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

Pallipadi, Venkatesh
2007-11-30T22:42:27Z

>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.

Thanks,
Venki