linux-kernel - Re: [PATCH v5 1/3] cpu/hotplug: Allow suspend/resume CPU to be specified

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Date:   Fri, 26 Aug 2016 11:19:53 +0100
From:   Will Deacon <will.deacon@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     James Morse <james.morse@....com>, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 1/3] cpu/hotplug: Allow suspend/resume CPU to be
 specified

On Fri, Aug 26, 2016 at 12:09:38PM +0200, Thomas Gleixner wrote:
> On Fri, 26 Aug 2016, Will Deacon wrote:
> > On Wed, Aug 17, 2016 at 01:50:25PM +0100, James Morse wrote:
> > > disable_nonboot_cpus() assumes that the lowest numbered online CPU is
> > > the boot CPU, and that this is the correct CPU to run any power
> > > management code on.
> > > 
> > > On x86 this is always correct, as CPU0 cannot (easily) by taken offline.
> > > 
> > > On arm64 CPU0 can be taken offline. For hibernate/resume this means we
> > > may hibernate on a CPU other than CPU0. If the system is rebooted with
> > > kexec 'CPU0' will be assigned to a different physical CPU. This
> > > complicates hibernate/resume as now we can't trust the CPU numbers.
> > > Arch code can find the correct physical CPU, and ensure it is online
> > > before resume from hibernate begins, but also needs to influence
> > > disable_nonboot_cpus()s choice of CPU.
> > > 
> > > Rename disable_nonboot_cpus() as freeze_secondary_cpus() and add an
> > > argument indicating which CPU should be left standing. Follow the logic
> > > in migrate_to_reboot_cpu() to use the lowest numbered online CPU if the
> > > requested CPU is not online.
> > > Add disable_nonboot_cpus() as an inline function that has the existing
> > > behaviour.
> > > 
> > > Signed-off-by: James Morse <james.morse@....com>
> > > Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> > > ---
> > > An alternative is to provide two functions calling a common function,
> > > but this would mean spilling the cpu_maps_update_begin() into these two.
> > > 
> > >  include/linux/cpu.h | 6 +++++-
> > >  kernel/cpu.c        | 9 +++++----
> > >  2 files changed, 10 insertions(+), 5 deletions(-)
> > 
> > Thomas, does this look ok to you? If so, would you prefer to merge this
> > series via -tip, or have us take this one via the arm64 tree?
> 
> You can take it via ARM64. It's not conflicting with the stuff I have in the
> pipeline.
> 
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Brill, thanks Thomas.

Will