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

[Mark Lord]

Pallipadi, Venkatesh wrote:
>  
>
> > 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.
..

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
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