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

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Message-ID: <20200115114112.GA3663@bogus>
Date:   Wed, 15 Jan 2020 11:41:12 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Hsin-Yi Wang <hsinyi@...omium.org>
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>,
        linux-kernel@...r.kernel.org, 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@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, Sudeep Holla <sudeep.holla@....com>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v5] reboot: support offline CPUs before reboot

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.

--
Regards,
Sudeep