On Wed, Mar 24, 2021 at 4:37 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Wed, Mar 24, 2021 at 4:23 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxx>
>
> This looks reasonable to me, but I'll give others some time to respond.
No comments, so applied as 5.12-rc material, thanks!
> > ---
> > 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(-)
> >
> > diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> > index c0538f82c9a2..57ef2094af93 100644
> > --- 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);
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 02813a7f3a7c..f877150a91da 100644
> > --- 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;
> > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > index d93e400940a3..eb1101b16c08 100644
> > --- 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 cpuidle_device *dev, int index)
> > wait_for_freeze();
> > } else
> > return -ENODEV;
> > +
> > +#ifdef CONFIG_X86
> > + /* If NMI wants to wake up CPU0, start CPU0. */
> > + if (wakeup_cpu0())
> > + start_cpu0();
> > +#endif
> > }
> >
> > /* Never reached */
> > --
> > 2.30.2
> >
