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Date: Mon, 11 Nov 2013 17:38:59 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Paul Turner <pjt@google.com>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Arjan van de Ven <arjan@linux.intel.com>, linux-pm@vger.kernel.org
Subject: Re: [RFC][PATCH v5 00/14] sched: packing tasks
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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.