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



On Fri, 30 Nov 2007 14:06:55 -0800
"Pallipadi, Venkatesh" <venkatesh.pallipadi@xxxxxxxxx> 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
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