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

[Boqun Feng]

Hi Yuyang,

On Fri, Jun 19, 2015 at 11:11:16AM +0800, Yuyang Du wrote:
> On Fri, Jun 19, 2015 at 03:57:24PM +0800, Boqun Feng wrote:
> > > 
> > > 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.
> > > 
> 
> Ok. By aggregate, the rewrite patch does not need it, because the cfs_rq's
> load is calculated at once with all its runnable and blocked tasks counted,
> assuming the all children's weights are up-to-date, of course. Please refer
> to the changelog to get an idea.
> 
> > 
> > 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 not the rewrite patch "lacks" aggregation, it is needless. The stock
> has to do a bottom-up update and aggregate, because 1) it updates the
> load at an entity granularity, 2) the blocked load is separate.

Yep, you are right, the aggregation is not necessary.

Let me see if I understand you, in the rewrite, when we
update_cfs_rq_load_avg() we need neither to aggregate child's load_avg,
nor to update cfs_rq->load.weight. Because:

1) For the load before cfs_rq->last_update_time, it's already in the
->load_avg, and decay will do the job.
2) For the load from cfs_rq->last_update_time to now, we calculate
with cfs_rq->load.weight, and the weight should be weight at
->last_update_time rather than now.

Right?

> 
> > > 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.
>  
> Then I don't need to say anything about this.
> 
> > 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.
>  
> If update_cfs_shares() is done here, it is good, but probably not necessary
> though. However, we do need to update_tg_load_avg() here, because if cfs_rq's

We may have another problem even we udpate_tg_load_avg(), because after
the loop, for each cfs_rq, ->load.weight is not up-to-date, right? So
next time before we update_cfs_rq_load_avg(), we need to guarantee that
the cfs_rq->load.weight is already updated, right? And IMO, we don't
have that guarantee yet, do we?

> load change, the parent tg's load_avg should change too. I will upload a next
> version soon.
> 
> In addition, an update to the stress + dbench test case:
> 
> I have a Core i7, not a Xeon Nehalem, and I have a patch that may not impact
> the result. Then, the dbench runs at very low CPU utilization ~1%. Boqun said
> this may result from cgroup control, the dbench I/O is low.
> 
> Anyway, I can't reproduce the results, the CPU0's util is 92+%, and other CPUs
> have ~100% util.

Thank you for looking into that problem, and I will test with your new
version of patch ;-)

Thanks,
Boqun

> 
> Thanks,
> Yuyang

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