[PATCH] RISC-V: Enable HOTPLUG_PARALLEL for secondary CPUs

[Atish Patra]

On 9/5/25 5:25 AM, Anup Patel wrote:
> The core kernel already supports parallel bringup of secondary
> CPUs (aka HOTPLUG_PARALLEL). The x86 and MIPS architectures
> already use HOTPLUG_PARALLEL and ARM is also moving toward it.
>
> On RISC-V, there is no arch specific global data accessed in the
> RISC-V secondary CPU bringup path so enabling HOTPLUG_PARALLEL for
> RISC-V would only requires:
> 1) Providing RISC-V specific arch_cpuhp_kick_ap_alive()
> 2) Calling cpuhp_ap_sync_alive() from smp_callin()
>
> This patch is tested natively with OpenSBI on QEMU RV64 virt machine
> with 64 cores and also tested with KVM RISC-V guest with 32 VCPUs.
>
> Signed-off-by: Anup Patel <apatel at ventanamicro.com>
> ---
>   arch/riscv/Kconfig          |  2 +-
>   arch/riscv/kernel/smpboot.c | 15 +++++++++++++++
>   2 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index a4b233a0659e..d5800d6f9a15 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -196,7 +196,7 @@ config RISCV
>   	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
>   	select HAVE_STACKPROTECTOR
>   	select HAVE_SYSCALL_TRACEPOINTS
> -	select HOTPLUG_CORE_SYNC_DEAD if HOTPLUG_CPU
> +	select HOTPLUG_PARALLEL if HOTPLUG_CPU
>   	select IRQ_DOMAIN
>   	select IRQ_FORCED_THREADING
>   	select KASAN_VMALLOC if KASAN
> diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
> index 601a321e0f17..d85916a3660c 100644
> --- a/arch/riscv/kernel/smpboot.c
> +++ b/arch/riscv/kernel/smpboot.c
> @@ -39,7 +39,9 @@
>   
>   #include "head.h"
>   
> +#ifndef CONFIG_HOTPLUG_PARALLEL
>   static DECLARE_COMPLETION(cpu_running);
> +#endif
>   
>   void __init smp_prepare_cpus(unsigned int max_cpus)
>   {
> @@ -179,6 +181,12 @@ static int start_secondary_cpu(int cpu, struct task_struct *tidle)
>   	return -EOPNOTSUPP;
>   }
>   
> +#ifdef CONFIG_HOTPLUG_PARALLEL
> +int arch_cpuhp_kick_ap_alive(unsigned int cpu, struct task_struct *tidle)
> +{
> +	return start_secondary_cpu(cpu, tidle);
> +}
> +#else
>   int __cpu_up(unsigned int cpu, struct task_struct *tidle)
>   {
>   	int ret = 0;
> @@ -199,6 +207,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
>   
>   	return ret;
>   }
> +#endif
>   
>   void __init smp_cpus_done(unsigned int max_cpus)
>   {
> @@ -225,6 +234,10 @@ asmlinkage __visible void smp_callin(void)
>   	mmgrab(mm);
>   	current->active_mm = mm;
>   
> +#ifdef CONFIG_HOTPLUG_PARALLEL
> +	cpuhp_ap_sync_alive();
> +#endif
> +
>   	store_cpu_topology(curr_cpuid);
>   	notify_cpu_starting(curr_cpuid);
>   
> @@ -243,7 +256,9 @@ asmlinkage __visible void smp_callin(void)
>   	 */
>   	local_flush_icache_all();
>   	local_flush_tlb_all();
> +#ifndef CONFIG_HOTPLUG_PARALLEL
>   	complete(&cpu_running);
> +#endif

LGTM.

Reviewed-by: Atish Patra <atishp at rivosinc.com>

Have you tried with 128 harts ? I was not able to boot 128 harts in Qemu 
with NR_CPUS=256.
This is unrelated to this patch though. I can reproduce the issue on 
upstream with 6.18-rc3.


>   	/*
>   	 * Disable preemption before enabling interrupts, so we don't try to
>   	 * schedule a CPU that hasn't actually started yet.