rcu: Make non-preemptive schedule be Tasks RCU quiescent state

Currently, a call to schedule() acts as a Tasks RCU quiescent state
only if a context switch actually takes place.  However, just the
call to schedule() guarantees that the calling task has moved off of
whatever tracing trampoline that it might have been one previously.
This commit therefore plumbs schedule()'s "preempt" parameter into
rcu_note_context_switch(), which then records the Tasks RCU quiescent
state, but only if this call to schedule() was -not- due to a preemption.

To avoid adding overhead to the common-case context-switch path,
this commit hides the rcu_note_context_switch() check under an existing
non-common-case check.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 20 insertions, 9 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e6146d0074f8..f531b29207da 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -363,15 +363,20 @@ static inline void rcu_init_nohz(void)
 #ifdef CONFIG_TASKS_RCU
 #define TASKS_RCU(x) x
 extern struct srcu_struct tasks_rcu_exit_srcu;
-#define rcu_note_voluntary_context_switch(t) \
+#define rcu_note_voluntary_context_switch_lite(t) \
 	do { \
-		rcu_all_qs(); \
 		if (READ_ONCE((t)->rcu_tasks_holdout)) \
 			WRITE_ONCE((t)->rcu_tasks_holdout, false); \
 	} while (0)
+#define rcu_note_voluntary_context_switch(t) \
+	do { \
+		rcu_all_qs(); \
+		rcu_note_voluntary_context_switch_lite(t); \
+	} while (0)
 #else /* #ifdef CONFIG_TASKS_RCU */
 #define TASKS_RCU(x) do { } while (0)
-#define rcu_note_voluntary_context_switch(t)	rcu_all_qs()
+#define rcu_note_voluntary_context_switch_lite(t)	do { } while (0)
+#define rcu_note_voluntary_context_switch(t)		rcu_all_qs()
 #endif /* #else #ifdef CONFIG_TASKS_RCU */
 
 /**
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 5219be250f00..74d9c3a1feee 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -92,10 +92,11 @@ static inline void kfree_call_rcu(struct rcu_head *head,
 	call_rcu(head, func);
 }
 
-static inline void rcu_note_context_switch(void)
-{
-	rcu_sched_qs();
-}
+#define rcu_note_context_switch(preempt) \
+	do { \
+		rcu_sched_qs(); \
+		rcu_note_voluntary_context_switch_lite(current); \
+	} while (0)
 
 /*
  * Take advantage of the fact that there is only one CPU, which
@@ -242,6 +243,10 @@ static inline bool rcu_is_watching(void)
 
 #endif /* #else defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */
 
+static inline void rcu_request_urgent_qs_task(struct task_struct *t)
+{
+}
+
 static inline void rcu_all_qs(void)
 {
 	barrier(); /* Avoid RCU read-side critical sections leaking across. */
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 63a4e4cf40a5..0bacb6b2af69 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -30,7 +30,7 @@
 #ifndef __LINUX_RCUTREE_H
 #define __LINUX_RCUTREE_H
 
-void rcu_note_context_switch(void);
+void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(u64 basem, u64 *nextevt);
 void rcu_cpu_stall_reset(void);
 
@@ -41,7 +41,7 @@ void rcu_cpu_stall_reset(void);
  */
 static inline void rcu_virt_note_context_switch(int cpu)
 {
-	rcu_note_context_switch();
+	rcu_note_context_switch(false);
 }
 
 void synchronize_rcu_bh(void);
@@ -108,6 +108,7 @@ void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 
 bool rcu_is_watching(void);
+void rcu_request_urgent_qs_task(struct task_struct *t);
 
 void rcu_all_qs(void);