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

["Rafael J. Wysocki" <rafael@kernel.org>]

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.

>
> 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.

[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.

>  }
>
>  /**
> --