linux-kernel - Re: [PATCH v2 3/5] kernel/cpu: Allow non-zero CPU to be primary for suspend / kexec freeze

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Date:   Thu, 25 Apr 2019 14:02:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 3/5] kernel/cpu: Allow non-zero CPU to be primary for
 suspend / kexec freeze

On Thu, Apr 11, 2019 at 01:34:46PM +1000, Nicholas Piggin wrote:
> This patch provides an arch option, ARCH_SUSPEND_NONZERO_CPU, to
> opt-in to allowing suspend to occur on one of the housekeeping CPUs
> rather than hardcoded CPU0.
> 
> This will allow CPU0 to be a nohz_full CPU with a later change.
> 
> It may be possible for platforms with hardware/firmware restrictions
> on suspend/wake effectively support this by handing off the final
> stage to CPU0 when kernel housekeeping is no longer required. Another
> option is to make housekeeping / nohz_full mask dynamic at runtime,
> but the complexity could not be justified at this time.

Should we not tie this into whatever already allows an achitecture to
hotplug CPU-0? For instance, x86 default disallows this but has
cpu0_hotpluggable to allow this.

Presumably POWER already allows hotplugging CPU-0 ?