Re: [PATCH -v6 2/5] Migrate shutdown/reboot to boot cpu.

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 24 Apr 2013 06:18:50 -0500 Robin Holt <holt@sgi.com> wrote:

> We recently noticed that reboot of a 1024 cpu machine takes approx 16
> minutes of just stopping the cpus.  The slowdown was tracked to commit
> f96972f.
> 
> The current implementation does all the work of hot removing the cpus
> before halting the system.  We are switching to just migrating to the
> boot cpu and then continuing with shutdown/reboot.
> 
> This also has the effect of not breaking x86's command line parameter for
> specifying the reboot cpu.  Note, this code was shamelessly copied from
> arch/x86/kernel/reboot.c with bits removed pertaining to the reboot_cpu
> command line parameter.
> 
> @@ -357,6 +357,24 @@ int unregister_reboot_notifier(struct notifier_block *nb)
>  }
>  EXPORT_SYMBOL(unregister_reboot_notifier);
>  
> +static void migrate_to_reboot_cpu(void)
> +{
> +	/* The boot cpu is always logical cpu 0 */
> +	int reboot_cpu_id = 0;
> +
> +	cpu_hotplug_disable();
> +
> +	/* Make certain the cpu I'm about to reboot on is online */
> +	if (!cpu_online(reboot_cpu_id))
> +		reboot_cpu_id = cpumask_first(cpu_online_mask);
> +
> +	/* Prevent races with other tasks migrating this task */
> +	current->flags |= PF_THREAD_BOUND;

PF_THREAD_BOUND was removed/replaced in linux-next's a9ab775bc
("workqueue: directly restore CPU affinity of workers from CPU_ONLINE")