Re: [PATCH v2] pseries/kexec: skip resetting CPUs added by firmware but not started by the kernel
From: Shivang Upadhyay
Date: Tue Mar 31 2026 - 02:46:43 EST
Hi Srikar,
Thanks for reviewing.
On Tue, 2026-03-31 at 10:03 +0530, Srikar Dronamraju wrote:
> * Shivang Upadhyay <shivangu@xxxxxxxxxxxxx> [2026-03-30 11:52:06]:
>
> > 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.
>
> What would happen if an non kexec crash nmi was delivered when we did
> a
> DLPAR operation and before the CPUs are initialized?
As per my understanding, similar hang like situation should happen
there also. But I think this case would be highly unlikely. Crash's
case is little special, because this is called for offline cpus also.
>
> >
> > 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@xxxxxxxxxxxxx>
> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> > Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxx>
> > Cc: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > Cc: Nysal Jan K.A. <nysal@xxxxxxxxxxxxx>
> > Cc: Vishal Chourasia <vishalc@xxxxxxxxxxxxx>
> > Cc: Ritesh Harjani <ritesh.list@xxxxxxxxx>
> > Cc: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
> > Reported-by: Anushree Mathur <anushree.mathur@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Shivang Upadhyay <shivangu@xxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxx/
> > ---
> > 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();
>
> Shouldn't we be checking got smp_ops->set_crash_nmi_ipi being non-
> NULL
> before calling. Dont we expect set_crash_nmi_ipi() to be NULL in
> PowerNV
> after this patch? Or what would happen if set_crash_nmi_ipi() was
> called in
> PowerNV code?
>
Yeah, that should be a Bug, Ill fix this in new patch. Thanks for
catching this.
> > 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;
> >
> > +
>
> Nit: Are we adding an unnecessary newline?
Ack.
>
> > +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;
> >
> > + 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) {
> > + 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) {
> > + plpar_signal_sys_r
> > eset(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,
> > --
> > 2.53.0
> >
Thanks
~Shivang.