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Date: Tue, 12 Nov 2013 11:36:31 +0100
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Subject: Re: [RFC][PATCH v5 00/14] sched: packing tasks
From: Vincent Guittot <vincent.guittot@linaro.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Paul Turner <pjt@google.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
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        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
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On 11 November 2013 17:38, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Nov 11, 2013 at 11:33:45AM +0000, Catalin Marinas wrote:
>> My understanding from the recent discussions is that the scheduler
>> should decide directly on the C-state (or rather the deepest C-state
>> possible since we don't want to duplicate the backend logic for
>> synchronising CPUs going up or down). This means that the scheduler
>> needs to know about C-state target residency, wake-up latency (I think
>> we can leave coupled C-states to the backend, there is some complex
>> synchronisation which I wouldn't duplicate).
>>
>> Alternatively (my preferred approach), we get the scheduler to predict
>> and pass the expected residency and latency requirements down to a
>> power driver and read back the actual C-states for making task
>> placement decisions. Some of the menu governor prediction logic could
>> be turned into a library and used by the scheduler. Basically what
>> this tries to achieve is better scheduler awareness of the current
>> C-states decided by a cpuidle/power driver based on the scheduler
>> constraints.
>
> Ah yes.. so I _think_ the scheduler wants to eventually know about idle
> topology constraints. But we can get there in a gradual fashion I hope.
>
> Like the package C states on x86 -- for those to be effective the
> scheduler needs to pack tasks and keep entire packages idle for as long
> as possible.

That's the purpose of patches 12, 13 and 14. To get the current wakeup
latency of a core and use it when selecting a core

>
>