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SubjectRe: [PATCH 2/3] x86, NMI: Add NMI IPI selftest

* Don Zickus <dzickus@redhat.com> wrote:

> The previous patch modified the stop cpus path to use NMI instead of IRQ
> as the way to communicate to the other cpus to shutdown. There were some
> concerns that various machines may have problems with using an NMI IPI.
>
> This patch creates a selftest to check if NMI IPI is working at boot.
> The idea is to help catch any issues before the machine panics and we
> learn the hard way. :-)
>
> If the selftest fails, the code tries to fall back to the original IRQ
> method.
>
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
> arch/x86/Kconfig.debug | 12 +++++++++
> arch/x86/include/asm/smp.h | 1 +
> arch/x86/kernel/smp.c | 5 ++++
> arch/x86/kernel/smpboot.c | 57 ++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 75 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
> index c0f8a5c..18261c2 100644
> --- a/arch/x86/Kconfig.debug
> +++ b/arch/x86/Kconfig.debug
> @@ -284,4 +284,16 @@ config DEBUG_STRICT_USER_COPY_CHECKS
>
> If unsure, or if you run an older (pre 4.4) gcc, say N.
>
> +config DEBUG_NMI_IPI_SELFTEST
> + bool "NMI IPI Selftest"
> + depends on DEBUG_KERNEL
> + ---help---
> + Enabling this option turns on a quick NMI IPI selftest to
> + verify that sending an NMI this way works for the panic and
> + kdump paths.
> +
> + This might help diagnose strange hangs in those paths.
> +
> + If unsure, say N.
> +
> endmenu
> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> index 73b11bc..916f89d 100644
> --- a/arch/x86/include/asm/smp.h
> +++ b/arch/x86/include/asm/smp.h
> @@ -149,6 +149,7 @@ static inline void arch_send_call_function_ipi_mask(const struct cpumask *mask)
> }
>
> void cpu_disable_common(void);
> +void native_smp_disable_nmi_ipi(void);
> void native_smp_prepare_boot_cpu(void);
> void native_smp_prepare_cpus(unsigned int max_cpus);
> void native_smp_cpus_done(unsigned int max_cpus);
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 7bdbf6a..f54e07a 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -247,6 +247,11 @@ static void native_irq_stop_other_cpus(int wait)
> local_irq_restore(flags);
> }
>
> +void native_smp_disable_nmi_ipi(void)
> +{
> + smp_ops.stop_other_cpus = native_irq_stop_other_cpus;
> +}
> +
> /*
> * Reschedule call back.
> */
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 9f548cb..dcb889f 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -71,6 +71,7 @@
>
> #include <asm/smpboot_hooks.h>
> #include <asm/i8259.h>
> +#include <asm/nmi.h>
>
> /* State of each CPU */
> DEFINE_PER_CPU(int, cpu_state) = { 0 };
> @@ -415,6 +416,57 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
> return cpu_llc_shared_mask(cpu);
> }
>
> +#ifdef CONFIG_DEBUG_NMI_IPI_SELFTEST
> +/* check to see if NMI IPIs work on this machine */
> +static DECLARE_BITMAP(nmi_ipi_mask, NR_CPUS) __read_mostly;
> +
> +static int check_nmi_ipi_callback(unsigned int val, struct pt_regs *regs)
> +{
> + int cpu = raw_smp_processor_id();
> +
> + if (cpumask_test_cpu(cpu, to_cpumask(nmi_ipi_mask))) {
> + cpumask_clear_cpu(cpu, to_cpumask(nmi_ipi_mask));
> + return NMI_HANDLED;
> + }
> +
> + return NMI_DONE;
> +}
> +
> +static bool check_nmi_ipi(void)
> +{
> + unsigned long timeout;
> +
> + if (num_online_cpus() > 1) {
> + cpumask_copy(to_cpumask(nmi_ipi_mask), cpu_online_mask);
> + cpumask_clear_cpu(smp_processor_id(), to_cpumask(nmi_ipi_mask));
> +
> + if (register_nmi_handler(NMI_LOCAL, check_nmi_ipi_callback,
> + NMI_FLAG_FIRST, "check_nmi_ipi"))
> + return false;
> +
> + /* sync above data before sending NMI */
> + wmb();
> +
> + apic->send_IPI_allbutself(NMI_VECTOR);
> +
> + /* Don't wait longer than a second */
> + timeout = USEC_PER_SEC;
> + while (!cpumask_empty(to_cpumask(nmi_ipi_mask)) && timeout--)
> + udelay(1);
> +
> + /* What happens if we timeout, do we still unregister?? */
> + unregister_nmi_handler(NMI_LOCAL, "check_nmi_ipi");
> +
> + if (!timeout) {
> + pr_warn("CPU does not support NMI IPIs\n");
> + return false;
> + }
> + }
> + pr_info("NMI IPIs selftest passed\n");
> + return true;
> +}
> +#endif
> +
> static void impress_friends(void)
> {
> int cpu;
> @@ -1142,6 +1194,11 @@ void __init native_smp_cpus_done(unsigned int max_cpus)
> {
> pr_debug("Boot done.\n");
>
> +#ifdef CONFIG_DEBUG_NMI_IPI_SELFTEST
> + /* if NMI IPI doesn't work, fallback to old irq method for panic */
> + if (check_nmi_ipi())
> + native_smp_disable_nmi_ipi();
> +#endif

Looks good in principle, but instead of the #ifdeffery i'd really
suggest to create a separate (small) .c module and .h header file for
this, to hide all those self-test details from the generic path. The
callback in native_smp_cpus_done() callback can be a plain
nmi_selftest_callback() call or such.

Thanks,

Ingo


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