Re: [RFC][PATCH v5 00/14] sched: packing tasks

[Arjan van de Ven <arjan@linux.intel.com>]

On 11/12/2013 3:14 PM, Catalin Marinas wrote:
> On 12 Nov 2013, at 16:48, Arjan van de Ven <arjan@linux.intel.com> wrote:
>> On 11/11/2013 10:18 AM, Catalin Marinas wrote:
>>> The ordering is based on the actual C-state, so a simple way is to wake
>>> up the CPU in the shallowest C-state. With asymmetric configurations
>>> (big.LITTLE) we have different costs for the same C-state, so this would
>>> come in handy.
>>
>> btw I was considering something else; in practice CPUs will be in the deepest state..
>> ... at which point I was going to go with some other metrics of what is best from a platform level
>
> I agree, other metrics are needed.  The problem is that we currently
> only have (relatively, guessed from the target residency) the cost of
> transition from a C-state to a P-state (for the latter, not sure which).
> But we don’t know what the power (saving) on that C-state is nor the one
> at a P-state (and vendors reluctant to provide such information). So the
> best the scheduler can do is optimise the wake-up cost and blindly assume
> that deeper C-state on a CPU is more efficient than lower P-states on two
> other CPUs (or the other way around).

for picking the cpu to wake on there are also low level physical kind of things
we'd want to take into account on the intel side.