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>

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 43b39d6..88e466b 100644 (file)
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -705,6 +705,7 @@ static void stack_proc(void *arg)
        struct task_struct *from = current, *to = arg;
 
        to->thread.saved_task = from;
+       rcu_switch_from(from);
        switch_to(from, to, from);
 }
 

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 20fb776..bbfe785 100644 (file)
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -184,6 +184,7 @@ static inline int rcu_preempt_depth(void)
 /* Internal to kernel */
 extern void rcu_sched_qs(int cpu);
 extern void rcu_bh_qs(int cpu);
+extern void rcu_preempt_note_context_switch(void);
 extern void rcu_check_callbacks(int cpu, int user);
 struct notifier_block;
 extern void rcu_idle_enter(void);

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e93df77..080b5bd 100644 (file)
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -87,10 +87,6 @@ static inline void kfree_call_rcu(struct rcu_head *head,
 
 #ifdef CONFIG_TINY_RCU
 
-static inline void rcu_preempt_note_context_switch(void)
-{
-}
-
 static inline void exit_rcu(void)
 {
 }
@@ -102,7 +98,6 @@ static inline int rcu_needs_cpu(int cpu)
 
 #else /* #ifdef CONFIG_TINY_RCU */
 
-void rcu_preempt_note_context_switch(void);
 extern void exit_rcu(void);
 int rcu_preempt_needs_cpu(void);
 
@@ -116,7 +111,6 @@ static inline int rcu_needs_cpu(int cpu)
 static inline void rcu_note_context_switch(int cpu)
 {
        rcu_sched_qs(cpu);
-       rcu_preempt_note_context_switch();
 }
 
 /*

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 81a173c..8f3fd94 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1905,12 +1905,22 @@ static inline void rcu_copy_process(struct task_struct *p)
        INIT_LIST_HEAD(&p->rcu_node_entry);
 }
 
+static inline void rcu_switch_from(struct task_struct *prev)
+{
+       if (prev->rcu_read_lock_nesting != 0)
+               rcu_preempt_note_context_switch();
+}
+
 #else
 
 static inline void rcu_copy_process(struct task_struct *p)
 {
 }
 
+static inline void rcu_switch_from(struct task_struct *prev)
+{
+}
+
 #endif
 
 #ifdef CONFIG_SMP

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d..6135150 100644 (file)
--- 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 cdd1be0..d6b70b0 100644 (file)
--- 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 c023464..b1ac22e 100644 (file)
--- 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 4603b9d..5d89eb9 100644 (file)
--- 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();