Re: [RFC V1] cpuidle: add idle routine registration and cleanup pm_idlepointer

[Trinabh Gupta]

On 10/20/2010 08:48 PM, Arjan van de Ven wrote:
> On 10/20/2010 8:12 AM, Trinabh Gupta wrote:
> > On 10/20/2010 12:31 AM, Trinabh Gupta wrote:
> > > On 10/20/2010 12:19 AM, Venkatesh Pallipadi wrote:
> > > > On Tue, Oct 19, 2010 at 11:38 AM, Arjan van de Ven
> > > > <arjan@xxxxxxxxxxxxxxx> wrote:
> > > > > On 10/19/2010 11:36 AM, Trinabh Gupta wrote:
> > > > > > The core of the kernel's idle routine on x86 presently depends on an
> > > > > > exported pm_idle function pointer that is unmanaged and causing
> > > > > > hazard to various subsystems when they save and restore it.
> > > > > > The first problem is that this exported pointer can be
> > > > > > modified/flipped
> > > > > > by any subsystem. There is no tracking or notification mechanism.
> > > > > > Secondly and more importantly, various subsystems save the value of
> > > > > > this pointer, flip it and later restore to the saved value. There is
> > > > > > no guarantee that the saved value is still valid. The problem has
> > > > > > been discussed in [2,3] and Peter Zijlstra suggested removing pm_idle
> > > > > > and implementing a list based registration [1].
> > > > > >
> > > > > > This patch is an initial RFC implementation for x86 architecture
> > > > > > only. This framework can be generalised for other archs and also
> > > > > > include the current cpuidle framework for managing multiple idle
> > > > > > routines.
> > > > > >
> > > > > > Tests done with the patch:
> > > > > > ------------------------
> > > > > > 1. Build (on 2.6.36-rc7) and booted on x86 with C1E as deepest idle
> > > > > > state and current_idle was selected to be mwait_idle.
> > > > > >
> > > > > > 2. Build (on 2.5.36-rc8) and booted on x86 (Nehalem) with ACPI C3 as
> > > > > > deepest sleep state. The current_idle was selected to be
> > > > > > cpuidle_idle_call which is the cpuidle subsystem that will further
> > > > > > select idle routines from {C1,C2,C3}.
> > > > > >
> > > > > > Future implementation will try to eliminate this hirearchy and have
> > > > > > a single registration and menu/idle cpuidle governor for selection
> > > > > > of idle routine.
> > > > >
> > > > >
> > > > > looks like you're duplicating the cpuidle subsystem
> > > > >
> > > > > how about biting the bullet and just always and only use the cpuidle
> > > > > subsystem for all idle on x86 ?
> > > >
> > > > I agree with Arjan.
> > > > If we have a default_cpuidle driver which parses idle= params, handles
> > > > various mwait quirks in x86 process*.c and registers with cpuidle, we
> > > > can then always call cpuidle idle routine on x86.
> > >
> > > This wouldn't duplicate code. It would move parts/functionality of
> > > cpuidle into the kernel, keeping governors alone as modules.
> > >
> > > If we directly call cpuidle_idle_call() then this may be too much
> > > overhead for architectures that have single idle routine i.e cases where
> > > cpuidle is not used and will be seen as a bloat. (c.f goals 4a,b).
> >
> > Hi Venki, Arjan
> >
> > Building cpuidle into the kernel would add ~7KB for everyone, even
> > x86 architectures having only single idle state/routine.
>
> but now you're duplicating this functionality adding code for everyone.

I do not intend to duplicate code. I am trying to do what Andi
suggested i.e make the registration part lightweight and *move*
it into the kernel so that we can do away with problems such as
exported pm_idle pointer. This implementation is an *intermediate*
step.

> 99.999% of the people today run cpuidle... (especially embedded x86
> where they really care about power)
> all x86 going forward also has > 1 idle option anyway.
>
> and you're adding and extra layer in the middle that just duplicates the
> layer that's in use in practice above it.

As I said, this would not duplicate rather split things - minimal
registration part within the kernel (so that problems like
pm_idle are resolved) and governors as module.

-Trinabh

> seriously, this sounds like the wrong tradeoff to make.
>
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