Re: [RFC] Extend mwait idle to optimize away IPIs when possible

[David Daney]

On 02/06/2012 05:24 PM, H. Peter Anvin wrote:
> On 02/06/2012 04:26 PM, David Daney wrote:
> > On 02/06/2012 12:42 PM, Venkatesh Pallipadi wrote:
> > [...]
> > > index 5bed94e..6a56cb0 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -170,6 +170,11 @@ config GENERIC_TIME_VSYSCALL
> > >        bool
> > >        default X86_64
> > >
> > > +config AVOID_IPI_ON_IDLE
> > > +    bool
> > > +    depends on X86_64
> > > +    default X86_64
> > > +
> >
> > Can you get rid of this part...
> >
> > >    config ARCH_HAS_CPU_RELAX
> > >        def_bool y
> > [...]
> > > diff --git a/arch/x86/kernel/acpi/cstate.c
> > > b/arch/x86/kernel/acpi/cstate.c
> > > index f50e7fb..340c4e9 100644
> > > --- a/arch/x86/kernel/acpi/cstate.c
> > > +++ b/arch/x86/kernel/acpi/cstate.c
> > > @@ -161,6 +161,14 @@ EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_probe);
> > >     */
> > >    void mwait_idle_with_hints(unsigned long ax, unsigned long cx)
> > >    {
> > > +#ifdef CONFIG_AVOID_IPI_ON_IDLE
> >
> >
> > ... and change all of these to #ifdef CONFIG_64BIT
> >
> > > +    int oldval, cpu = smp_processor_id();
> > > +
>
> Why should this be 64-bit specific??

Well I don't know that part.  The gist of my comment was to get rid of 
an unneeded new config variable.

If this works just as well for 32-bit, then by all means do it there too.

In any event, I think you want the kernel to automatically do it if the 
CPU is capable, and not have a manual config setting you have to fiddle 
with.

David Daney
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