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

From: Bharata B Rao
Date: Fri Feb 25 2011 - 08:59:16 EST

On Thu, Feb 24, 2011 at 07:10:58PM -0800, Paul Turner wrote:
> On Wed, Feb 23, 2011 at 5:32 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> > On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote:
> >> +     update_cfs_load(cfs_rq, 0);
> >> +
> >> +     /* prevent previous buddy nominations from re-picking this se */
> >> +     clear_buddies(cfs_rq_of(se), se);
> >> +
> >> +     /*
> >> +      * It's possible for the current task to block and re-wake before task
> >> +      * switch, leading to a throttle within enqueue_task->update_curr()
> >> +      * versus an an entity that has not technically been enqueued yet.
> >
> > I'm not quite seeing how this would happen.. care to expand on this?
> >
> I'm not sure the example Bharata gave is correct -- I'm going to treat
> that discussion separately as it's not the intent here.

Just for the record, my examples were not given for the above question from

I answered two questions and I am tempted to stand by those until proven
wrong :)

1. Why do we have cfs_rq_throtted() check in dequeue_task_fair() ?
( => How could we be running if our parent was throttled ?)

Consider the following hierarchy.

Root Group
Group 1 (Bandwidth constrained group)
Group 2 (Infinite runtime group)

Assume both the groups have tasks in them.

When Group 1 is throttled, its cfs_rq is marked throttled, and is removed from
Root group's runqueue. But leaf tasks in Group 2 continue to be enqueued in
Group 1's runqueue.

Load balancer kicks in on CPU A and figures out that it can pull a few tasks
from CPU B (busiest_cpu). It iterates through all the task groups
(load_balance_fair) and considers Group 2 also. It tries to pull a task from
CPU B's cfs_rq for Group 2. I don't see anything that would prevent the
load balancer from bailing out here. Note that Group 2 is technically
not throttled, only its parent Group 1 is. Load balancer goes ahead and
starts pulling individual tasks from Group 2's cfs_rq on CPU B. This
results in dequeuing of task whose hierarchy is throttled.

When load balancer iterates through Group 1's cfs_rqs, the situation is
different because we have already marked Group 1's cfs_rqs as throttled.
And we check this in load_balance_fair() and bail out from pulling tasks
from throttled hierarchy.

This is my understanding. Let me know what I miss. Specifically I would
like to understand how do you ensure that load balancer doesn't consider
tasks from throttled cfs_rqs for pulling.

2. Why there is cfs_rq_throttled() check in account_cfs_rq_quota() ?

In addition to the case you described, I believe the situation I described
is also valid.

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