[RFC PATCH v2 11/35] arch_topology: Make register_cpu_capacity_sysctl() tolerant to late CPUs

[Russell King (Oracle)]

On Thu, Sep 14, 2023 at 01:01:26PM +0100, Jonathan Cameron wrote:
> On Wed, 13 Sep 2023 16:37:59 +0000
> James Morse <james.morse at arm.com> wrote:
> 
> > register_cpu_capacity_sysctl() adds a property to sysfs that describes
> > the CPUs capacity. This is done from a subsys_initcall() that assumes
> > all possible CPUs are registered.
> > 
> > With CPU hotplug, possible CPUs aren't registered until they become
> > present, (or for arm64 enabled). This leads to messages during boot:
> > | register_cpu_capacity_sysctl: too early to get CPU1 device!
> > and once these CPUs are added to the system, the file is missing.
> > 
> > Move this to a cpuhp callback, so that the file is created once
> > CPUs are brought online. This covers CPUs that are added late by
> > mechanisms like hotplug.
> > One observable difference is the file is now missing for offline CPUs.
> > 
> > Signed-off-by: James Morse <james.morse at arm.com>
> > ---
> > If the offline CPUs thing is a problem for the tools that consume
> > this value, we'd need to move cpu_capacity to be part of cpu.c's
> > common_cpu_attr_groups.
> 
> I think we should do that anyway and then use an is_visible() if we want to
> change whether it is visible in offline cpus.
> 
> Dynamic sysfs file creation is horrible - particularly when done
> from an totally different file from where the rest of the attributes
> are registered.  I'm curious what the history behind that is.
> 
> Whilst here, why is there a common_cpu_attr_groups which is
> identical to the hotpluggable_cpu_attr_groups in base/cpu.c?
> 
> 
> +CC GregKH
> Given changes in drivers/base/

It would be good to have a comment on this from Greg before I post
an updated series of James' patches with most of the comments
addressed, possibly later today.

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!