linux-kernel - Re: [PATCH v1] cpu: Add missing check to cpuhp_smt

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Message-ID: <CAJZ5v0jyN0=aGFOwE8fzuXi=1LgiLR5wgvvsAihGB0qpUp=mUQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 15:13:41 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux PM <linux-pm@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Christian Loehle <christian.loehle@....com>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v1] cpu: Add missing check to cpuhp_smt_enable()

On Fri, Sep 5, 2025 at 9:39 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Fri, Aug 29 2025 at 22:01, Rafael J. Wysocki wrote:
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -2710,6 +2710,12 @@
> >       cpu_maps_update_begin();
> >       cpu_smt_control = CPU_SMT_ENABLED;
> >       for_each_present_cpu(cpu) {
> > +             /*
> > +              * Avoid accidentally onlining primary thread CPUs that have
> > +              * been taken offline.
> > +              */
> > +             if (topology_is_primary_thread(cpu))
> > +                     continue;
>
> Hmm. That's kinda solving the problem, but I think the proper solution
> would be to implement topology_is_core_online() for x86.
>
> The above is preventing the primary thread to be onlined, but then
> onlines the corresponding hyperthread, which does not really make sense.

Well, manual online can be used for onlining the secondary thread of a
core where the primary thread is offline, so this is technically
possible already.

> Something like the completely untested below.

So given the above, shouldn't topology_is_core_online() check if any
thread in the given core is online?

> ---
> diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
> index 6c79ee7c0957..21041898157a 100644
> --- a/arch/x86/include/asm/topology.h
> +++ b/arch/x86/include/asm/topology.h
> @@ -231,6 +231,16 @@ static inline bool topology_is_primary_thread(unsigned int cpu)
>  }
>  #define topology_is_primary_thread topology_is_primary_thread
>
> +int topology_get_primary_thread(unsigned int cpu);
> +
> +static inline bool topology_is_core_online(unsigned int cpu)
> +{
> +       int pcpu = topology_get_primary_thread(cpu);
> +
> +       return pcpu >= 0 ? cpu_online(pcpu) : false;
> +}
> +#define topology_is_core_online topology_is_core_online
> +
>  #else /* CONFIG_SMP */
>  static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
>  static inline int topology_max_smt_threads(void) { return 1; }
> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
> index e35ccdc84910..6073a16628f9 100644
> --- a/arch/x86/kernel/cpu/topology.c
> +++ b/arch/x86/kernel/cpu/topology.c
> @@ -372,6 +372,19 @@ unsigned int topology_unit_count(u32 apicid, enum x86_topology_domains which_uni
>         return topo_unit_count(lvlid, at_level, apic_maps[which_units].map);
>  }
>
> +#ifdef CONFIG_SMP
> +int topology_get_primary_thread(unsigned int cpu)
> +{
> +       u32 apic_id = cpuid_to_apicid[cpu];
> +
> +       /*
> +        * Get the core domain level APIC id, which is the primary thread
> +        * and return the CPU number assigned to it.
> +        */
> +       return topo_lookup_cpuid(topo_apicid(apic_id, TOPO_CORE_DOMAIN));
> +}
> +#endif
> +
>  #ifdef CONFIG_ACPI_HOTPLUG_CPU
>  /**
>   * topology_hotplug_apic - Handle a physical hotplugged APIC after boot
>
>