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

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Message-ID: <CAJZ5v0ik7fRKwH3CnXysvBJkkdJbWP-6iL=zBF0o92rR+nHTKg@mail.gmail.com>
Date: Tue, 2 Sep 2025 17:05:04 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Christian Loehle <christian.loehle@....com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v1] cpu: Add missing check to cpuhp_smt_enable()

On Fri, Aug 29, 2025 at 10:01 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Christian has reported that commit a430c11f4015 ("intel_idle: Rescan
> "dead" SMT siblings during initialization") broke the use case in
> which both nosmt and maxcpus were added to the kernel command line
> because it caused CPUs that were not SMT siblings to be brought
> online during the intel_idle driver initialization in violation of the
> maxcpus limit.
>
> The underlying reason for this is a missing topology_is_primary_thread()
> check in cpuhp_smt_enable() which causes that function to put online
> more CPUs than just the SMT siblings that it is supposed to handle.
>
> Add the missing check to address the issue.
>
> Fixes: a430c11f4015 ("intel_idle: Rescan "dead" SMT siblings during initialization")
> Fixes: f694481b1d31 ("ACPI: processor: Rescan "dead" SMT siblings during initialization")
> Closes: https://lore.kernel.org/linux-pm/724616a2-6374-4ba3-8ce3-ea9c45e2ae3b@arm.com/
> Reported-by: Christian Loehle <christian.loehle@....com>
> Tested-by: Christian Loehle <christian.loehle@....com>
> Cc: 6.16+ <stable@...r.kernel.org> # 6.16+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

I'm wondering if there are any objections or concerns.

Since this is a regression in 6.16, it would be good to get it fixed in 6.17-rc.

> ---
>  kernel/cpu.c |    6 ++++++
>  1 file changed, 6 insertions(+)
>
> --- 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;
>                 /* Skip online CPUs and CPUs on offline nodes */
>                 if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
>                         continue;
>
>
>
>