linux-kernel - Re: [PATCH 04/11] sched: unify imbalance bias for target group

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Message-ID: <20140225141407.GT9987@twins.programming.kicks-ass.net>
Date:	Tue, 25 Feb 2014 15:14:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Shi <alex.shi@...aro.org>
Cc:	mingo@...hat.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, daniel.lezcano@...aro.org,
	fweisbec@...il.com, linux@....linux.org.uk, tony.luck@...el.com,
	fenghua.yu@...el.com, james.hogan@...tec.com, jason.low2@...com,
	viresh.kumar@...aro.org, hanjun.guo@...aro.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...ux.intel.com, pjt@...gle.com,
	fengguang.wu@...el.com, linaro-kernel@...ts.linaro.org,
	wangyun@...ux.vnet.ibm.com, mgorman@...e.de
Subject: Re: [PATCH 04/11] sched: unify imbalance bias for target group

On Tue, Feb 25, 2014 at 09:50:47AM +0800, Alex Shi wrote:
> Old code considers the bias in source/target_load already. but still
> use imbalance_pct as last check in idlest/busiest group finding. It is
> also a kind of redundant job. If we bias imbalance in source/target_load,
> we'd better not use imbalance_pct again.
> 
> After cpu_load array removed, it is nice time to unify the target bias
> consideration. So I remove the imbalance_pct from last check and add the
> live bias using.
> 
> On wake_affine, since all archs' wake_idx is 0, current logical is just
> want to prefer current cpu. so we follows this logical. Just renaming the
> target_load/source_load to wegithed_cpuload for more exact meaning.
> Thanks for reminding from Morten!
> 

So this patch is weird..

So the original bias in the source/target load is purely based on actual
load figures. It only pulls-down/pulls-up resp. the long term avg with a
shorter term average; iow. it allows the source to decrease faster and
the target to increase faster, giving a natural inertia (ie. a
resistance to movement).

Therefore this gives rise to a conservative imbalance.

Then at the end we use the imbalance_pct thing as a normal hysteresis
control to avoid the rapid state switching associated with a single
control point system.

You completely wreck that, you also don't give a coherent model back.

The movement of imbalance_pct into target_load() doesn't make sense to
me either; it's an (expensive) no-op afaict. Seeing how:

  100 * source_load() < imb_pct * target_load()

is very much equal to:

  source_load() < (imb_pct * target_load()) / 100;

Except you get to do that div all over the place.

It also completely muddles the fact that its a normal hysteresis
control. Not a load bias. A fixed bias can never replace the inertial
control we had; it doesn't make sense as a replacement.

Not to mention you seem to ignore all concerns wrt the use of longer
term averages for the bigger domains.

Now I'm all for removing code; and so far the numbers aren't bad; but I
don't like the complete muddle you make of things at all.
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