linux-kernel - Re: [ PATCH 0/8] sched: remove cpu

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Date:	Mon, 17 Mar 2014 11:24:46 +0800
From:	Alex Shi <alex.shi@...aro.org>
To:	mingo@...hat.com, peterz@...radead.org, morten.rasmussen@....com
CC:	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, alex.shi@...aro.org,
	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 0/8] sched: remove cpu_load array

On 03/13/2014 01:57 PM, Alex Shi wrote:
> In the cpu_load decay usage, we mixed the long term, short term load with
> balance bias, randomly pick a big/small value from them according to balance 
> destination or source. This mix is wrong, the balance bias should be based
> on task moving cost between cpu groups, not on random history or instant load.
> History load maybe diverage a lot from real load, that lead to incorrect bias.
> 
> In fact, the cpu_load decays can be replaced by the sched_avg decay, that 
> also decays load on time. The balance bias part can fullly use fixed bias --
> imbalance_pct, which is already used in newly idle, wake, forkexec balancing
> and numa balancing scenarios.
> 
> Currently the only working idx is busy_idx and idle_idx.
> As to busy_idx:
> We mix history load decay and bias together. The ridiculous thing is, when 
> all cpu load are continuous stable, long/short term load is same. then we 
> lose the bias meaning, so any minimum imbalance may cause unnecessary task
> moving. To prevent this funny thing happen, we have to reuse the 
> imbalance_pct again in find_busiest_group().  But that clearly causes over
> bias in normal time. If there are some burst load in system, it is more worse.
> 

Any comments?

> As to idle_idx:
> Though I have some cencern of usage corretion, 
> https://lkml.org/lkml/2014/3/12/247, but since we are working on cpu
> idle migration into scheduler. The problem will be reconsidered. We don't
> need to care it now.
> 
> This patch removed the cpu_load idx decay, since it can be replaced by
> sched_avg feature. and left the imbalance_pct bias untouched, since only 
> idle_idx missed it, but it is fine. and will be reconsidered soon.
> 
> 
> V5,
> 1, remove unify bias patch and biased_load function. Thanks for PeterZ's 
> comments!
> 2, remove get_sd_load_idx() in the 1st patch as SrikarD's suggestion.
> 3, remove LB_BIAS feature, it is not needed now.


-- 
Thanks
    Alex
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