linux-kernel - Re: [RFC PATCH v3 5/6] sched: pack the idle load balance

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Date:	Fri, 5 Apr 2013 13:08:02 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Paul Turner <pjt@...gle.com>,
	Santosh <santosh.shilimkar@...com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Chander Kashyap <chander.kashyap@...aro.org>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <len.brown@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [RFC PATCH v3 5/6] sched: pack the idle load balance

Peter,

After some toughts about your comments,I can update the buddy cpu
during ILB or periofdic LB to a new idle core and extend the packing
mechanism  Does this additional mechanism sound better for you ?

Vincent

On 26 March 2013 15:42, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2013-03-26 at 15:03 +0100, Vincent Guittot wrote:
>> > But ha! here's your NO_HZ link.. but does the above DTRT and ensure
>> > that the ILB is a little core when possible?
>>
>> The loop looks for an idle CPU as close as possible to the buddy CPU
>> and the buddy CPU is the 1st CPU has been chosen. So if your buddy is
>> a little and there is an idle little, the ILB will be this idle
>> little.
>
> Earlier you wrote:
>
>>       | Cluster 0   | Cluster 1   |
>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>> -----------------------------------
>> buddy | CPU0 | CPU0 | CPU0 | CPU2 |
>
> So extrapolating that to a 4+4 big-little you'd get something like:
>
>       |   little  A9  ||   big A15     |
>       | 0 | 1 | 2 | 3 || 4 | 5 | 6 | 7 |
> ------+---+---+---+---++---+---+---+---+
> buddy | 0 | 0 | 0 | 0 || 0 | 4 | 4 | 4 |
>
> Right?
>
> So supposing the current ILB is 6, we'll only check 4, not 0-3, even
> though there might be a perfectly idle cpu in there.
>
> Also, your scheme fails to pack when cpus 0,4 are filled, even when
> there's idle cores around.
>
> If we'd use the ILB as packing cpu, we would simply select a next pack
> target once the old one fills up.
>
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