linux-kernel - Re: [PATCH] powerpc/kexec: Enable SMT before waking offline CPUs

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Message-ID: <f1544738-bc22-4b6c-b643-7e7076217395@linux.ibm.com>
Date: Tue, 28 Oct 2025 10:14:01 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: "Nysal Jan K.A." <nysal@...ux.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Sachin P Bappalige <sachinpb@...ux.ibm.com>, stable@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Thomas Gleixner
 <tglx@...utronix.de>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/kexec: Enable SMT before waking offline CPUs

Hello Nysal,

On 25/10/25 13:35, Nysal Jan K.A. wrote:
> If SMT is disabled or a partial SMT state is enabled, when a new kernel
> image is loaded for kexec, on reboot the following warning is observed:
>
> kexec: Waking offline cpu 228.
> WARNING: CPU: 0 PID: 9062 at arch/powerpc/kexec/core_64.c:223 kexec_prepare_cpus+0x1b0/0x1bc
> [snip]
>   NIP kexec_prepare_cpus+0x1b0/0x1bc
>   LR  kexec_prepare_cpus+0x1a0/0x1bc
>   Call Trace:
>    kexec_prepare_cpus+0x1a0/0x1bc (unreliable)
>    default_machine_kexec+0x160/0x19c
>    machine_kexec+0x80/0x88
>    kernel_kexec+0xd0/0x118
>    __do_sys_reboot+0x210/0x2c4
>    system_call_exception+0x124/0x320
>    system_call_vectored_common+0x15c/0x2ec
>
> This occurs as add_cpu() fails due to cpu_bootable() returning false for
> CPUs that fail the cpu_smt_thread_allowed() check or non primary
> threads if SMT is disabled.
>
> Fix the issue by enabling SMT and resetting the number of SMT threads to
> the number of threads per core, before attempting to wake up all present
> CPUs.
>
> Fixes: 38253464bc82 ("cpu/SMT: Create topology_smt_thread_allowed()")
> Reported-by: Sachin P Bappalige <sachinpb@...ux.ibm.com>
> Cc: stable@...r.kernel.org # v6.6+
> Signed-off-by: Nysal Jan K.A. <nysal@...ux.ibm.com>
> ---
>   arch/powerpc/kexec/core_64.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c
> index 222aa326dace..ff6df43720c4 100644
> --- a/arch/powerpc/kexec/core_64.c
> +++ b/arch/powerpc/kexec/core_64.c
> @@ -216,6 +216,11 @@ static void wake_offline_cpus(void)
>   {
>   	int cpu = 0;
>   
> +	lock_device_hotplug();
> +	cpu_smt_num_threads = threads_per_core;
> +	cpu_smt_control = CPU_SMT_ENABLED;


Above variables are  #define if CONFIG_SMP and CONFIG_HOTPLUG_SMT is not 
there.

I think the above code should go under #if defined(CONFIG_SMP) && 
defined(CONFIG_HOTPLUG_SMT).

Seems like the build failure reported below is also pointing the same issue:
https://lore.kernel.org/all/202510280824.Fe2D1Sbw-lkp@intel.com/

> +	unlock_device_hotplug();
> +
>   	for_each_present_cpu(cpu) {
>   		if (!cpu_online(cpu)) {
>   			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",