Re: + restore-missing-sysfs-max_cstate-attr.patch added to -mm tree

[Mark Lord <lkml@rtr.ca>]

Pallipadi, Venkatesh wrote:
>  
> 
>> On Fri, 30 Nov 2007 14:06:55 -0800
>> "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com> wrote:
>>
>> Please dont go off-list like this.  I put Mark's original 
>> mailing list cc's
>> back.
> 
> Sorry for missing some cc's earlier. I blindly did a reply-all to the
> mm-commits mail I got.
> 
>>> I will have to Nack this. The reason max_cstate was initentionally
>>> removed due to couple of reasons:
>> It broke userspace without any warning or migration period, afaict.
> 
> Yes. That's true. I will have to take the blame for that. It has been
> known for a while during cpuidle development. But, it was never
> documented as deprecating.
>  
>>> 1) All in kernel users of max_cstate should rather be using
>>> pm_qos/latency interfaces. All such max_cstate usages must already be
>>> migrated.
>> That code isn't merged.
> 
> All kernel part is already merged. I mean, there are do drivers that
> depend on max_cstate. They use latency_notifier thing today and their
> migration to pm_qos part is not merged yet.
> 
>>> 2) Supporting max_cstate as a dynamic parameter cleanly is no longer
>>> possible in acpi/processor_idle.c as the C-state policy has moved to
>>> cpuidle instead. It can be done if it is needed. But, just 
>> below patch
>>> will not really work with cpuidle.
>>>
>>> Selecting max_cstate at boot time as a debug option still 
>> works without
>>> this patch.
>>>
>>> So, just this patch will not get back the functionality with cpuidle.
>>> Infact changing it at run time will have no effect. Question 
>> however is:
>>> Is there a real need to revive this parameter so that user can change
>>> max_cstate at run time?
>> It is not known whether Mark is actually writing to this 
>> thing.  Perhaps
>> read-only permissions would be a suitable fix?
>>
> 
> Exporting it as read only should be OK. We also need to know if there
> are hard user space dependency on writing to this from userspace.
..

Well, actually..  my scripts have a firm need to write "1" to it,
and then later restore the original value.

This is needed to *greatly* speed up an otherwise sluggish binary I use,
as well as whenever I want to semi-accurately benchmark I/O.

Is there another way to achieve exactly the same behaviour?

Thanks