On 09/11/2012 01:42 AM, Andrew Theurer wrote:

On Mon, 2012-09-10 at 19:12 +0200, Peter Zijlstra wrote:

quoted

On Mon, 2012-09-10 at 22:26 +0530, Srikar Dronamraju wrote:

quoted

quoted

+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;

Peter,

Should we also add a check if the runq has a skip buddy (as pointed out
by Raghu) and return if the skip buddy is already set.

Oh right, I missed that suggestion.. the performance improvement went
from 81% to 139% using this, right?

It might make more sense to keep that separate, outside of this
function, since its not a strict prerequisite.

quoted

quoted

+     if (task_running(p_rq, p) || p->state)
+             return false;
+
+     return true;
+}

quoted

quoted

@@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p,

bool preempt)

quoted

       rq = this_rq();

  again:
+     /* optimistic test to avoid taking locks */
+     if (!__yield_to_candidate(curr, p))
+             goto out_irq;
+

So add something like:

	/* Optimistic, if we 'raced' with another yield_to(), don't bother */
	if (p_rq->cfs_rq->skip)
		goto out_irq;

quoted

quoted

       p_rq = task_rq(p);
       double_rq_lock(rq, p_rq);

But I do have a question on this optimization though,.. Why do we check
p_rq->cfs_rq->skip and not rq->cfs_rq->skip ?

That is, I'd like to see this thing explained a little better.

Does it go something like: p_rq is the runqueue of the task we'd like to
yield to, rq is our own, they might be the same. If we have a ->skip,
there's nothing we can do about it, OTOH p_rq having a ->skip and
failing the yield_to() simply means us picking the next VCPU thread,
which might be running on an entirely different cpu (rq) and could
succeed?

Here's two new versions, both include a __yield_to_candidate(): "v3"
uses the check for p_rq->curr in guest mode, and "v4" uses the cfs_rq
skip check.  Raghu, I am not sure if this is exactly what you want
implemented in v4.

Andrew, Yes that is what I had. I think there was a mis-understanding. 
My intention was to if there is a directed_yield happened in runqueue 
(say rqA), do not bother to directed yield to that. But unfortunately as 
PeterZ pointed that would have resulted in setting next buddy of a 
different run queue than rqA.
So we can drop this "skip" idea. Pondering more over what to do? can we 
use next buddy itself ... thinking..