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

[Atish Patra <atish.patra@linux.dev>]

On 10/28/25 1:48 AM, Anup Patel wrote:
> On Tue, Oct 28, 2025 at 2:05 PM Atish Patra <atish.patra@linux.dev> wrote:
>>
>> 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@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@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.
> I have tried upto 96 harts and that works fine.

Yeah. I had tried upto 92 as well.

> For 128 harts, the memory used by OpenSBI goes beyond 2MB so
> OpenSBI can't run from the first 2MB of DRAM (0x80000000) as the
> first booting stage. Try with U-Boot SPL loading OpenSBI from FIT image.

Ahh yes. That makes sense. I will try with U-Boot SPL.

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

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