linux-kernel - Re: [PATCH v8 2/4] sched: Rewrite runnable load and utilization average tracking

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Date:	Fri, 19 Jun 2015 15:57:24 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, pjt@...gle.com, bsegall@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, len.brown@...el.com,
	rafael.j.wysocki@...el.com, fengguang.wu@...el.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH v8 2/4] sched: Rewrite runnable load and utilization
 average tracking

Hi Yuyang,

On Fri, Jun 19, 2015 at 07:05:54AM +0800, Yuyang Du wrote:
> On Fri, Jun 19, 2015 at 02:00:38PM +0800, Boqun Feng wrote:
> > However, update_cfs_rq_load_avg() only updates cfs_rq->avg, the change
> > won't be contributed or aggregated to cfs_rq's parent in the
> > for_each_leaf_cfs_rq loop, therefore that's actually not a bottom-up
> > update.
> > 
> > To fix this, I think we can add a update_cfs_shares(cfs_rq) after
> > update_cfs_rq_load_avg(). Like:
> > 
> >  	for_each_leaf_cfs_rq(rq, cfs_rq) {
> > -		/*
> > -		 * Note: We may want to consider periodically releasing
> > -		 * rq->lock about these updates so that creating many task
> > -		 * groups does not result in continually extending hold time.
> > -		 */
> > -		__update_blocked_averages_cpu(cfs_rq->tg, rq->cpu);
> > +		/* throttled entities do not contribute to load */
> > +		if (throttled_hierarchy(cfs_rq))
> > +			continue;
> > +
> > +		update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
> > +		update_cfs_share(cfs_rq);
> >  	}
> > 
> > However, I think update_cfs_share isn't cheap, because it may do a
> > bottom-up update once called. So how about just update the root cfs_rq?
> > Like:
> > 
> > -	/*
> > -	 * Iterates the task_group tree in a bottom up fashion, see
> > -	 * list_add_leaf_cfs_rq() for details.
> > -	 */
> > -	for_each_leaf_cfs_rq(rq, cfs_rq) {
> > -		/*
> > -		 * Note: We may want to consider periodically releasing
> > -		 * rq->lock about these updates so that creating many task
> > -		 * groups does not result in continually extending hold time.
> > -		 */
> > -		__update_blocked_averages_cpu(cfs_rq->tg, rq->cpu);
> > -	}
> > +	update_cfs_rq_load_avg(rq_clock_task(rq), rq->cfs_rq);
> 
> Hi Boqun,
> 
> Did I get you right:
> 
> This rewrite patch does not NEED to aggregate entity's load to cfs_rq,
> but rather directly update the cfs_rq's load (both runnable and blocked),
> so there is NO NEED to iterate all of the cfs_rqs.

Actually, I'm not sure whether we NEED to aggregate or NOT.

> 
> So simply updating the top cfs_rq is already equivalent to the stock.
> 

The stock does have a bottom up update, so simply updating the top
cfs_rq is not equivalent to it. Simply updateing the top cfs_rq is
equivalent to the rewrite patch, because the rewrite patch lacks of the
aggregation.

> It is better if we iterate the cfs_rq to update the actually weight
> (update_cfs_share), because the weight may have already changed, which
> would in turn change the load. But update_cfs_share is not cheap.
> 
> Right?

You get me right for most part ;-)

My points are:

1. We *may not* need to aggregate entity's load to cfs_rq in
update_blocked_averages(), simply updating the top cfs_rq may be just
fine, but I'm not sure, so scheduler experts' insights are needed here.

2. Whether we need to aggregate or not, the update_blocked_averages() in
the rewrite patch could be improved. If we need to aggregate, we have to
add something like update_cfs_shares(). If we don't need, we can just
replace the loop with one update_cfs_rq_load_avg() on root cfs_rq.

I think we'd better to figure out the "may not" part in point 1 first to
get a reasonable implemenation of update_blocked_averages().

Is that clear now?

Thanks and Best Regards,
Boqun

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