[PATCH RFC 3/3] arm64: Add HOTPLUG_PARALLEL support for secondary CPUs

[Thomas Gleixner]

On Thu, Jun 11 2026 at 21:38, Jinjie Ruan wrote:
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -113,6 +113,7 @@ config ARM64
>  	select CPUMASK_OFFSTACK if NR_CPUS > 256
>  	select DCACHE_WORD_ACCESS
>  	select HAVE_EXTRA_IPI_TRACEPOINTS
> +	select HOTPLUG_PARALLEL if SMP && HOTPLUG_CPU

Why do you tie that to HOTPLUG_CPU? HOTPLUG_CPU lets you unplug/plug
CPUs at runtime, but if its disabled then a SMP system still has to
bring up the APs. So why should that fall back to the existing variant?

> +#ifdef CONFIG_HOTPLUG_PARALLEL
> +extern struct secondary_data cpu_boot_data[NR_CPUS];
> +#endif
> +
>  extern struct secondary_data secondary_data;
>  extern long __early_cpu_boot_status;
>  extern void secondary_entry(void);
> @@ -124,7 +128,11 @@ static inline void __noreturn cpu_park_loop(void)
>  
>  static inline void update_cpu_boot_status(unsigned int cpu, int val)
>  {
> +#ifdef CONFIG_HOTPLUG_PARALLEL
> +	WRITE_ONCE(cpu_boot_data[cpu].status, val);
> +#else
>  	WRITE_ONCE(secondary_data.status, val);
> +#endif

You're really a great fan of #ifdefs, right?

Just convert it over to the parallel mode unconditionally and get rid of
the existing cruft.

>  	/*
>  	 * TTBR0 is only used for the identity mapping at this stage. Make it
>  	 * point to zero page to avoid speculatively fetching new entries.
> @@ -254,7 +276,9 @@ asmlinkage notrace void secondary_start_kernel(void)
>  					 read_cpuid_id());
>  	update_cpu_boot_status(cpu, CPU_BOOT_SUCCESS);
>  	set_cpu_online(cpu, true);
> +#ifndef CONFIG_HOTPLUG_PARALLEL
>  	complete(&cpu_running);
> +#endif

Just for the record. You can get rid of this completion w/o PARALLEL
hotplug by selecting HOTPLUG_SPLIT_STARTUP and implementing the
kick/sync parts.
  
Thanks,

        tglx