sched: Enhance the pre/post scheduling logic

We currently have an explicit "needs_post" vtable method which
returns a stack variable for whether we should later run
post-schedule.  This leads to an awkward exchange of the
variable as it bubbles back up out of the context switch. Peter
Zijlstra observed that this information could be stored in the
run-queue itself instead of handled on the stack.

Therefore, we revert to the method of having context_switch
return void, and update an internal rq->post_schedule variable
when we require further processing.

In addition, we fix a race condition where we try to access
current->sched_class without holding the rq->lock.  This is
technically racy, as the sched-class could change out from
under us.  Instead, we reference the per-rq post_schedule
variable with the runqueue unlocked, but with preemption
disabled to see if we need to reacquire the rq->lock.

Finally, we clean the code up slightly by removing the #ifdef
CONFIG_SMP conditionals from the schedule() call, and implement
some inline helper functions instead.

This patch passes checkpatch, and rt-migrate.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090729150422.17691.55590.stgit@dev.haskins.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -616,6 +616,7 @@ struct rq {
 
 	unsigned char idle_at_tick;
 	/* For active balancing */
+	int post_schedule;
 	int active_balance;
 	int push_cpu;
 	/* cpu of this runqueue: */
@@ -2839,17 +2840,11 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
  * with the lock held can cause deadlocks; see schedule() for
  * details.)
  */
-static int finish_task_switch(struct rq *rq, struct task_struct *prev)
+static void finish_task_switch(struct rq *rq, struct task_struct *prev)
 	__releases(rq->lock)
 {
 	struct mm_struct *mm = rq->prev_mm;
 	long prev_state;
-	int post_schedule = 0;
-
-#ifdef CONFIG_SMP
-	if (current->sched_class->needs_post_schedule)
-		post_schedule = current->sched_class->needs_post_schedule(rq);
-#endif
 
 	rq->prev_mm = NULL;
 
@@ -2880,10 +2875,44 @@ static int finish_task_switch(struct rq *rq, struct task_struct *prev)
 		kprobe_flush_task(prev);
 		put_task_struct(prev);
 	}
-
-	return post_schedule;
 }
 
+#ifdef CONFIG_SMP
+
+/* assumes rq->lock is held */
+static inline void pre_schedule(struct rq *rq, struct task_struct *prev)
+{
+	if (prev->sched_class->pre_schedule)
+		prev->sched_class->pre_schedule(rq, prev);
+}
+
+/* rq->lock is NOT held, but preemption is disabled */
+static inline void post_schedule(struct rq *rq)
+{
+	if (rq->post_schedule) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&rq->lock, flags);
+		if (rq->curr->sched_class->post_schedule)
+			rq->curr->sched_class->post_schedule(rq);
+		spin_unlock_irqrestore(&rq->lock, flags);
+
+		rq->post_schedule = 0;
+	}
+}
+
+#else
+
+static inline void pre_schedule(struct rq *rq, struct task_struct *p)
+{
+}
+
+static inline void post_schedule(struct rq *rq)
+{
+}
+
+#endif
+
 /**
  * schedule_tail - first thing a freshly forked thread must call.
  * @prev: the thread we just switched away from.
@@ -2892,14 +2921,14 @@ asmlinkage void schedule_tail(struct task_struct *prev)
 	__releases(rq->lock)
 {
 	struct rq *rq = this_rq();
-	int post_schedule;
 
-	post_schedule = finish_task_switch(rq, prev);
+	finish_task_switch(rq, prev);
 
-#ifdef CONFIG_SMP
-	if (post_schedule)
-		current->sched_class->post_schedule(rq);
-#endif
+	/*
+	 * FIXME: do we need to worry about rq being invalidated by the
+	 * task_switch?
+	 */
+	post_schedule(rq);
 
 #ifdef __ARCH_WANT_UNLOCKED_CTXSW
 	/* In this case, finish_task_switch does not reenable preemption */
@@ -2913,7 +2942,7 @@ asmlinkage void schedule_tail(struct task_struct *prev)
  * context_switch - switch to the new MM and the new
  * thread's register state.
  */
-static inline int
+static inline void
 context_switch(struct rq *rq, struct task_struct *prev,
 	       struct task_struct *next)
 {
@@ -2960,7 +2989,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 	 * CPUs since it called schedule(), thus the 'rq' on its stack
 	 * frame will be invalid.
 	 */
-	return finish_task_switch(this_rq(), prev);
+	finish_task_switch(this_rq(), prev);
 }
 
 /*
@@ -5371,7 +5400,6 @@ asmlinkage void __sched schedule(void)
 {
 	struct task_struct *prev, *next;
 	unsigned long *switch_count;
-	int post_schedule = 0;
 	struct rq *rq;
 	int cpu;
 
@@ -5403,10 +5431,7 @@ need_resched_nonpreemptible:
 		switch_count = &prev->nvcsw;
 	}
 
-#ifdef CONFIG_SMP
-	if (prev->sched_class->pre_schedule)
-		prev->sched_class->pre_schedule(rq, prev);
-#endif
+	pre_schedule(rq, prev);
 
 	if (unlikely(!rq->nr_running))
 		idle_balance(cpu, rq);
@@ -5422,25 +5447,17 @@ need_resched_nonpreemptible:
 		rq->curr = next;
 		++*switch_count;
 
-		post_schedule = context_switch(rq, prev, next); /* unlocks the rq */
+		context_switch(rq, prev, next); /* unlocks the rq */
 		/*
 		 * the context switch might have flipped the stack from under
 		 * us, hence refresh the local variables.
 		 */
 		cpu = smp_processor_id();
 		rq = cpu_rq(cpu);
-	} else {
-#ifdef CONFIG_SMP
-		if (current->sched_class->needs_post_schedule)
-			post_schedule = current->sched_class->needs_post_schedule(rq);
-#endif
+	} else
 		spin_unlock_irq(&rq->lock);
-	}
 
-#ifdef CONFIG_SMP
-	if (post_schedule)
-		current->sched_class->post_schedule(rq);
-#endif
+	post_schedule(rq);
 
 	if (unlikely(reacquire_kernel_lock(current) < 0))
 		goto need_resched_nonpreemptible;
@@ -9403,6 +9420,7 @@ void __init sched_init(void)
 #ifdef CONFIG_SMP
 		rq->sd = NULL;
 		rq->rd = NULL;
+		rq->post_schedule = 0;
 		rq->active_balance = 0;
 		rq->next_balance = jiffies;
 		rq->push_cpu = 0;