Re: [patch 2/5] x86_64: CPU hotplug support.

[Ashok Raj <ashok.raj@intel.com>]

On Thu, Jun 02, 2005 at 02:19:55PM -0600, Zwane Mwaikambo wrote:
> On Thu, 2 Jun 2005, Ashok Raj wrote:
> 
> > @@ -445,8 +477,10 @@ void __cpuinit start_secondary(void)
> >  	/*
> >  	 * Allow the master to continue.
> >  	 */
> > +	lock_ipi_call_lock();
> >  	cpu_set(smp_processor_id(), cpu_online_map);
> >  	mb();
> > +	unlock_ipi_call_lock();
> 
> What's that? Is this another smp_call_function race workaround? I thought 
> there was an additional patch to avoid the broadcast.

The other patch avoids sending to offline cpu's, but we read cpu_online_map
and clear self bit in smp_call_function. If a cpu comes online, dont we 
want this cpu to take part in smp_call_function?

if we dont care about this new CPU participating, and if cpu_set() is atomic
(for all NR_CPUS) we dont need to hold call_lock, otherwise we need to hold
this as well.

> 
> > +#include <asm/nmi.h>
> > +/* We don't actually take CPU down, just spin without interrupts. */
> > +static inline void play_dead(void)
> > +{
> > +	idle_task_exit();
> > +	mb();
> > +	/* Ack it */
> > +	__get_cpu_var(cpu_state) = CPU_DEAD;
> > +
> > +	local_irq_disable();
> > +	while (1)
> > +		safe_halt();
> > +}
> 
> Might as well drop the local_irq_disable since safe_halt enables 
> interrupts.
> 

Will do. I think we could cleanup more of the cpu state before we do safe_halt.

But i think i need to look how much more state can be cleaned out. 

In Ia64, we have a clear SAL handoff state, and essentially the cpu is handed
off as OS got it back to BIOS land. Althoght that might be a hard goal 
i need to look at what states _must_ be cleaned out as next step.


-- 
Cheers,
Ashok Raj
- Open Source Technology Center