linux-kernel - Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

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Message-ID: <20140928142726.GH5182@n2100.arm.linux.org.uk>
Date:	Sun, 28 Sep 2014 15:27:26 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Li Yang <leoli@...escale.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Chenhui Zhao <chenhui.zhao@...escale.com>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Zhuoyu.Zhang@...escale.com" <Zhuoyu.Zhang@...escale.com>,
	"Jason.Jin@...escale.com" <Jason.Jin@...escale.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

On Sun, Sep 28, 2014 at 06:57:18PM +0800, Li Yang wrote:
> On Fri, Sep 26, 2014 at 9:20 PM, Mark Rutland <mark.rutland@....com> wrote:
> >> > This looks to be a carbon copy of the vexpress pseudo-hotplug in
> >> > arch/arm/mach-vexpress/hotplug.c, which is obviously broken in the way
> >> > you describe above. Perhaps we should go about ripping that out?
> >>
> >> The Versatile Express does not support suspend so the only problem case
> >> is kexec.  However, isn't this support needed for big.LITTLE, and as
> >> the Versatile Express is the platform which these features get developed
> >> on, having working CPU hotplug seems rather fundamental for ARM kernel
> >> feature development.
> >>
> >> In that regard, Versatile Express is something of a special case.
> >
> > It is admittedly helpful during development to perform pseudo-hotplug on
> > Versatile Express. I have a local patch adding vexpress_cpu_disable so I
> > can test for bugs that only trigger if CPU0 is hotplugged.
> >
> > Given that, perhaps we should make it clearer that Versatile Express is
> > not a reference implementation for CPU hotplug; add some Kconfig (e.g.
> > VEXPRESS_PSEUDO_HOTPLUG) that depends on !KEXEC && !SUSPEND, and putting
> > a note in hotplug.c stating it's not suitable as a reference
> > implementation.
> >
> > ...but perhaps that's overkill.
> 
> I agree that the pseudo-hotplug is breaking the kexec.  However, I
> don't think it breaks the system suspend case as tasks and interrupts
> have been moved out of the non-booting CPU.  The pseudo-hotplug would
> be even better than real hotplug as it introduces less latency.

Sorry, that's not an acceptable reason to use it.

NAK.

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