RE: [PATCH v5 03/12] x86/topology.c: Support functions for CPU0 online/offline

[Ben Hutchings <bhutchings@solarflare.com>]

On Tue, 2012-01-24 at 22:31 +0000, Yu, Fenghua wrote:
> > On Wed, 2012-01-11 at 09:04 -0800, Fenghua Yu wrote:
> > > From: Fenghua Yu <fenghua.yu@intel.com>
> > >
> > > If CONFIG_BOOTPARAM_HOTPLUG_CPU is turned on, CPU0 hotplug feature is
> > enabled
> > > by default.
> > >
> > > If CONFIG_BOOTPARAM_HOTPLUG_CPU is not turned on, CPU0 hotplug
> > feature is not
> > > enabled by default. The kernel parameter cpu0_hotplug can enable CPU0
> > hotplug
> > > feature at boot.
> > [...]
> > >  int __ref arch_register_cpu(int num)
> > >  {
> > >  	/*
> > > -	 * CPU0 cannot be offlined due to several
> > > -	 * restrictions and assumptions in kernel. This basically
> > > -	 * doesn't add a control file, one cannot attempt to offline
> > > -	 * BSP.
> > > +	 * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
> > > +	 * depends on BSP. PIC interrupts depend on BSP.
> > >  	 *
> > > -	 * Also certain PCI quirks require not to enable hotplug control
> > > -	 * for all CPU's.
> > > +	 * If the BSP depencies are under control, one can tell kernel to
> > > +	 * enable BSP hotplug. This basically adds a control file and
> > > +	 * one can attempt to offline BSP.
> > >  	 */
> > > -	if (num)
> > > +	if (num || cpu0_hotpluggable)
> > >  		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
> > >
> > >  	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
> > 
> > This change belongs at the end of the series.  It should not be
> > possible
> > to enable CPU0 hotplug until after the hotplug logic can do it
> > correctly, and this might break bisection.
> 
> Quote from https://www.linux.com/how-to-participate-in-the-linux-community
> "It can be tempting to add a whole new infrastructure with a series of
> patches, but to leave that infrastructure unused until the final patch
> in the series enables the whole thing. This temptation should be
> avoided if possible; if that series adds regressions, bisection will
> finger the last patch as the one which caused the problem, even though
> the real bug is elsewhere. Whenever possible, a patch which adds new
> code should make that code active immediately."
> 
> So this patch currently is in the right place in the patch set unless
> I miss something.

You're giving undue weight to that guidance.  It is far more important
that you do not enable features that don't work!

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.