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

[Jinjie Ruan]

On 6/18/2026 11:49 PM, Thomas Gleixner wrote:
> 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?

That's a very good point. Parallel bringup of APs during early boot
should indeed benefit SMP systems even if runtime CPU hotplug
(HOTPLUG_CPU) is disabled. I will decouple this optimization from
HOTPLUG_CPU and tie it strictly to SMP. Thanks for catching this!

> 
>> +#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.

Converting this unconditionally to use cpu_boot_data makes the code so
much cleaner. Thanks for the guidance!

> 
>>  	/*
>>  	 * 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.

I will look into selecting HOTPLUG_SPLIT_STARTUP and cleaning up this
completion mechanism either as a prerequisite cleanup patch. For now, I
will make sure to eliminate the ugly #ifndef as suggested earlier.

>   
> Thanks,
> 
>         tglx
> 
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