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

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Message-ID: <20091014131854.GI24787@MAIL.13thfloor.at>
Date:	Wed, 14 Oct 2009 15:18:54 +0200
From:	Herbert Poetzl <herbert@...hfloor.at>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>
Subject: Re: [RFC v2 PATCH 4/8] sched: Enforce hard limits by throttling

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) ...

> 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 ...

best,
Herbert

> > This added return value really utterly craps up the code and I'm not
> > going to take it.
> 
> OK :) I will work towards making them more acceptable in future iterations.
> 
> > 
> > What I'm not seeing is why all this code looks so very much different
> > from the rt bits.
> 
> Throttling code here looks different than rt for the following reasons:
> 
> - As I mentioned earlier, I update rq->running during throttling which
> is not done in rt afaics.
> - There are special conditions to prevent movement of tasks in and out
> of the throttled groups during load balancing and migration.
> - rt dequeues the throttled entity by walking the entity hierachy from
> update_curr_rt(). But I found it difficult to do the same in cfs because
> update_curr() is called from many different places and from places where
> we are actually walking the entity hiearchy. A second walk (in update_curr)
> of the hiearchy while we are in the middle of a hierarchy walk didn't look
> all that good. So I resorted to just marking the entity as throttled in
> update_curr() and later doing the dequeing from put_prev_entity() ?
> Isn't this acceptable ?
> 
> Regards,
> Bharata.
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