linux-kernel - Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

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Message-ID: <20140107125930.GW31570@twins.programming.kicks-ass.net>
Date:	Tue, 7 Jan 2014 13:59:30 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Alex Shi <alex.shi@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"jason.low2@...com" <jason.low2@...com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

On Tue, Jan 07, 2014 at 12:55:18PM +0000, Morten Rasmussen wrote:
> My understanding is that should_we_balance() decides which cpu is
> eligible for doing the load balancing for a given domain (and the
> domains above). That is, only one cpu in a group is allowed to load
> balance between the local group and other groups. That cpu would
> therefore be reponsible for pulling enough load that the groups are
> balanced even if it means temporarily overloading itself. The other cpus
> in the group will take care of load balancing the extra load within the
> local group later.

Correct.

> I may have missed something, but I don't understand the reason for the
> performance improvements that you are reporting. I see better numbers
> for a few benchmarks, but I still don't understand why the code makes
> sense after the cleanup. If we don't understand why it works, we cannot
> be sure that it doesn't harm other benchmarks. There is always a chance
> that we miss something but, IMHO, not having any idea to begin with
> increases the chances for problems later significantly. So why not get
> to the bottom of the problem of cleaning up cpu_load?
> 
> Have you done more extensive benchmarking? Have you seen any regressions
> in other benchmarks?

I only remember hackbench numbers and that generally fares well with a
more aggressive balancer since it has no actual work to speak of the
migration penalty is very low and because there's a metric ton of tasks
the aggressive leveling makes for more coherent 'throughput'.
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