[v1] powerpc/kexec: Enable SMT before waking offline CPUs

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

Posted by Nysal Jan K.A. 3 months, 2 weeks ago

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+
Signed-off-by: Nysal Jan K.A. <nysal@linux.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;
+	unlock_device_hotplug();
+
 	for_each_present_cpu(cpu) {
 		if (!cpu_online(cpu)) {
 			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
-- 
2.51.0

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

Posted by Sourabh Jain 3 months, 1 week ago

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@linux.ibm.com>
> Cc: stable@vger.kernel.org # v6.6+
> Signed-off-by: Nysal Jan K.A. <nysal@linux.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",

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

Posted by Srikar Dronamraju 3 months, 1 week ago

* Nysal Jan K.A. <nysal@linux.ibm.com> [2025-10-25 13:35:09]:

> 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;
> +	unlock_device_hotplug();
> +

Nit: can we add a comment mentioning why we do it and probably move to a
small helper function of its own.

Otherwise looks good.

Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>

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

-- 
Thanks and Regards
Srikar Dronamraju

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

Posted by kernel test robot 3 months, 1 week ago

Hi Nysal,

kernel test robot noticed the following build errors:

[auto build test ERROR on powerpc/next]
[also build test ERROR on powerpc/fixes linus/master v6.18-rc3 next-20251027]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Nysal-Jan-K-A/powerpc-kexec-Enable-SMT-before-waking-offline-CPUs/20251025-160821
base:   https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git next
patch link:    https://lore.kernel.org/r/20251025080512.85690-1-nysal%40linux.ibm.com
patch subject: [PATCH] powerpc/kexec: Enable SMT before waking offline CPUs
config: powerpc64-randconfig-001-20251028 (https://download.01.org/0day-ci/archive/20251028/202510280824.Fe2D1Sbw-lkp@intel.com/config)
compiler: clang version 22.0.0git (https://github.com/llvm/llvm-project e1ae12640102fd2b05bc567243580f90acb1135f)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251028/202510280824.Fe2D1Sbw-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202510280824.Fe2D1Sbw-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from <built-in>:3:
   In file included from include/linux/compiler_types.h:171:
   include/linux/compiler-clang.h:37:9: warning: '__SANITIZE_THREAD__' macro redefined [-Wmacro-redefined]
      37 | #define __SANITIZE_THREAD__
         |         ^
   <built-in>:353:9: note: previous definition is here
     353 | #define __SANITIZE_THREAD__ 1
         |         ^
>> arch/powerpc/kexec/core_64.c:220:22: error: expression is not assignable
     220 |         cpu_smt_num_threads = threads_per_core;
         |         ~~~~~~~~~~~~~~~~~~~ ^
   arch/powerpc/kexec/core_64.c:221:18: error: expression is not assignable
     221 |         cpu_smt_control = CPU_SMT_ENABLED;
         |         ~~~~~~~~~~~~~~~ ^
   1 warning and 2 errors generated.


vim +220 arch/powerpc/kexec/core_64.c

   204	
   205	/*
   206	 * We need to make sure each present CPU is online.  The next kernel will scan
   207	 * the device tree and assume primary threads are online and query secondary
   208	 * threads via RTAS to online them if required.  If we don't online primary
   209	 * threads, they will be stuck.  However, we also online secondary threads as we
   210	 * may be using 'cede offline'.  In this case RTAS doesn't see the secondary
   211	 * threads as offline -- and again, these CPUs will be stuck.
   212	 *
   213	 * So, we online all CPUs that should be running, including secondary threads.
   214	 */
   215	static void wake_offline_cpus(void)
   216	{
   217		int cpu = 0;
   218	
   219		lock_device_hotplug();
 > 220		cpu_smt_num_threads = threads_per_core;
   221		cpu_smt_control = CPU_SMT_ENABLED;
   222		unlock_device_hotplug();
   223	
   224		for_each_present_cpu(cpu) {
   225			if (!cpu_online(cpu)) {
   226				printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
   227				       cpu);
   228				WARN_ON(add_cpu(cpu));
   229			}
   230		}
   231	}
   232	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

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

Posted by Nysal Jan K.A. 3 months, 1 week ago

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();
+	cpu_smt_num_threads = threads_per_core;
+	cpu_smt_control = CPU_SMT_ENABLED;
+	unlock_device_hotplug();
+#endif
+}
+
 /*
  * 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();
+
 	for_each_present_cpu(cpu) {
 		if (!cpu_online(cpu)) {
 			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
-- 
2.47.3

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

Posted by Madhavan Srinivasan 1 month, 1 week ago

On Tue, 28 Oct 2025 16:25:12 +0530, 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
> 
> [...]

Applied to powerpc/fixes.

[1/1] powerpc/kexec: Enable SMT before waking offline CPUs
      https://git.kernel.org/powerpc/c/c2296a1e42418556efbeb5636c4fa6aa6106713a

cheers

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

Posted by Sourabh Jain 1 month, 2 weeks ago


On 28/10/25 16:25, 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();
> +	cpu_smt_num_threads = threads_per_core;
> +	cpu_smt_control = CPU_SMT_ENABLED;
> +	unlock_device_hotplug();
> +#endif
> +}
> +
>   /*
>    * 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();
> +
>   	for_each_present_cpu(cpu) {
>   		if (!cpu_online(cpu)) {
>   			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",

The fix looks good to me. I also tested it with QEMU using different
SMT values with the kexec_load and kexec_file_load system calls, and no
warnings were observed during kexec.

Feel free to add:
Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>

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

Posted by Shrikanth Hegde 3 months, 1 week ago

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",

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

Posted by Nysal Jan K.A. 3 months, 1 week ago

hi Shrikanth,

On Tue, Oct 28, 2025 at 10:56:05PM +0530, Shrikanth Hegde wrote:
> Hi Nysal.
> 
> On 10/28/25 4:25 PM, Nysal Jan K.A. wrote:

[snip]

> > --- 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.
> 

Agree, we can look at it as a separate patch.

> > +	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()
> 

There is some reasoning in 4d37cc2dc3df, which made the switch to use the core
device API, against calling cpu_up() directly. The other issue is 
cpuhp_smt_enable() can skip bringing up a CPU in certain cases, for example
when a core is offline.

> > +
> >   /*
> >    * 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",
> 

Thanks for the review.

--Nysal