From: Vitaly Kuznetsov vkuznets@redhat.com
commit 8cdddd182bd7befae6af49c5fd612893f55d6ccb upstream.
Commit 496121c02127 ("ACPI: processor: idle: Allow probing on platforms with one ACPI C-state") broke CPU0 hotplug on certain systems, e.g. I'm observing the following on AWS Nitro (e.g r5b.xlarge but other instance types are affected as well):
# echo 0 > /sys/devices/system/cpu/cpu0/online # echo 1 > /sys/devices/system/cpu/cpu0/online <10 seconds delay> -bash: echo: write error: Input/output error
In fact, the above mentioned commit only revealed the problem and did not introduce it. On x86, to wakeup CPU an NMI is being used and hlt_play_dead()/mwait_play_dead() loops are prepared to handle it:
/* * If NMI wants to wake up CPU0, start CPU0. */ if (wakeup_cpu0()) start_cpu0();
cpuidle_play_dead() -> acpi_idle_play_dead() (which is now being called on systems where it wasn't called before the above mentioned commit) serves the same purpose but it doesn't have a path for CPU0. What happens now on wakeup is: - NMI is sent to CPU0 - wakeup_cpu0_nmi() works as expected - we get back to while (1) loop in acpi_idle_play_dead() - safe_halt() puts CPU0 to sleep again.
The straightforward/minimal fix is add the special handling for CPU0 on x86 and that's what the patch is doing.
Fixes: 496121c02127 ("ACPI: processor: idle: Allow probing on platforms with one ACPI C-state") Signed-off-by: Vitaly Kuznetsov vkuznets@redhat.com Cc: 5.10+ stable@vger.kernel.org # 5.10+ Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/smp.h | 1 + arch/x86/kernel/smpboot.c | 2 +- drivers/acpi/processor_idle.c | 7 +++++++ 3 files changed, 9 insertions(+), 1 deletion(-)
--- a/arch/x86/include/asm/smp.h +++ b/arch/x86/include/asm/smp.h @@ -132,6 +132,7 @@ void native_play_dead(void); void play_dead_common(void); void wbinvd_on_cpu(int cpu); int wbinvd_on_all_cpus(void); +bool wakeup_cpu0(void);
void native_smp_send_reschedule(int cpu); void native_send_call_func_ipi(const struct cpumask *mask); --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1659,7 +1659,7 @@ void play_dead_common(void) local_irq_disable(); }
-static bool wakeup_cpu0(void) +bool wakeup_cpu0(void) { if (smp_processor_id() == 0 && enable_start_cpu0) return true; --- a/drivers/acpi/processor_idle.c +++ b/drivers/acpi/processor_idle.c @@ -29,6 +29,7 @@ */ #ifdef CONFIG_X86 #include <asm/apic.h> +#include <asm/cpu.h> #endif
#define _COMPONENT ACPI_PROCESSOR_COMPONENT @@ -541,6 +542,12 @@ static int acpi_idle_play_dead(struct cp wait_for_freeze(); } else return -ENODEV; + +#if defined(CONFIG_X86) && defined(CONFIG_HOTPLUG_CPU) + /* If NMI wants to wake up CPU0, start CPU0. */ + if (wakeup_cpu0()) + start_cpu0(); +#endif }
/* Never reached */