linux-kernel - Re: [PATCH v5] reboot: support offline CPUs before reboot

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Date:   Thu, 16 Jan 2020 17:25:43 +0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Sudeep Holla <sudeep.holla@....com>
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 v5] reboot: support offline CPUs before reboot

On Wed, Jan 15, 2020 at 7:41 PM Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Wed, Jan 15, 2020 at 02:34:10PM +0800, Hsin-Yi Wang 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.
> >
>
> What's the timing impact on systems with large number of CPUs(say 256 or
> more) ? I remember we added some change to reduce the wait times for
> offlining CPUs in system suspend path on arm64, still not negligible.
>

This is not the final solution, but I would still provided some data
points here:

Tested on my arm64 with 4 cpu: 2 a53 and 2 a72.
Offlining 3 cpu takes about 60~65 ms
Offlining 2 cpu(a53+a72 or a72+a72) takes about 42~47 ms
Offlining 1 cpu(a53 or a72) takes about 23~25 ms.

It would take longer time for systems with large number of CPUs.