Re: sched: deep power-saving states

[Gregory Haskins]

Arjan van de Ven wrote:
> On Wed, 22 Oct 2008 10:05:21 -0400
> Gregory Haskins <ghaskins@xxxxxxxxxx> wrote:
>
>   
>> Arjan van de Ven wrote:
>>     
>>> On Wed, 22 Oct 2008 09:42:52 -0400
>>> Gregory Haskins <gregory.haskins.ml@xxxxxxxxx> wrote:
>>>
>>>   
>>>       
>>>> What I was thinking is that a simple mechanism to quantify the
>>>> power-state penalty would be to add those states as priority
>>>> levels in the cpupri namespace.  E.g. We could substitute
>>>> IDLE-RUNNING for IDLE, and add IDLE-PS1, IDLE-PS2, .. IDLE-PSn,
>>>> OTHER, RT1, .. RT99.  This means the scheduler would favor waking
>>>> an IDLE-RUNNING core over an IDLE-PS1-PSn, etc.  The question in
>>>> my mind is: can the power-states be determined in a static fashion
>>>> such that we know what value to quantify the idle state before we
>>>> enter it?  Or is it more dynamic (e.g. the longer it is in an
>>>> MWAIT, the deeper the sleep gets). 
>>>>         
>>> it's a little dynamic, but just assuming the worst will be a very
>>> good approximation of reality. And we know what we're getting into
>>> in that sense.
>>>   
>>>       
>> Ok, but if we just assume the worst case always, how do I
>> differentiate between, say, IDLE-RUNNING and IDLE-PSn?  If I assign
>> them all to IDLE-PSn apriori its no better than the basic single IDLE
>> state we support today.  Or am I misunderstanding you?
>>     
>
> eh yes I wasn't very clear; it's pre-coffee time here ;)
>
> we know *for each C state* we go in, what its maximum latency is.
> Now, that is the *maximum*; there are times where it'll be less
> (there are several steps for going into a C-state hardware wise, and if
> an interrupt comes in before they're all completed, getting out of it
> means not having to undo ALL the steps, so it'll be faster)
>   

[Adding Peter Zijlstra to the thread]

Ah, yes of course!   That makes sense.  So I have to admit I am fairly
ignorant of the ACPI C-state stuff, so I just read up on it.  In the
context of what you said, it makes perfect sense to me now.

IIUC, the OS selects which C-state it will enter at idle points based on
some internal criteria (TBD).  All we have to do is remap the cpupri
"IDLE" state to something like IDLE-C1, IDLE-C2, ..., IDLE-Cn and have
the cpupri map get updated coincident with the pm_idle() call.  Then the
scheduler will naturally favor cores that are in lighter sleep over
cores in deep sleep.

I am not sure if this is exactly what you were getting at during the
conf, since it doesnt really consider deep-sleep latency times
directly.  But I think this is a step in the right direction.

-Greg

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