[PATCH RFC 1/3] cpu/hotplug: Introduce CONFIG_PARALLEL_SMT_PRIMARY_FIRST

[Thomas Gleixner]

On Thu, Jun 11 2026 at 21:38, Jinjie Ruan wrote:

> During parallel CPU bringup, x86 requires primary SMT threads to boot
> first to avoid siblings stopping during microcode updates. This constraint
> is architecture-specific and unnecessary for other platforms
> like arm64.
>
> Introduce CONFIG_PARALLEL_SMT_PRIMARY_FIRST to decouple this constraint.
> Platforms requiring this temporal order (e.g., x86) can select it
> in Kconfig. Other architectures (e.g., arm64) can leave it unselected
> to entirely bypass the SMT branch via the preprocessor.
>
> Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
> ---
>  arch/Kconfig       | 4 ++++
>  arch/mips/Kconfig  | 1 +
>  arch/riscv/Kconfig | 1 +
>  arch/x86/Kconfig   | 1 +
>  kernel/cpu.c       | 6 +++++-
>  5 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index e86880045158..0365d2df2659 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -102,6 +102,10 @@ config HOTPLUG_PARALLEL
>  	bool
>  	select HOTPLUG_SPLIT_STARTUP
>  
> +config PARALLEL_SMT_PRIMARY_FIRST

Proper namespaces are overrated, right?

All related options start with HOTPLUG_....

> +	bool
> +	depends on HOTPLUG_PARALLEL
> +
>  config GENERIC_IRQ_ENTRY
>  	bool
>  
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 4364f3dba688..84e11ac0cf71 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -642,6 +642,7 @@ config EYEQ
>  	select MIPS_CPU_SCACHE
>  	select MIPS_GIC
>  	select MIPS_L1_CACHE_SHIFT_7
> +	select PARALLEL_SMT_PRIMARY_FIRST if HOTPLUG_PARALLEL
>  	select PCI_DRIVERS_GENERIC
>  	select SMP_UP if SMP
>  	select SWAP_IO_SPACE
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d235396c4514..0cc49aecc841 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -210,6 +210,7 @@ config RISCV
>  	select OF
>  	select OF_EARLY_FLATTREE
>  	select OF_IRQ
> +	select PARALLEL_SMT_PRIMARY_FIRST if HOTPLUG_PARALLEL

Why does RISCV need this? It does not select HOTPLUG_SMT to begin with.

> +#ifdef CONFIG_PARALLEL_SMT_PRIMARY_FIRST
>  #ifdef CONFIG_HOTPLUG_SMT

More #ifdeffery is better, right?

>  static inline bool cpuhp_smt_aware(void)
>  {
> @@ -1811,7 +1812,8 @@ static inline const struct cpumask *cpuhp_get_primary_thread_mask(void)
>  {
>  	return cpu_none_mask;
>  }
> -#endif
> +#endif /* CONFIG_HOTPLUG_SMT */
> +#endif /* CONFIG_PARALLEL_SMT_PRIMARY_FIRST */
>  
>  bool __weak arch_cpuhp_init_parallel_bringup(void)
>  {
> @@ -1837,6 +1839,7 @@ static bool __init cpuhp_bringup_cpus_parallel(unsigned int ncpus)
>  	if (!__cpuhp_parallel_bringup)
>  		return false;
>  
> +#ifdef CONFIG_PARALLEL_SMT_PRIMARY_FIRST

Seriously?

>  	if (cpuhp_smt_aware()) {
>  		const struct cpumask *pmask = cpuhp_get_primary_thread_mask();
>  		static struct cpumask tmp_mask __initdata;
> @@ -1857,6 +1860,7 @@ static bool __init cpuhp_bringup_cpus_parallel(unsigned int ncpus)
>  		cpumask_andnot(&tmp_mask, mask, pmask);
>  		mask = &tmp_mask;
>  	}
> +#endif /* CONFIG_PARALLEL_SMT_PRIMARY_FIRST */

Something simple like the uncompiled below should just work, no?

---
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -102,6 +102,10 @@ config HOTPLUG_PARALLEL
 	bool
 	select HOTPLUG_SPLIT_STARTUP
 
+config HOTPLUG_PARALLEL_SMT
+	bool
+	select HOTPLUG_PARALLEL
+
 config GENERIC_IRQ_ENTRY
 	bool
 
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -657,7 +657,7 @@ config EYEQ
 	select USB_UHCI_BIG_ENDIAN_DESC if CPU_BIG_ENDIAN
 	select USB_UHCI_BIG_ENDIAN_MMIO if CPU_BIG_ENDIAN
 	select USE_OF
-	select HOTPLUG_PARALLEL if HOTPLUG_CPU
+	select HOTPLUG_PARALLEL_SMT if HOTPLUG_CPU
 	help
 	  Select this to build a kernel supporting EyeQ SoC from Mobileye.
 
@@ -2295,7 +2295,6 @@ config MIPS_CPS
 	select MIPS_CM
 	select MIPS_CPS_PM if HOTPLUG_CPU
 	select SMP
-	select HOTPLUG_SMT if HOTPLUG_PARALLEL
 	select HOTPLUG_CORE_SYNC_DEAD if HOTPLUG_CPU
 	select SYNC_R4K if (CEVT_R4K || CSRC_R4K)
 	select SYS_SUPPORTS_HOTPLUG_CPU
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -304,7 +304,7 @@ config X86
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
 	select VDSO_GETRANDOM			if X86_64
-	select HOTPLUG_PARALLEL			if SMP && X86_64
+	select HOTPLUG_PARALLEL_SMT		if SMP && X86_64
 	select HOTPLUG_SMT			if SMP
 	select HOTPLUG_SPLIT_STARTUP		if SMP && X86_32
 	select IRQ_FORCED_THREADING
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1792,7 +1792,7 @@ static int __init parallel_bringup_parse
 }
 early_param("cpuhp.parallel", parallel_bringup_parse_param);
 
-#ifdef CONFIG_HOTPLUG_SMT
+#ifdef CONFIG_HOTPLUG_PARALLEL_SMT
 static inline bool cpuhp_smt_aware(void)
 {
 	return cpu_smt_max_threads > 1;