rcu: Move PREEMPT_RCU preemption to switch_to() invocation

Currently, PREEMPT_RCU readers are enqueued upon entry to the scheduler.
This is inefficient because enqueuing is required only if there is a
context switch, and entry to the scheduler does not guarantee a context
switch.

The commit therefore moves the enqueuing to immediately precede the
call to switch_to() from the scheduler.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 4 insertions, 13 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d3922c..61351505ec78 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -192,7 +192,6 @@ void rcu_note_context_switch(int cpu)
 {
         trace_rcu_utilization("Start context switch");
         rcu_sched_qs(cpu);
-        rcu_preempt_note_context_switch(cpu);
         trace_rcu_utilization("End context switch");
 }
 EXPORT_SYMBOL_GPL(rcu_note_context_switch);
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index cdd1be0a4072..d6b70b08a01a 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -423,7 +423,6 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
 /* Forward declarations for rcutree_plugin.h */
 static void rcu_bootup_announce(void);
 long rcu_batches_completed(void);
-static void rcu_preempt_note_context_switch(int cpu);
 static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
 #ifdef CONFIG_HOTPLUG_CPU
 static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp,
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c023464816be..b1ac22e6fa31 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -153,7 +153,7 @@ static void rcu_preempt_qs(int cpu)
  *
  * Caller must disable preemption.
  */
-static void rcu_preempt_note_context_switch(int cpu)
+void rcu_preempt_note_context_switch(void)
 {
         struct task_struct *t = current;
         unsigned long flags;
@@ -164,7 +164,7 @@ static void rcu_preempt_note_context_switch(int cpu)
             (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
 
                 /* Possibly blocking in an RCU read-side critical section. */
-                rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
+                rdp = __this_cpu_ptr(rcu_preempt_state.rda);
                 rnp = rdp->mynode;
                 raw_spin_lock_irqsave(&rnp->lock, flags);
                 t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
@@ -228,7 +228,7 @@ static void rcu_preempt_note_context_switch(int cpu)
          * means that we continue to block the current grace period.
          */
         local_irq_save(flags);
-        rcu_preempt_qs(cpu);
+        rcu_preempt_qs(smp_processor_id());
         local_irq_restore(flags);
 }
 
@@ -1018,14 +1018,6 @@ void rcu_force_quiescent_state(void)
 EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
 
 /*
- * Because preemptible RCU does not exist, we never have to check for
- * CPUs being in quiescent states.
- */
-static void rcu_preempt_note_context_switch(int cpu)
-{
-}
-
-/*
  * Because preemptible RCU does not exist, there are never any preempted
  * RCU readers.
  */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4603b9d8f30a..5d89eb93f7e4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2083,6 +2083,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 #endif
 
         /* Here we just switch the register state and the stack. */
+        rcu_switch_from(prev);
         switch_to(prev, next, prev);
 
         barrier();