Re: [RFC][PATCH] Improving directed yield scalability for PLE handler

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2012-09-10 at 08:16 -0500, Andrew Theurer wrote:
> > > @@ -4856,8 +4859,6 @@ again:
> > >     if (curr->sched_class != p->sched_class)
> > >             goto out;
> > > 
> > > -   if (task_running(p_rq, p) || p->state)
> > > -           goto out;
> > 
> > Is it possible that by this time the current thread takes double rq
> > lock, thread p could actually be running?  i.e is there merit to keep
> > this check around even with your similar check above?
> 
> I think that's a good idea.  I'll add that back in. 

Right, it needs to still be there, the test before acquiring p_rq is an
optimistic test to avoid work, but you have to still test it once you
acquire p_rq since the rest of the code relies on this not being so.

How about something like this instead.. ?

---
 kernel/sched/core.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c46a011..c9ecab2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4300,6 +4300,23 @@ void __sched yield(void)
 }
 EXPORT_SYMBOL(yield);
 
+/*
+ * Tests preconditions required for sched_class::yield_to().
+ */
+static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p)
+{
+	if (!curr->sched_class->yield_to_task)
+		return false;
+
+	if (curr->sched_class != p->sched_class)
+		return false;
+
+	if (task_running(p_rq, p) || p->state)
+		return false;
+
+	return true;
+}
+
 /**
  * yield_to - yield the current processor to another thread in
  * your thread group, or accelerate that thread toward the
@@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 	rq = this_rq();
 
 again:
+	/* optimistic test to avoid taking locks */
+	if (!__yield_to_candidate(curr, p))
+		goto out_irq;
+
 	p_rq = task_rq(p);
 	double_rq_lock(rq, p_rq);
 	while (task_rq(p) != p_rq) {
@@ -4330,14 +4351,9 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 		goto again;
 	}
 
-	if (!curr->sched_class->yield_to_task)
-		goto out;
-
-	if (curr->sched_class != p->sched_class)
-		goto out;
-
-	if (task_running(p_rq, p) || p->state)
-		goto out;
+	/* validate state, holding p_rq ensures p's state cannot change */
+	if (!__yield_to_candidate(curr, p))
+		goto out_unlock;
 
 	yielded = curr->sched_class->yield_to_task(rq, p, preempt);
 	if (yielded) {
@@ -4350,8 +4366,9 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 			resched_task(p_rq->curr);
 	}
 
-out:
+out_unlock:
 	double_rq_unlock(rq, p_rq);
+out_irq:
 	local_irq_restore(flags);
 
 	if (yielded)