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

Anup Patel
2025-10-28T08:49:01Z

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.

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.

Regards,
Anup

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