linux-kernel - Re: [RFC PATCH v2 3/6] sched: pack small tasks

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Date:	Thu, 13 Dec 2012 16:48:23 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-dev@...ts.linaro.org, peterz@...radead.org,
	mingo@...nel.org, linux@....linux.org.uk, pjt@...gle.com,
	santosh.shilimkar@...com, Morten.Rasmussen@....com,
	chander.kashyap@...aro.org, cmetcalf@...era.com,
	tony.luck@...el.com, preeti@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	len.brown@...el.com, arjan@...ux.intel.com,
	amit.kucheria@...aro.org, viresh.kumar@...aro.org
Subject: Re: [RFC PATCH v2 3/6] sched: pack small tasks

On 13 December 2012 15:53, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> On 13 December 2012 15:25, Alex Shi <alex.shi@...el.com> wrote:
>> On 12/13/2012 06:11 PM, Vincent Guittot wrote:
>>> On 13 December 2012 03:17, Alex Shi <alex.shi@...el.com> wrote:
>>>> On 12/12/2012 09:31 PM, Vincent Guittot wrote:
>>>>> During the creation of sched_domain, we define a pack buddy CPU for each CPU
>>>>> when one is available. We want to pack at all levels where a group of CPU can
>>>>> be power gated independently from others.
>>>>> On a system that can't power gate a group of CPUs independently, the flag is
>>>>> set at all sched_domain level and the buddy is set to -1. This is the default
>>>>> behavior.
>>>>> On a dual clusters / dual cores system which can power gate each core and
>>>>> cluster independently, the buddy configuration will be :
>>>>>
>>>>>       | Cluster 0   | Cluster 1   |
>>>>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>>>>> -----------------------------------
>>>>> buddy | CPU0 | CPU0 | CPU0 | CPU2 |
>>>>>
>>>>> Small tasks tend to slip out of the periodic load balance so the best place
>>>>> to choose to migrate them is during their wake up. The decision is in O(1) as
>>>>> we only check again one buddy CPU
>>>>
>>>> Just have a little worry about the scalability on a big machine, like on
>>>> a 4 sockets NUMA machine * 8 cores * HT machine, the buddy cpu in whole
>>>> system need care 64 LCPUs. and in your case cpu0 just care 4 LCPU. That
>>>> is different on task distribution decision.
>>>
>>> The buddy CPU should probably not be the same for all 64 LCPU it
>>> depends on where it's worth packing small tasks
>>
>> Do you have further ideas for buddy cpu on such example?
>
> yes, I have several ideas which were not really relevant for small
> system but could be interesting for larger system
>
> We keep the same algorithm in a socket but we could either use another
> LCPU in the targeted socket (conf0) or chain the socket (conf1)
> instead of packing directly in one LCPU
>
> The scheme below tries to summaries the idea:
>
> Socket      | socket 0 | socket 1   | socket 2   | socket 3   |
> LCPU        | 0 | 1-15 | 16 | 17-31 | 32 | 33-47 | 48 | 49-63 |
> buddy conf0 | 0 | 0    | 1  | 16    | 2  | 32    | 3  | 48    |
> buddy conf1 | 0 | 0    | 0  | 16    | 16 | 32    | 32 | 48    |
> buddy conf2 | 0 | 0    | 16 | 16    | 32 | 32    | 48 | 48    |
>
> But, I don't know how this can interact with NUMA load balance and the
> better might be to use conf3.

I mean conf2 not conf3

>
>>>
>>> Which kind of sched_domain configuration have you for such system ?
>>> and how many sched_domain level have you ?
>>
>> it is general X86 domain configuration. with 4 levels,
>> sibling/core/cpu/numa.
>>>
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