Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entitieswhich exceed their local quota

[Bharata B Rao]

On Thu, Feb 24, 2011 at 12:05:01PM +0100, Peter Zijlstra wrote:
> On Thu, 2011-02-24 at 10:51 +0530, Bharata B Rao wrote:
> > Hi Peter,
> > 
> > I will only answer a couple of your questions and let Paul clarify the rest...
> > 
> > On Wed, Feb 23, 2011 at 02:32:13PM +0100, Peter Zijlstra wrote:
> > > On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote:
> > > 
> > > 
> > > > @@ -1363,6 +1407,9 @@ enqueue_task_fair(struct rq *rq, struct 
> > > >  			break;
> > > >  		cfs_rq = cfs_rq_of(se);
> > > >  		enqueue_entity(cfs_rq, se, flags);
> > > > +		/* don't continue to enqueue if our parent is throttled */
> > > > +		if (cfs_rq_throttled(cfs_rq))
> > > > +			break;
> > > >  		flags = ENQUEUE_WAKEUP;
> > > >  	}
> > > >  
> > > > @@ -1390,8 +1437,11 @@ static void dequeue_task_fair(struct rq 
> > > >  		cfs_rq = cfs_rq_of(se);
> > > >  		dequeue_entity(cfs_rq, se, flags);
> > > >  
> > > > -		/* Don't dequeue parent if it has other entities besides us */
> > > > -		if (cfs_rq->load.weight)
> > > > +		/*
> > > > +		 * Don't dequeue parent if it has other entities besides us,
> > > > +		 * or if it is throttled
> > > > +		 */
> > > > +		if (cfs_rq->load.weight || cfs_rq_throttled(cfs_rq))
> > > >  			break;
> > > >  		flags |= DEQUEUE_SLEEP;
> > > >  	}
> > > 
> > > How could we even be running if our parent was throttled?
> > 
> > The task isn't running actually. One of its parents up in the heirarchy has
> > been throttled and been already dequeued. Now this task sits on its immediate
> > parent's runqueue which isn't throttled but not really running also since
> > the hierarchy is throttled. In this situation, load balancer can try to pull
> > this task. When that happens, load balancer tries to dequeue it and this
> > check will ensure that we don't attempt to dequeue a group entity in our
> > hierarchy which has already been dequeued.
> 
> That's insane, its throttled, that means it should be dequeued and
> should thus invisible for the load-balancer. If it is visible the
> load-balancer will try and move tasks around to balance load, but all in
> vain, it'll move phantom loads around and get most confused at best.

We can't walk the se hierarchy downwards and hence can't really dequeue
the entire hierarchy if any one entity in the hierarchy is throttled.
However this semantics of retaining the child entities enqueued while
dequeuing the entities upwards of a throttled entity makes our life simple
during unthrottling. We just have to enqueue the entities upwards the
throttled entity and the rest of the entities downwards automatically become
available.

While I admit that our load balancing semantics wrt thorttled entities are
not consistent (we don't allow pulling of tasks directly from throttled
cfs_rqs, while allow pulling of tasks from a throttled hierarchy as in the
above case), I am beginning to think if it works out to be advantageous.
Is there a chance that the task gets to run on other CPU where the hierarchy
isn't throttled since runtime is still available ?

> 
> Pure and utter suckage if you ask me.
> 
> > > > @@ -1438,10 +1524,16 @@ static void account_cfs_rq_quota(struct 
> > > >  
> > > >  	cfs_rq->quota_used += delta_exec;
> > > >  
> > > > -	if (cfs_rq->quota_used < cfs_rq->quota_assigned)
> > > > +	if (cfs_rq_throttled(cfs_rq) ||
> > > > +		cfs_rq->quota_used < cfs_rq->quota_assigned)
> > > >  		return;
> > > 
> > > So we are throttled but running anyway, I suppose this comes from the PI
> > > ceiling muck?
> > 
> > When a cfs_rq is throttled, its representative se (and all its parent
> > se's) get dequeued and the task is marked for resched. But the task entity is
> > still on its throttled parent's cfs_rq (=> task->se.on_rq = 1). Next during
> > put_prev_task_fair(), we enqueue the task back on its throttled parent's
> > cfs_rq at which time we end up calling update_curr() on throttled cfs_rq.
> > This check would help us bail out from that situation.
> 
> But why bother with this early exit? At worst you'll call
> tg_request_cfs_quota() in vain, at best you'll find there is runtime
> because the period tick just happened on another cpu and you're good to
> go, yay!

I see your point. I had this check in my version of hard limits patches earlier
for the reason I described above. Lets see if Paul had any other reason
to retain this check.

Regards,
Bharata.
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