RE: [patch 1/1] Hot plug CPU to support physical add of new processors (i386)

["Protasevich, Natalie" <Natalie.Protasevich@UNISYS.com>]

> Hallo Natalie,
> 
> On Wednesday 31 August 2005 14:13, 
> Natalie.Protasevich@unisys.com wrote:
> > Current IA32 CPU hotplug code doesn't allow bringing up processors 
> > that were not present in the boot configuration. To make 
> existing hot 
> > plug facility more practical for physical hot plug, possible 
> > processors should be encountered during boot for potentual hot 
> > add/replace/remove. On ES7000, ACPI marks all the sockets that are 
> > empty or not assigned to the partitionas as "disabled".
> 
> Good idea. In fact I always hated the behaviour of the 
> existing hotplug code that assumes all possible CPUs can be 
> hotplugged.
> It would be much nicer to be told be the firmware what CPUs 
> are hotpluggable. It would be great if all ia32/x86-64 
> hotplug capable BIOS behaved like your.
> 
> 
> >  	struct warm_boot_cpu_info info;
> >  	struct work_struct task;
> >  	int	apicid, ret;
> > +	extern u8 bios_cpu_apicid[NR_CPUS];
> 
> This should be in some header.
> 
> >
> >  	lock_cpu_hotplug();
> > -	apicid = x86_cpu_to_apicid[cpu];
> > +	apicid = bios_cpu_apicid[cpu];
> 
> Why this change? It seems unrelated.

The problem is that x86_cpu_to_apicid[] only gets set for processors
that successfully came online in do_boot_cpu():
        
        ... 
        if (boot_error) {
                /* Try to put things back the way they were before ...
*/
                unmap_cpu_to_logical_apicid(cpu);
                cpu_clear(cpu, cpu_callout_map); /* was set here
(do_boot_cpu()) */
                cpu_clear(cpu, cpu_initialized); /* was set by
cpu_init() */
                cpucount--;
        } else {
                x86_cpu_to_apicid[cpu] = apicid;
                cpu_set(cpu, cpu_present_map);
        }
        ...

And in case of the "new" CPU the "inactive" APIC ID is needed, which was
recorded in bios_cpu_apicid[].

> >  	if (apicid == BAD_APICID) {
> >  		ret = -ENODEV;
> >  		goto exit;
> > diff -puN arch/i386/mach-default/topology.c~hotcpu-i386
> > arch/i386/mach-default/topology.c ---
> > 
> linux-2.6.13-rc6-mm2/arch/i386/mach-default/topology.c~hotc
> pu-i386	2005-08-
> >31 04:17:20.957019600 -0700 +++
> > linux-2.6.13-rc6-mm2-root/arch/i386/mach-default/topology.c	
> 2005-08-31
> > 04:22:13.020619184 -0700 @@ -76,7 +76,7 @@ static int __init
> > topology_init(void)
> >  	for_each_online_node(i)
> >  		arch_register_node(i);
> >
> > -	for_each_present_cpu(i)
> > +	for_each_cpu(i)
> 
> This looks wrong. The CPUs should be in the present mask
> if it's present. Followup code similar.

I changed it from present to possible CPUS to create nodes for both
present and absent CPUs, the latter have online=0. This way we can bring
them up if they become physically available. 
 
> BTW shouldn't there be some attribute in sysfs that says
> "CPU disabled"?

I think that would be great. And maybe __cpu_up() should log a message.

> -Andi
> 
-
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