Re: [PATCH 7/7 v3] sched: fix wrong utilization accounting when switching to fair class

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 16, 2016 at 04:23:16PM +0200, Vincent Guittot wrote:
> On 16 September 2016 at 14:16, Peter Zijlstra <peterz@infradead.org> wrote:

> >> > Also, the normalize comment in dequeue_entity() worries me, 'someone'
> >> > didn't update that when he moved update_min_vruntime() around.
> >
> > I now worry more, so we do:
> >
> >         dequeue_task := dequeue_task_fair (p == current)
> >           dequeue_entity
> >             update_curr()
> >               update_min_vruntime()
> >             vruntime -= min_vruntime
> >             update_min_vruntime()
> >               // use cfs_rq->curr, which we just normalized !
> 
> yes but does it really change the cfs_rq->min_vruntime in this case ?

So let me see; it does:

	vruntime = cfs_rq->min_vruntime;

	if (curr) // true
	  vruntime = curr->vruntime; // == vruntime - min_vruntime

	if (leftmost) // possible
	  if (curr) // true
	    vruntime = min_vruntime(vruntime, se->vruntime);
	      if (se->vruntime - (curr->vruntime - min_vruntime)) < 0 // false

	min_vruntime = max_vruntime(min_vruntime, vruntime);
	  if ((curr->vruntime - min_vruntime) - min_vruntime) > 0)


The problem is that double subtraction of min_vruntime can wrap.
The thing is, min_vruntime is the 0-point in our modular space, it
normalizes vruntime (ideally min_vruntime would be our 0-lag point,
resulting in vruntime - min_vruntime being the lag).

The moment min_vruntime grows past S64_MAX/2 -2*min_vruntime wraps into
positive space again and the test above becomes true and we'll select
the normalized @curr vruntime as new min_vruntime and weird stuff will
happen.


Also, even it things magically worked out, its still very icky to mix
the normalized vruntime into things.

> >         put_prev_task := put_prev_task_fair
> >           put_prev_entity
> >             cfs_rq->curr = NULL;
> >
> >
> > Now the point of the latter update_min_vruntime() is to advance
> > min_vruntime when the task we removed was the one holding it back.
> >
> > However, it means that if we do dequeue+enqueue, we're further in the
> > future (ie. we get penalized).
> >
> > So I'm inclined to simply remove the (2nd) update_min_vruntime() call.
> > But as said above, my brain isn't co-operating much today.

OK, so not sure we can actually remove it, we do want it to move
min_vruntime forward (sometimes). We just don't want it to do so when
DEQUEUE_SAVE -- we want to get back where we left off, nor do we want to
muck about with touching normalized values.

Another fun corner case is DEQUEUE_SLEEP; in that case we do not
normalize, but we still want advance min_vruntime if this was the one
holding it back.

I ended up with the below, but I'm not sure I like it much. Let me prod
a wee bit more to see if there's not something else we can do.

Google has this patch-set replacing min_vruntime with an actual global
0-lag, which greatly simplifies things. If only they'd post it sometime
:/ /me prods pjt and ben with a sharp stick :-)

---
 kernel/sched/fair.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 986c10c25176..77566a340cbf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -462,17 +462,23 @@ static inline int entity_before(struct sched_entity *a,
 
 static void update_min_vruntime(struct cfs_rq *cfs_rq)
 {
+	struct sched_entity *curr = cfs_rq->curr;
+
 	u64 vruntime = cfs_rq->min_vruntime;
 
-	if (cfs_rq->curr)
-		vruntime = cfs_rq->curr->vruntime;
+	if (curr) {
+		if (curr->on_rq)
+			vruntime = curr->vruntime;
+		else
+			curr = NULL;
+	}
 
 	if (cfs_rq->rb_leftmost) {
 		struct sched_entity *se = rb_entry(cfs_rq->rb_leftmost,
 						   struct sched_entity,
 						   run_node);
 
-		if (!cfs_rq->curr)
+		if (!curr)
 			vruntime = se->vruntime;
 		else
 			vruntime = min_vruntime(vruntime, se->vruntime);
@@ -3483,8 +3489,16 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	/* return excess runtime on last dequeue */
 	return_cfs_rq_runtime(cfs_rq);
 
-	update_min_vruntime(cfs_rq);
 	update_cfs_shares(cfs_rq);
+
+	/*
+	 * Now advance min_vruntime if @se was the entity holding it back,
+	 * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case we'll be
+	 * put back on, and if we advance min_vruntime, we'll be placed back
+	 * further than we started -- ie. we'll be penalized.
+	 */
+	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
+		update_min_vruntime(cfs_rq);
 }
 
 /*