Re: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm)

[Trinabh Gupta <trinabh@linux.vnet.ibm.com>]

On 03/31/2011 07:47 AM, Len Brown wrote:
>>>> Maybe there is some other way to handle asymmetry ??
>
> I mis-spoke on asymmetry.
>
> Moorestown is already an example of an asymmetric system,
> since its deepest c-state is available on cpu0, but not on cpu1.
> So it needs different tables for each cpu.
>
> I think what would work is a default c-state table for the system,
> and the ability of a per-cpu override table.  I think that would
> gracefully handle the case of many identical cpus, and also systems
> with different tables per cpu.

Hi Len,

What would happen if a cpu enters a state which wasn't
reported for it? I am wondering if an approach like union
of states of each would work.

Can we handle asymmetry through checking and demotion inside the
routine itself; just like you are proposing as dev->prepare
alternative? But I guess this may not be efficient if this
happens often.

I am not sure if having a per-cpu override would be very tidy
(ideas ?); and much better than per-cpu stuff. So just want to check
what would be the best way forward?

>
> The same goes for write-access to the tables.
> In the typical case, a single table can be shared for the entire system
> and nobody will be writing to it.  However, with the governor changes
> to call dev->prepare and sift through all the states to find the
> legal one with the lowest power_usage... There is software today
> out of tree that updates that power_usage entry from prepare().
>
> As I mentioned, I'm not fond of that mechanism - it looks racey
> to me.  I'd rather see the capability of a drivers idle handler
> to demote to another handler in the driver and for the accounting
> to not get messed up when that happens.  I think the way to do that
> is to let the driver do the accounting rather than doing it in
> the cpuidle caller.

I agree with this; we should move update of statistics inside the
driver routines (enter routines). They already take device/stats
structure as parameter.

Thanks,
-Trinabh