Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

[Li Yang]

On Fri, Sep 26, 2014 at 9:20 PM, Mark Rutland <mark.rutland@arm.com> wrote:
>> > This looks to be a carbon copy of the vexpress pseudo-hotplug in
>> > arch/arm/mach-vexpress/hotplug.c, which is obviously broken in the way
>> > you describe above. Perhaps we should go about ripping that out?
>>
>> The Versatile Express does not support suspend so the only problem case
>> is kexec.  However, isn't this support needed for big.LITTLE, and as
>> the Versatile Express is the platform which these features get developed
>> on, having working CPU hotplug seems rather fundamental for ARM kernel
>> feature development.
>>
>> In that regard, Versatile Express is something of a special case.
>
> It is admittedly helpful during development to perform pseudo-hotplug on
> Versatile Express. I have a local patch adding vexpress_cpu_disable so I
> can test for bugs that only trigger if CPU0 is hotplugged.
>
> Given that, perhaps we should make it clearer that Versatile Express is
> not a reference implementation for CPU hotplug; add some Kconfig (e.g.
> VEXPRESS_PSEUDO_HOTPLUG) that depends on !KEXEC && !SUSPEND, and putting
> a note in hotplug.c stating it's not suitable as a reference
> implementation.
>
> ...but perhaps that's overkill.

I agree that the pseudo-hotplug is breaking the kexec.  However, I
don't think it breaks the system suspend case as tasks and interrupts
have been moved out of the non-booting CPU.  The pseudo-hotplug would
be even better than real hotplug as it introduces less latency.

Regards,
Leo

Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

[Russell King - ARM Linux]

On Sun, Sep 28, 2014 at 06:57:18PM +0800, Li Yang wrote:
> On Fri, Sep 26, 2014 at 9:20 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> >> > This looks to be a carbon copy of the vexpress pseudo-hotplug in
> >> > arch/arm/mach-vexpress/hotplug.c, which is obviously broken in the way
> >> > you describe above. Perhaps we should go about ripping that out?
> >>
> >> The Versatile Express does not support suspend so the only problem case
> >> is kexec.  However, isn't this support needed for big.LITTLE, and as
> >> the Versatile Express is the platform which these features get developed
> >> on, having working CPU hotplug seems rather fundamental for ARM kernel
> >> feature development.
> >>
> >> In that regard, Versatile Express is something of a special case.
> >
> > It is admittedly helpful during development to perform pseudo-hotplug on
> > Versatile Express. I have a local patch adding vexpress_cpu_disable so I
> > can test for bugs that only trigger if CPU0 is hotplugged.
> >
> > Given that, perhaps we should make it clearer that Versatile Express is
> > not a reference implementation for CPU hotplug; add some Kconfig (e.g.
> > VEXPRESS_PSEUDO_HOTPLUG) that depends on !KEXEC && !SUSPEND, and putting
> > a note in hotplug.c stating it's not suitable as a reference
> > implementation.
> >
> > ...but perhaps that's overkill.
> 
> I agree that the pseudo-hotplug is breaking the kexec.  However, I
> don't think it breaks the system suspend case as tasks and interrupts
> have been moved out of the non-booting CPU.  The pseudo-hotplug would
> be even better than real hotplug as it introduces less latency.

Sorry, that's not an acceptable reason to use it.

NAK.

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