Re: [PATCH v2] pseries/kexec: skip resetting CPUs added by firmware but not started by the kernel

Shrikanth Hegde
2026-03-30T08:50:12Z

Hi Shivang.

Few nitpicks.

On 3/30/26 11:52 AM, Shivang Upadhyay wrote:
> During DLPAR operations, the newly added CPUs start in halted mode.
> The kernel then takes some time to initialize those CPUs internally and
> start them using the "start-cpu" RTAS call. However, if a kexec crash
> occurs in this window (before the new CPU has been initialized),
> the kexec NMI will try to reset all other CPUs from the crashing CPU.
> This leads to firmware starting the uninitialized CPUs as well.
>
> This can cause the kdump kernel to hang during bring-up.
>
> Sample Log:
> [175993.028231][ T1502] NIP [00007fffb953f394] 0x7fffb953f394
> [175993.028314][ T1502] LR [00007fffb953f394] 0x7fffb953f394
> [175993.028390][ T1502] --- interrupt: 3000
> [ 5.519483][ T1] Processor 0 is stuck.
> [ 11.089481][ T1] Processor 1 is stuck.
>
> To fix this, only issue the system-reset hcall to CPUs that have
> actually been started by the kernel.
>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Srikar Dronamraju <srikar@linux.ibm.com>
> Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
> Cc: Nysal Jan K.A. <nysal@linux.ibm.com>
> Cc: Vishal Chourasia <vishalc@linux.ibm.com>
> Cc: Ritesh Harjani <ritesh.list@gmail.com>
> Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
> Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
> Signed-off-by: Shivang Upadhyay <shivangu@linux.ibm.com>
> ---
> Changelog:
>
> V2:
> * added set_crash_nmi_ipi to saperate crash's case from other nmi_ipi
> users
>
> V1:
> * https://lore.kernel.org/all/20251205142825.44698-1-shivangu@linux.ibm.com/
> ---
> arch/powerpc/include/asm/smp.h | 1 +
> arch/powerpc/kernel/smp.c | 1 +
> arch/powerpc/platforms/pseries/smp.c | 29 +++++++++++++++++++++++++++-
> 3 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
> index e41b9ea42122..cb74201f5674 100644
> --- a/arch/powerpc/include/asm/smp.h
> +++ b/arch/powerpc/include/asm/smp.h
> @@ -47,6 +47,7 @@ struct smp_ops_t {
> void (*cause_ipi)(int cpu);
> #endif
> int (*cause_nmi_ipi)(int cpu);
> + void (*set_crash_nmi_ipi)(void);
> void (*probe)(void);
> int (*kick_cpu)(int nr);
> int (*prepare_cpu)(int nr);
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 3467f86fd78f..3390ee8adf79 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -594,6 +594,7 @@ void crash_send_ipi(void (*crash_ipi_callback)(struct pt_regs *))
> {
> int cpu;
>
> + smp_ops->set_crash_nmi_ipi();
> smp_send_nmi_ipi(NMI_IPI_ALL_OTHERS, crash_ipi_callback, 1000000);
> if (kdump_in_progress() && crash_wake_offline) {
> for_each_present_cpu(cpu) {
> diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
> index db99725e752b..c6c2baacca9a 100644
> --- a/arch/powerpc/platforms/pseries/smp.c
> +++ b/arch/powerpc/platforms/pseries/smp.c
> @@ -51,6 +51,9 @@
> */
> static cpumask_var_t of_spin_mask;
>
> +
> +static int crash_nmi_ipi;
> +
> /* Query where a cpu is now. Return codes #defined in plpar_wrappers.h */
> int smp_query_cpu_stopped(unsigned int pcpu)
> {
> @@ -171,12 +174,35 @@ static void dbell_or_ic_cause_ipi(int cpu)
> ic_cause_ipi(cpu);
> }
>
> +static void pseries_set_crash_nmi_ipi(void)
> +{
> + crash_nmi_ipi = 1;
> +}
> +
> static int pseries_cause_nmi_ipi(int cpu)
> {
> int hwcpu;
> + int k, curcpu;

Please follow inverted christmas tree for variables.

Can the below block be re-written with less indentations?
One level indentation removal could be.

if (cpu == NMI_IPI_ALL_OTHERS && crash_nmi_ipi)
hwcpu = <>
else if (cpu == NMI_IPI_ALL_OTHERS)
hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
else
<existing remaining logic>

>
> + curcpu = smp_processor_id();
> if (cpu == NMI_IPI_ALL_OTHERS) {
> - hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
> + if (crash_nmi_ipi) {
> + for_each_present_cpu(k) {
> + if (k != curcpu) {

Maybe below to reduce one more indentation level.

if ( k == curcpu)
continue;

> + hwcpu = get_hard_smp_processor_id(k);
> +
> + /* it is possible that cpu is present,
> + * but not started yet.
> + */
> +
> + if (paca_ptrs[hwcpu]->cpu_start == 1) {

It either 1 or 0. So if(paca_ptrs[hwcpu]->cpu_start) is enough.

> + plpar_signal_sys_reset(hwcpu);
> + }
> + }
> + }
> + return 1;
> + } else
> + hwcpu = H_SIGNAL_SYS_RESET_ALL_OTHERS;
> } else {
> if (cpu < 0) {
> WARN_ONCE(true, "incorrect cpu parameter %d", cpu);
> @@ -243,6 +269,7 @@ static struct smp_ops_t pseries_smp_ops = {
> .message_pass = NULL, /* Use smp_muxed_ipi_message_pass */
> .cause_ipi = NULL, /* Filled at runtime by pSeries_smp_probe() */
> .cause_nmi_ipi = pseries_cause_nmi_ipi,
> + .set_crash_nmi_ipi = pseries_set_crash_nmi_ipi,
> .probe = pSeries_smp_probe,
> .prepare_cpu = pseries_smp_prepare_cpu,
> .kick_cpu = smp_pSeries_kick_cpu,