[RFC PATCH 2/2] tiny_rcu: resched when call_rcu_sched() on idle_task

[Lai Jiangshan]

For rcu_sched in UP, context-switch = QS = GP, thus we can force a
context-switch when call_rcu_sched() is happened on idle_task.
After doing so, rcu_idle/irq_enter/exit() are useless, so we can simply
make these functions empty.

More important, this change does not change the functionality logically.
Note: raise_softirq(RCU_SOFTIRQ)/rcu_sched_qs() in rcu_idle_enter() and
outmost rcu_irq_exit() will have to wake up the ksoftirqd
(due to in_interrupt() == 0).

Before this patch		After this patch:
call_rcu_sched() in idle;	call_rcu_sched() in idle
				  set resched
do other stuffs;		do other stuffs
outmost rcu_irq_exit()		outmost rcu_irq_exit() (empty function)
  (or rcu_idle_enter())		  (or rcu_idle_enter(), also empty function)
				start to resched. (see above)
  rcu_sched_qs()		rcu_sched_qs()
    QS,and GP and advance cb	  QS,and GP and advance cb
    wake up the ksoftirqd	    wake up the ksoftirqd
      set resched
resched to ksoftirqd (or other)	resched to ksoftirqd (or other)

These two code patches are almost the same.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
 kernel/rcu/rcu.h         |    6 +++
 kernel/rcu/tiny.c        |   98 +++-------------------------------------------
 kernel/rcu/tiny_plugin.h |    2 +-
 kernel/rcu/tree.c        |    6 ---
 4 files changed, 13 insertions(+), 99 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 07bb02e..80adef7 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -137,4 +137,10 @@ int rcu_jiffies_till_stall_check(void);
 
 void rcu_early_boot_tests(void);
 
+/*
+ * This function really isn't for public consumption, but RCU is special in
+ * that context switches can allow the state machine to make progress.
+ */
+extern void resched_cpu(int cpu);
+
 #endif /* __LINUX_RCU_H */
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index f8e19ac..004cfe1 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -47,54 +47,14 @@ static void __call_rcu(struct rcu_head *head,
 		       void (*func)(struct rcu_head *rcu),
 		       struct rcu_ctrlblk *rcp);
 
-static long long rcu_dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
-
 #include "tiny_plugin.h"
 
-/* Common code for rcu_idle_enter() and rcu_irq_exit(), see kernel/rcu/tree.c. */
-static void rcu_idle_enter_common(long long newval)
-{
-	if (newval) {
-		RCU_TRACE(trace_rcu_dyntick(TPS("--="),
-					    rcu_dynticks_nesting, newval));
-		rcu_dynticks_nesting = newval;
-		return;
-	}
-	RCU_TRACE(trace_rcu_dyntick(TPS("Start"),
-				    rcu_dynticks_nesting, newval));
-	if (IS_ENABLED(CONFIG_RCU_TRACE) && !is_idle_task(current)) {
-		struct task_struct *idle __maybe_unused = idle_task(smp_processor_id());
-
-		RCU_TRACE(trace_rcu_dyntick(TPS("Entry error: not idle task"),
-					    rcu_dynticks_nesting, newval));
-		ftrace_dump(DUMP_ALL);
-		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
-			  current->pid, current->comm,
-			  idle->pid, idle->comm); /* must be idle task! */
-	}
-	rcu_sched_qs();
-	barrier();
-	rcu_dynticks_nesting = newval;
-}
-
 /*
  * Enter idle, which is an extended quiescent state if we have fully
- * entered that mode (i.e., if the new value of dynticks_nesting is zero).
+ * entered that mode.
  */
 void rcu_idle_enter(void)
 {
-	unsigned long flags;
-	long long newval;
-
-	local_irq_save(flags);
-	WARN_ON_ONCE((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) == 0);
-	if ((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) ==
-	    DYNTICK_TASK_NEST_VALUE)
-		newval = 0;
-	else
-		newval = rcu_dynticks_nesting - DYNTICK_TASK_NEST_VALUE;
-	rcu_idle_enter_common(newval);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
@@ -103,55 +63,14 @@ EXPORT_SYMBOL_GPL(rcu_idle_enter);
  */
 void rcu_irq_exit(void)
 {
-	unsigned long flags;
-	long long newval;
-
-	local_irq_save(flags);
-	newval = rcu_dynticks_nesting - 1;
-	WARN_ON_ONCE(newval < 0);
-	rcu_idle_enter_common(newval);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_irq_exit);
 
