linux-kernel - Re: [PATCH RFC v2] reboot: hotplug cpus in migrate_to_reboot

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Date:   Fri, 10 Jan 2020 18:58:30 +0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Aaro Koskinen <aaro.koskinen@...ia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guenter Roeck <groeck@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2] reboot: hotplug cpus in migrate_to_reboot_cpu()

On Fri, Jan 10, 2020 at 4:15 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Hsin-Yi Wang <hsinyi@...omium.org> writes:
>
> > @@ -220,8 +221,6 @@ void migrate_to_reboot_cpu(void)
> >       /* The boot cpu is always logical cpu 0 */
> >       int cpu = reboot_cpu;
> >
> > -     cpu_hotplug_disable();
> > -
> >       /* Make certain the cpu I'm about to reboot on is online */
> >       if (!cpu_online(cpu))
> >               cpu = cpumask_first(cpu_online_mask);
> > @@ -231,6 +230,11 @@ void migrate_to_reboot_cpu(void)
> >
> >       /* Make certain I only run on the appropriate processor */
> >       set_cpus_allowed_ptr(current, cpumask_of(cpu));
> > +
> > +     /* Hotplug other cpus if possible */
> > +#ifdef CONFIG_HOTPLUG_CPU
> > +     offline_secondary_cpus(cpu);
> > +#endif
>
> In general I like the idea, but shouldn't this remove the architecture
> code as a follow up?
>
> Also this needs to be explicitely enabled per architecture (opt-in) and
> not as an unconditional operation for all architectures which support
> CPU hotplug.
>
> Thanks,
>
>         tglx

Thanks for your comment.

I'll make this another config and depends on HOTPLUG_CPU.

I did some check on how architectures implement smp_send_stop. Most
architecture would loop through all other cpus in cpu_online_mask and
call ipi stop to each. If they enable this config, the loop would be
empty loop. In case they don't enable this config, they still need
original smp_send_stop (or as a fallback if some cpus fails to
offline).
Currently there are 2 architectures (csky, alpha) that call ipi on
possible cpus (instead of online cpus). Besides this, I'm not sure if
there's other architecture code that should be cleaned up.

Thanks,

Hsin-Yi