linux-kernel - Re: [RFC PATCH v2 11/35] arch_topology: Make register_cpu_capacity

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Message-ID: <2023102128-banter-circular-85a2@gregkh>
Date:   Sat, 21 Oct 2023 12:56:55 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        James Morse <james.morse@....com>, linux-pm@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-acpi@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com
Subject: Re: [RFC PATCH v2 11/35] arch_topology: Make
 register_cpu_capacity_sysctl() tolerant to late CPUs

On Fri, Oct 20, 2023 at 12:53:29PM +0100, Russell King (Oracle) wrote:
> 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@....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@....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.

Sorry, I don't see what I am supposed to comment on, so please just send
a new series and I'll look at that.

thanks,

greg k-h