linux-kernel - Re: [PATCH RESEND v4] reboot: support offline CPUs before reboot

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Date:   Fri, 17 Jan 2020 12:20:56 +0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Rob Landley <rob@...dley.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Pavankumar Kondeti <pkondeti@...eaurora.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Aaro Koskinen <aaro.koskinen@...ia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Will Deacon <will@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        James Morse <james.morse@....com>,
        Mark Rutland <mark.rutland@....com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Guenter Roeck <groeck@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>, linux-csky@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390@...r.kernel.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH RESEND v4] reboot: support offline CPUs before reboot

On Fri, Jan 17, 2020 at 12:16 PM Rob Landley <rob@...dley.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