[PATCH v3 12/12] arm64: Add HOTPLUG_PARALLEL support for secondary CPUs

[Jinjie Ruan]

On 6/24/2026 5:25 PM, Jinjie Ruan wrote:
> Support for parallel secondary CPU bringup is already utilized by x86,
> MIPS and RISC-V. This patch brings this capability to the arm64
> architecture.
> 
> To fully enable HOTPLUG_PARALLEL, this patch implements an arm64-specific
> arch_cpuhp_init_parallel_bringup() handler.
> 
> In parallel bringup, early `set_cpu_present(cpu, 0)` inside
> cpu_die_early() removes the secondary CPU prematurely, causing the primary
> CPU's second-stage cpuhp_bringup_mask() sweep to skip it and drop
> failure logs.


Hi, Will,

In parallel bringup, cpu_die_early() calls set_cpu_present(cpu, 0),
which removes the failing secondary CPU from the mask. This causes the
primary CPU's cpuhp_bringup_mask() sweep to skip it and drop any failure
log.

Have you seen this before, and what do you think about the fix?


1830  */
1831 static bool __init cpuhp_bringup_cpus_parallel(unsigned int ncpus)
1832 {
1833 >-------const struct cpumask *mask = cpu_present_mask;
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...

1861 >-------/* Bring the not-yet started CPUs up */
1862 >-------cpuhp_bringup_mask(mask, ncpus, CPUHP_BP_KICK_AP);
                               ^^^^^
1863 >-------cpuhp_bringup_mask(mask, ncpus, CPUHP_ONLINE);
                                ^^^^^

Best regards,
Jinjie

> 
> Remove this early unregistration from the secondary CPU, deferring the
> set_cpu_present(cpu, 0) call to the primary CPU's cleanup path to ensure
> robust parallel boot timeout detection.
> 
> Tested natively with ATF on QEMU arm64 virt machine with 64 cores
> and also tested with KVM arm64 guest with 128 vCPUs.
> 
> Tested-by: Michael Kelley <mhklinux at outlook.com>
> Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
> ---
>  arch/arm64/Kconfig      |  2 +-
>  arch/arm64/kernel/smp.c | 12 ++++++++++--
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 24496e9967a8..a9d8030e7492 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -231,7 +231,7 @@ config ARM64
>  	select HAVE_KPROBES
>  	select HAVE_KRETPROBES
>  	select HAVE_GENERIC_VDSO
> -	select HOTPLUG_SPLIT_STARTUP if SMP
> +	select HOTPLUG_PARALLEL if SMP
>  	select HOTPLUG_SMT if HOTPLUG_CPU
>  	select IRQ_DOMAIN
>  	select IRQ_FORCED_THREADING
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 98ddbe50081d..a973b2d3bab1 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -93,6 +93,15 @@ static inline int op_cpu_kill(unsigned int cpu)
>  }
>  #endif
>  
> +extern const struct cpu_operations cpu_psci_ops;
> +
> +/* Establish whether parallel bringup can be supported. */
> +bool __init arch_cpuhp_init_parallel_bringup(void)
> +{
> +	const struct cpu_operations *ops = get_secondary_cpu_ops();
> +
> +	return ops == &cpu_psci_ops;
> +}
>  
>  /*
>   * Boot a secondary CPU, and assign it the specified idle task.
> @@ -137,6 +146,7 @@ void arch_cpuhp_cleanup_kick_cpu(unsigned int cpu, bool is_alive)
>  	 * We failed to synchronise with the CPU, so check if it left us
>  	 * any breadcrumbs.
>  	 */
> +	set_cpu_present(cpu, 0);
>  	cpu_boot_data[cpu].task = NULL;
>  	status = READ_ONCE(cpu_boot_data[cpu].status);
>  	if (status == CPU_MMU_OFF)
> @@ -416,8 +426,6 @@ void __noreturn cpu_die_early(void)
>  
>  	pr_crit("CPU%d: will not boot\n", cpu);
>  
> -	/* Mark this CPU absent */
> -	set_cpu_present(cpu, 0);
>  	rcutree_report_cpu_dead();
>  
>  	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {