linux-kernel - Re: [PATCH v5 5/7] sched: compute runnable load avg in cpu_load and cpu_avg_load_per

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Date:	Mon, 6 May 2013 12:19:36 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
Cc:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 5/7] sched: compute runnable load avg in cpu_load and
 cpu_avg_load_per_task

On Mon, May 06, 2013 at 01:46:19AM -0700, Paul Turner wrote:
> On Sun, May 5, 2013 at 6:45 PM, Alex Shi <alex.shi@...el.com> wrote:
> > @@ -2536,7 +2536,7 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
> >  void update_idle_cpu_load(struct rq *this_rq)
> >  {
> >         unsigned long curr_jiffies = ACCESS_ONCE(jiffies);
> > -       unsigned long load = this_rq->load.weight;
> > +       unsigned long load = (unsigned long)this_rq->cfs.runnable_load_avg;
> 
> We should be minimizing:
>   Variance[ for all i ]{ cfs_rq[i]->runnable_load_avg +
> cfs_rq[i]->blocked_load_avg }
> 
> blocked_load_avg is the expected "to wake" contribution from tasks
> already assigned to this rq.
> 
> e.g. this could be:
>   load = this_rq->cfs.runnable_load_avg + this_rq->cfs.blocked_load_avg;
> 
> Although, in general I have a major concern with the current implementation:
> 
> The entire reason for stability with the bottom up averages is that
> when load migrates between cpus we are able to migrate it between the
> tracked sums.
> 
> Stuffing observed averages of these into the load_idxs loses that
> mobility; we will have to stall (as we do today for idx > 0) before we
> can recognize that a cpu's load has truly left it; this is a very
> similar problem to the need to stably track this for group shares
> computation.

Ah indeed. I overlooked that.

> To that end, I would rather see the load_idx disappear completely:
>  (a) We can calculate the imbalance purely from delta (runnable_avg +
> blocked_avg)
>  (b) It eliminates a bad tunable.

So I suspect (haven't gone back in history to verify) that load_idx mostly
comes from the fact that our balance passes happen more and more slowly the
bigger the domains get.

In that respect it makes sense to equate load_idx to sched_domain::level;
higher domains balance slower and would thus want a longer-term average to base
decisions on.

So what we would want is means to get sane longer term averages.
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