Re: [PATCH v5] reboot: support offline CPUs before reboot

[Hsin-Yi Wang <hsinyi@chromium.org>]

On Wed, Jan 15, 2020 at 5:49 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Wed, Jan 15, 2020 at 7:35 AM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
> >
> > Currently system reboots uses architecture specific codes (smp_send_stop)
> > to offline non reboot CPUs. Most architecture's implementation is looping
> > through all non reboot online CPUs and call ipi function to each of them. Some
> > architecture like arm64, arm, and x86... would set offline masks to cpu without
> > really offline them. This causes some race condition and kernel warning comes
> > out sometimes when system reboots.
> >
> > This patch adds a config ARCH_OFFLINE_CPUS_ON_REBOOT, which would offline cpus in
> > migrate_to_reboot_cpu(). If non reboot cpus are all offlined here, the loop for
> > checking online cpus would be an empty loop. If architecture don't enable this
> > config, or some cpus somehow fails to offline, it would fallback to ipi
> > function.
> >
> > Opt in this config for architectures that support CONFIG_HOTPLUG_CPU.
> >
> > Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> > ---
> > Change from v4:
> > * fix a few nits: naming, comments, remove Kconfig text...
> >
> > Change from v3:
> > * Opt in config for architectures that support CONFIG_HOTPLUG_CPU
> > * Merge function offline_secondary_cpus() and freeze_secondary_cpus()
> >   with an additional flag.
>
> This does not seem to be a very good idea, since
> freeze_secondary_cpus() does much more than you need for reboot.
>
> For reboot, you basically only need to do something like this AFAICS:
>
> cpu_maps_update_begin();
>
> for_each_online_cpu(i) {
>         if (i != cpu)
>                 _cpu_down(i, 1, CPUHP_OFFLINE);
> }
> cpu_hotplug_disabled++;
>
> cpu_maps_update_done();
>
> And you may put this into a function defined outside of CONFIG_PM_SLEEP.
>
v2's implementation is similar to this. The conclusion in v2[1] is
that since these 2 functions are similar, we should merge them. I'm
fine both ways but slightly prefer v2's. Maybe wait for others to
comment?

[1] https://lore.kernel.org/lkml/87muarpcwm.fsf@vitty.brq.redhat.com/
> >
> > Change from v2:
> > * Add another config instead of configed by CONFIG_HOTPLUG_CPU
>
> So why exactly is this new Kconfig option needed?
>
> Everybody supporting CPU hotplug seems to opt in anyway.
>
Currently we opt-in for all arch that supports HOTPLUG_CPU, but if
some arch decides that this would make reboot slow (or maybe other
reasons), they can choose to opt-out.
I have only tested on arm64 and x86 for now.
> [cut]
>
> >
> > -int freeze_secondary_cpus(int primary)
> > +int freeze_secondary_cpus(int primary, bool reboot)
> >  {
> >         int cpu, error = 0;
> >
> > @@ -1237,11 +1237,13 @@ int freeze_secondary_cpus(int primary)
> >                 if (cpu == primary)
> >                         continue;
> >
> > -               if (pm_wakeup_pending()) {
> > +#ifdef CONFIG_PM_SLEEP
> > +               if (!reboot && pm_wakeup_pending()) {
> >                         pr_info("Wakeup pending. Abort CPU freeze\n");
> >                         error = -EBUSY;
> >                         break;
> >                 }
> > +#endif
>
> Please avoid using #ifdefs in function bodies.  This makes the code
> hard to maintain in the long term.
>
> >
> >                 trace_suspend_resume(TPS("CPU_OFF"), cpu, true);
> >                 error = _cpu_down(cpu, 1, CPUHP_OFFLINE);
> > @@ -1250,7 +1252,9 @@ int freeze_secondary_cpus(int primary)
> >                         cpumask_set_cpu(cpu, frozen_cpus);
> >                 else {
> >                         pr_err("Error taking CPU%d down: %d\n", cpu, error);
> > -                       break;
> > +                       /* When rebooting, offline as many CPUs as possible. */
> > +                       if (!reboot)
> > +                               break;
> >                 }
> >         }
> >
> > diff --git a/kernel/reboot.c b/kernel/reboot.c
> > index c4d472b7f1b4..12f643b66e57 100644
> > --- a/kernel/reboot.c
> > +++ b/kernel/reboot.c
> > @@ -7,6 +7,7 @@
> >
> >  #define pr_fmt(fmt)    "reboot: " fmt
> >
> > +#include <linux/cpu.h>
> >  #include <linux/ctype.h>
> >  #include <linux/export.h>
> >  #include <linux/kexec.h>
> > @@ -220,7 +221,9 @@ void migrate_to_reboot_cpu(void)
> >         /* The boot cpu is always logical cpu 0 */
> >         int cpu = reboot_cpu;
> >
> > +#if !IS_ENABLED(CONFIG_ARCH_OFFLINE_CPUS_ON_REBOOT)
> >         cpu_hotplug_disable();
> > +#endif
>
> You can write this as
>
> if (!IS_ENABLED(CONFIG_ARCH_OFFLINE_CPUS_ON_REBOOT))
>         cpu_hotplug_disable();
>
> That's what IS_ENABLED() is there for.
>
> >
> >         /* Make certain the cpu I'm about to reboot on is online */
> >         if (!cpu_online(cpu))
> > @@ -231,6 +234,11 @@ void migrate_to_reboot_cpu(void)
> >
> >         /* Make certain I only run on the appropriate processor */
> >         set_cpus_allowed_ptr(current, cpumask_of(cpu));
> > +
> > +#if IS_ENABLED(CONFIG_ARCH_OFFLINE_CPUS_ON_REBOOT)
> > +       /* Offline other cpus if possible */
> > +       freeze_secondary_cpus(cpu, true);
> > +#endif
>
> The above comment applies here too.
>
> >  }
> >
> >  /**
> > --