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

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Date:	Fri, 30 Nov 2007 14:20:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	lkml@....ca, abelay@...ell.com, lenb@...nel.org, mlord@...ox.com,
	rjw@...k.pl, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org
Subject: 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@...el.com> wrote:

Please dont go off-list like this.  I put Mark's original mailing list cc's
back.

> 
> 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.

> 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.

> 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?


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