleanup(int cpu)
+{
+	struct rcu_data *rdp;
+	int ret;
+	struct rcu_node *rnp;
+	struct rcu_state *rsp = &rcu_state;
+
+	rdp = per_cpu_ptr(rsp->rda, cpu);
+	rnp = rdp->mynode;
+	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
+		return;
+	ret = smp_call_function_single(cpu, sync_sched_exp_handler, rsp, 0);
+	WARN_ON_ONCE(ret);
+}
+
 /*
- * Wait for an rcu-preempt grace period, but make it happen quickly.
- * But because preemptible RCU does not exist, map to rcu-sched.
- */
+ * Because a context switch is a grace period for RCU-sched, any blocking
+ * grace-period wait automatically implies a grace period if there
+ * is only one CPU online at any point time during execution of either
+ * synchronize_sched() or synchronize_rcu_bh().  It is OK to occasionally
+ * incorrectly indicate that there are multiple CPUs online when there
+ * was in fact only one the whole time, as this just adds some overhead:
+ * RCU still operates correctly.
+ */
+static int rcu_blocking_is_gp(void)
+{
+	int ret;
+
+	might_sleep();  /* Check for RCU read-side critical section. */
+	preempt_disable();
+	ret = num_online_cpus() <= 1;
+	preempt_enable();
+	return ret;
+}
+
+/* PREEMPT=n implementation of synchronize_rcu_expedited(). */
 void synchronize_rcu_expedited(void)
 {
-	synchronize_sched_expedited();
+	struct rcu_state *rsp = &rcu_state;
+
+	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
+			 lock_is_held(&rcu_lock_map) ||
+			 lock_is_held(&rcu_sched_lock_map),
+			 "Illegal synchronize_sched_expedited() in RCU read-side critical section");
+
+	/* If only one CPU, this is automatically a grace period. */
+	if (rcu_blocking_is_gp())
+		return;
+
+	_synchronize_rcu_expedited(rsp, sync_sched_exp_handler);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);