Re: [PATCH v2] powerpc/kexec: Enable SMT before waking offline CPUs

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Nysal.

On 10/28/25 4:25 PM, 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@linux.ibm.com>
> Cc: stable@vger.kernel.org # v6.6+
> Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>
> Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
> ---
>   arch/powerpc/kexec/core_64.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
> 
> diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c
> index 222aa326dace..825ab8a88f18 100644
> --- a/arch/powerpc/kexec/core_64.c
> +++ b/arch/powerpc/kexec/core_64.c
> @@ -202,6 +202,23 @@ static void kexec_prepare_cpus_wait(int wait_state)
>   	mb();
>   }
>   
> +
> +/*
> + * The add_cpu() call in wake_offline_cpus() can fail as cpu_bootable()
> + * returns false for CPUs that fail the cpu_smt_thread_allowed() check
> + * or non primary threads if SMT is disabled. Re-enable SMT and set the
> + * number of SMT threads to threads per core.
> + */
> +static void kexec_smt_reenable(void)
> +{
> +#if defined(CONFIG_SMP) && defined(CONFIG_HOTPLUG_SMT)
> +	lock_device_hotplug();

I was looking at usage of lock_device_hotplug, looks like a good candidate for
guard() use case. Could be done on its own patch/series.

> +	cpu_smt_num_threads = threads_per_core;
> +	cpu_smt_control = CPU_SMT_ENABLED;
> +	unlock_device_hotplug();
> +#endif
> +}


Will this work too? It might be better since we anyway going to bring that CPU up
by doing add_cpu afterwords.

	cpu_smt_num_threads = threads_per_core;
	cpuhp_smt_enable()

> +
>   /*
>    * We need to make sure each present CPU is online.  The next kernel will scan
>    * the device tree and assume primary threads are online and query secondary
> @@ -216,6 +233,8 @@ static void wake_offline_cpus(void)
>   {
>   	int cpu = 0;
>   
> +	kexec_smt_reenable();
> +

If we do above, just change the below logic to complain if any present CPU is offline.

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