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

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Message-ID: <20150621224306.GC3933@intel.com>
Date:	Mon, 22 Jun 2015 06:43:06 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Boqun Feng <boqun.feng@...il.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

On Fri, Jun 19, 2015 at 08:22:07PM +0800, Boqun Feng wrote:
> > 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?
 
Yes.

> > 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?

If we update weight, we must update load_avg. But if we update load_avg, we may need
to update weight. Yes, your comment here is valid, but we already update the shares
as needed in the cases when they are "active", update_blocked_averages() is
largely for inactive group entities, so we should be fine here.
 
> > 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 ;-)
 
That would be good. I played the dbench "as is", and its output looks pretty fine.

Thanks,
Yuyang
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