Re: [RFC/PATCH 3/4] introduce cpu_add and cpu_remove

[Matthew Dobson <colpatch@us.ibm.com>]

On Thu, 2004-11-04 at 17:57, Ashok Raj wrote:
> On Sun, Oct 24, 2004 at 05:42:31AM -0400, Nathan Lynch wrote:
> > 
> > These functions safely update cpu_present_map (i.e. with the
> > cpucontrol semaphore held) and register or unregister the cpu device
> > as needed.  These are needed by systems which can add or remove cpus
> > from the system after boot (e.g. ppc64 and ia64), and are intended to
> > be called from the platform-specific code such as the ACPI or Open
> > Firmware layers.
> > 
> > Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
> > 
> > 
> > ---
> > 
> > 
> > +
> > +/*
> > + * Add a cpu to the system.  Return the number of the cpu added,
> > + * or NR_CPUS if no more slots available.
> > + */
> > +unsigned int cpu_add(void)
> > +{
> > +	unsigned int cpu = NR_CPUS;
> > +
> > +	lock_cpu_hotplug();
> > +
> > +	if (num_present_cpus() == num_possible_cpus())
> > +goto out;
> > +
> > +	for_each_cpu(cpu)
> > +		if (!cpu_present(cpu))
> > +			break;
> 
>      could we simplify this by
> 
>    cpus_compliment(cpu_compliment_map, cpu_present_map);
>    cpu = first_cpu(cpu_compliment_map);

Well, since for_each_cpu() is defined like this:
#define for_each_cpu(cpu)         for_each_cpu_mask((cpu), cpu_possible_map)

We could do:
cpus_andnot(new_cpu_map, cpu_possible_map, cpu_present_map);
cpu = first_cpu(new_cpu_map);

Or maybe even:
unsigned int cpu_add(void)
{
	unsigned int cpu = NR_CPUS;

	lock_cpu_hotplug();

	if (num_present_cpus() == num_possible_cpus())
		goto out;

	cpus_andnot(new_cpu_map, cpu_possible_map, cpu_present_map);
	for_each_cpu_mask(new_cpu_map)
		if (!register_cpu(cpu)) {
			cpu_set(cpu, cpu_present_map);
			goto out;
		}

	cpu = NR_CPUS;
out:
	unlock_cpu_hotplug();
	return cpu;
}

since we want to try all possible but !present CPUs until we exhaust
them all or find one to bring online.

Simply complimenting the cpu_present_map could easily return CPUs which
aren't 'present' and aren't even 'possible' since cpu_possible_map
doesn't necessarily equal 0xFFFFFFFF (or however many FF's for your
particular platform! ;)

And FWIW, I like where you're going with this, Nathan.  I wrote a lot of
the original system topology code for sysfs, most of which is thankfully
gone now! ;)  I had hoped to clean up some of the ickier stuff, but
haven't gotten around to it.

-Matt