-/* Common code for rcu_idle_exit() and rcu_irq_enter(), see kernel/rcu/tree.c. */
-static void rcu_idle_exit_common(long long oldval)
-{
-	if (oldval) {
-		RCU_TRACE(trace_rcu_dyntick(TPS("++="),
-					    oldval, rcu_dynticks_nesting));
-		return;
-	}
-	RCU_TRACE(trace_rcu_dyntick(TPS("End"), oldval, rcu_dynticks_nesting));
-	if (IS_ENABLED(CONFIG_RCU_TRACE) && !is_idle_task(current)) {
-		struct task_struct *idle __maybe_unused = idle_task(smp_processor_id());
-
-		RCU_TRACE(trace_rcu_dyntick(TPS("Exit error: not idle task"),
-			  oldval, rcu_dynticks_nesting));
-		ftrace_dump(DUMP_ALL);
-		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
-			  current->pid, current->comm,
-			  idle->pid, idle->comm); /* must be idle task! */
-	}
-}
-
 /*
  * Exit idle, so that we are no longer in an extended quiescent state.
  */
 void rcu_idle_exit(void)
 {
-	unsigned long flags;
-	long long oldval;
-
-	local_irq_save(flags);
-	oldval = rcu_dynticks_nesting;
-	WARN_ON_ONCE(rcu_dynticks_nesting < 0);
-	if (rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK)
-		rcu_dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
-	else
-		rcu_dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
-	rcu_idle_exit_common(oldval);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
@@ -160,15 +79,6 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
  */
 void rcu_irq_enter(void)
 {
-	unsigned long flags;
-	long long oldval;
-
-	local_irq_save(flags);
-	oldval = rcu_dynticks_nesting;
-	rcu_dynticks_nesting++;
-	WARN_ON_ONCE(rcu_dynticks_nesting == 0);
-	rcu_idle_exit_common(oldval);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_irq_enter);
 
@@ -179,7 +89,7 @@ EXPORT_SYMBOL_GPL(rcu_irq_enter);
  */
 bool notrace __rcu_is_watching(void)
 {
-	return rcu_dynticks_nesting;
+	return true;
 }
 EXPORT_SYMBOL(__rcu_is_watching);
 
@@ -352,6 +262,10 @@ static void __call_rcu(struct rcu_head *head,
 void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 {
 	__call_rcu(head, func, &rcu_sched_ctrlblk);
+	if (unlikely(is_idle_task(current))) {
+		/* force scheduling for rcu_sched_qs() */
+		resched_cpu(0);
+	}
 }
 EXPORT_SYMBOL_GPL(call_rcu_sched);
 
diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
index 858c565..b5fc5e5 100644
--- a/kernel/rcu/tiny_plugin.h
+++ b/kernel/rcu/tiny_plugin.h
@@ -147,7 +147,7 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
 	js = ACCESS_ONCE(rcp->jiffies_stall);
 	if (*rcp->curtail && ULONG_CMP_GE(j, js)) {
 		pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)\n",
-		       rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting,
+		       rcp->name, rcp->ticks_this_gp, DYNTICK_TASK_EXIT_IDLE,
 		       jiffies - rcp->gp_start, rcp->qlen);
 		dump_stack();
 	}
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3f5dd16..b5ef579 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -940,12 +940,6 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 }
 
 /*
- * This function really isn't for public consumption, but RCU is special in
- * that context switches can allow the state machine to make progress.
- */
-extern void resched_cpu(int cpu);
-
-/*
  * Return true if the specified CPU has passed through a quiescent
  * state by virtue of being in or having passed through an dynticks
  * idle state since the last call to dyntick_save_progress_counter()
-- 
1.7.4.4

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