linux-kernel - Re: [RFC PATCH] cpuidle: allow per cpu latencies

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Date:	Fri, 06 Apr 2012 17:35:46 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	"Shilimkar, Santosh" <santosh.shilimkar@...com>
CC:	Peter De Schrijver <pdeschrijver@...dia.com>,
	Kevin Hilman <khilman@...com>, Len Brown <len.brown@...el.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	linux-kernel@...r.kernel.org,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	linux-tegra@...r.kernel.org, Colin Cross <ccross@...roid.com>,
	Olof Johansson <olof@...om.net>,
	linux-arm-kernel@...ts.infradead.org,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [RFC PATCH] cpuidle: allow per cpu latencies

On 04/06/2012 12:32 PM, Shilimkar, Santosh wrote:
> Peter,
>
> On Thu, Apr 5, 2012 at 7:07 PM, Arjan van de Ven<arjan@...ux.intel.com>  wrote:
>> On 4/5/2012 2:53 AM, Peter De Schrijver wrote:
>>> This patch doesn't update all cpuidle device registrations. I will do that
>>
>> question is if you want to do per cpu latencies, or if you want to have
>> both types of C state in one big table, and have each of the tegra cpyu
>> types pick half of them...
>>
>>
> Indeed !! That should work.
> I thought the C-states are always per CPU based and during the
> cpuidle registration you can register C-state accordingly based on the
> specific CPU types with different latencies if needed.
>
> Am I missing something ?

That was the case before the cpuidle_state were moved from the 
cpuidle_device to the cpuidle_driver structure [1].

That had the benefit of using a single latencies array instead of 
multiple copy of the same array, which was the case until today.

I looked at the white paper for the tegra3 and understand this is no 
longer true because of the 4-plus-1 architecture [2].

With the increasing number of SoCs, we have a lot of new cpuidle drivers 
and each time we modify something in the cpuidle core, that impacts all 
the cpuidle drivers.

My feeling is we are going back and forth when patching the cpuidle core 
and may be it is time to define a clear semantic before patching again 
the cpuidle, no ?

What could nice is to have:

  * in case of the same latencies for all cpus, use a single array

  * in case of different latencies, group the same latencies into a 
single array (I assume this is the case for 4-plus-1, right ?)

May be we can move the cpuidle_state to a per_cpu pointer like 
cpuidle_devices in cpuidle.c and then add:

register_latencies(struct cpuidle_latencies l, int cpu);

If we have the same latencies for all the cpus, then we can register the 
same array, which is only a pointer.

Thanks
   -- Daniel


[1] https://lkml.org/lkml/2011/10/3/57
[2] 
http://www.nvidia.com/content/PDF/tegra_white_papers/Variable-SMP-A-Multi-Core-CPU-Architecture-for-Low-Power-and-High-Performance.pdf


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