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From:   Hsin-Yi Wang <hsinyi@chromium.org>
Date:   Fri, 17 Jan 2020 12:20:56 +0800
Message-ID: <CAJMQK-iH7FPgj2cNZr==Szd8bnRyHC7=GzQt6-zsFq3eCdWJ1Q@mail.gmail.com>
Subject: Re: [PATCH RESEND v4] reboot: support offline CPUs before reboot
To:     Rob Landley <rob@landley.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Will Deacon <will@kernel.org>,
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On Fri, Jan 17, 2020 at 12:16 PM Rob Landley <rob@landley.net> wrote:
>
> On 1/14/20 5:06 AM, Hsin-Yi Wang wrote:
> > 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.
>
> I'm curious:
>
> > +# Select to do a full offline on secondary CPUs before reboot.
> > +config ARCH_OFFLINE_CPUS_ON_REBOOT
> > +     bool "Support for offline CPUs before reboot"
> > +     depends on HOTPLUG_CPU
>
> The new symbol can't be selected without the other symbol.
>
> > +     select ARCH_OFFLINE_CPUS_ON_REBOOT if HOTPLUG_CPU
>
> And the other symbol automatically selects the new one.
>
> Why are you adding a second symbol that means the same thing as the existing symbol?
>

I should make the arch selecting this symbol in other patches and let
the arch decides if they want to opt in, as Thomas pointed out in v5:
https://lore.kernel.org/lkml/8736cgxmxi.fsf@nanos.tec.linutronix.de/

Current solution is not sufficient since it only solve problems for
system that enables HOTPLUG_CPU.

> > +#if defined(CONFIG_PM_SLEEP_SMP) || defined(CONFIG_ARCH_OFFLINE_CPUS_ON_REBOOT)
> > +extern int freeze_secondary_cpus(int primary, bool reboot);
> > +#endif
>
> Couldn't that just test HOTPLUG_CPU? What's the second symbol for? (You can have
> empty stub functions when architectures don't support a thing...)
>
> Rob