Re: [RFC v2 PATCH 4/8] sched: Enforce hard limits by throttling

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Wed, Oct 14, 2009 at 03:18:54PM +0200, Herbert Poetzl wrote:
> On Wed, Oct 14, 2009 at 05:20:03PM +0530, Bharata B Rao wrote:
> > On Wed, Oct 14, 2009 at 11:17:44AM +0200, Peter Zijlstra wrote:
> > > On Wed, 2009-10-14 at 09:11 +0530, Bharata B Rao wrote:
> > > > On Tue, Oct 13, 2009 at 04:27:00PM +0200, Peter Zijlstra wrote:
> > > > > On Wed, 2009-09-30 at 18:22 +0530, Bharata B Rao wrote:
> > > > > 
> > > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > > > index 0f1ea4a..77ace43 100644
> > > > > > --- a/include/linux/sched.h
> > > > > > +++ b/include/linux/sched.h
> > > > > > @@ -1024,7 +1024,7 @@ struct sched_domain;
> > > > > >  struct sched_class {
> > > > > >     const struct sched_class *next;
> > > > > >  
> > > > > > -   void (*enqueue_task) (struct rq *rq, struct task_struct *p, int wakeup);
> > > > > > +   int (*enqueue_task) (struct rq *rq, struct task_struct *p, int wakeup);
> > > > > >     void (*dequeue_task) (struct rq *rq, struct task_struct *p, int sleep);
> > > > > >     void (*yield_task) (struct rq *rq);
> > > > > >  
> > > > > 
> > > > > I really hate this, it uglfies all the enqueue code in a horrid way
> > > > > (which is most of this patch).
> > > > > 
> > > > > Why can't we simply enqueue the task on a throttled group just like rt?
> > > > 
> > > > We do enqueue a task to its group even if the group is throttled. However such
> > > > throttled groups are not enqueued further. In such scenarios, even though the
> > > > task enqueue to its parent group succeeded, it really didn't add any task to
> > > > the cpu runqueue (rq). So we need to identify this condition and don't
> > > > increment rq->running. That is why this return value is needed.
> > > 
> > > I would still consider those tasks running, the fact that they don't get
> > > to run is a different matter.
> 
> > Ok, that's how rt also considers them I realize. I thought that we
> > should update rq->running when tasks go off the runqueue due to
> > throttling. When a task is throttled, it is no doubt present on its
> > group's cfs_rq, but it doesn't contribute to the CPU load as the
> > throttled group entity isn't there on any cfs_rq. rq->running is used
> > to obtain a few load balancing metrics and they might go wrong if
> > rq->running isn't uptodate.
> 
> for all practical purposes throttled tasks _are_ running
> (i.e. they would like to run, but the hardware/software
> doesn't allow them to do more work) ...

Ok, I will take a re-look at this and see if I too can consider them
as running and don't touch rq->running which should simply some code.

> 
> > Do you still think we shouldn't update rq->running ? If so, I can get rid
> > of this return value change.
> 
> Linux-VServer marked throttled tasks as 'H' (on hold)
> but counted them as running, which seems to work fine
> and reflect the expected behaviour ...

So a new task state 'H' ?

Thanks for your comments.

Regards,
Bharata.