Re: [discussion]sched: a rough proposal to enable power saving in scheduler

[Rik van Riel <riel@redhat.com>]

On 08/16/2012 10:01 AM, Arjan van de Ven wrote:
>> *Power policy*:
>>
>> So how is power policy different? As Peter says,'pack more than spread
>> more'.
>
> this is ... a dubiously general statement.
>
> for good power, at least on Intel cpus, you want to spread. Parallelism is efficient.
>
> the only thing you do not want to do, is wake cpus up for
> tasks that only run extremely briefly (think "100 usec" or less).
>
> so maybe the balance interval is slightly different, or more, you don't balance tasks that
> historically ran only for brief periods

This makes me think that maybe, in addition to tracking
the idle residency time in the c-state governor, we may
also want to track the average run times in the scheduler.

The c-state governor can call the scheduler code before
putting a CPU to sleep, to indicate (1) the wakeup latency
of the CPU, and (2) whether TLB and/or cache get invalidated.

At wakeup time, the scheduler can check whether the CPU
the to-be-woken process ran on is in a deeper sleep state,
and whether the typical run time for the process significantly
exceeds the wakeup latency of the CPU it last ran on.

If the process typically runs for a short interval, and/or
the process's CPU lost its cached state, it may be better
to run the just-woken task on the CPU that is doing the
waking up, instead of on the CPU where it used to run.

Does that make sense?

Am I overlooking any factors?