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SubjectRe: [PATCH][2.5.32] CPU frequency and voltage scaling (0/4)

On Wed, 28 Aug 2002, Dominik Brodowski wrote:
>
> The following patches add CPU frequency and volatage scaling
> support (Intel SpeedStep, AMD PowerNow, etc.) to kernel 2.5.32

The thing is, this interface appears fundamentally broken with respect to
CPU's that change their frequency on the fly. I happen to know one such
CPU rather well myself.

What is this interface supposed to do about a CPU that can change its
frequency dynamically several hundred times a second? Having the OS
control it simply isn't an option - the overhead of the control is _way_
more than is acceptable at that level.

In short, this interface is too broken to be called generic.

A quote from Peter Anvin:

"What is worse is that the interface is, in my opinion, fundamentally
broken for *ALL* CPUs. It doesn't present a policy interface to the
kernel, instead it presents a frequency-setting interface and expect
the policy to be done in userspace. The kernel is the only part of the
system which has sufficient information (idle times of all CPUs, for
example) to do a decent job managing the CPU frequency efficiently.
On Transmeta CPUs this policy should simply be passed down to CMS, of
course; on other CPUs the kernel needs to manage it."

In other words: there is no valid way that a _user_ can set the policy
right now: the user can set the frequency, but since any sane policy
depends on how busy the CPU is, the user isn't even, the right person to
_do_ that, since the user doesn't _know_.

Also note that policy is not just about how busy the CPU is, but also
about how _hot_ the CPU is. Again, a user-mode application (that maybe
polls the situation every minute or so), simply _cannot_ handle this
situation. You need to have the ability to poll the CPU tens of times a
second to react to heat events, and clearly user mode cannot do that
without impacting performance in a big way.

The interface needs to be improved upon. It is simply _not_ valid to say
"run at this speed" as the primary policy.

Linus

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