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SubjectRe: CPU hotplug on i386
From
Date
Hi.

On Thu, 2005-03-17 at 01:40, Rafael J. Wysocki wrote:
> On Wednesday, 16 of March 2005 14:21, Pavel Machek wrote:
> > Hi!
> >
> > I tried to solve long-standing uglyness in swsusp cmp code by calling
> > cpu hotplug... only to find out that CONFIG_CPU_HOTPLUG is not
> > available on i386. Is there way to enable CPU_HOTPLUG on i386?
>
> Heh, that's exactly what I was thinking about. ;-)
>
> AFAICS we don't need the full CPU hotplug to do this. For suspend, we need to
> enable the CPU hotplug-related code in sched.c and cpu.c, and we need to
> implement the functions __cpu_disable() and __cpu_die() (called from within
> cpu.c) on each architecture for which we want swsusp to work on SMP.
>
> If that's acceptable, the CPU hotplug code in sched.c and cpu.c may be
> enabled by changing some #ifdefs there. For example, we could replace the
>
> #idef CONFIG_CPU_HOTPLUG
>
> with
>
> #if defined(CONFIG_CPU_HOTPLUG) || (defined(CONFIG_SOFTWARE_SUSPEND)
> && defined(CONFIG_SMP))
>
> wherever necessary.
>
> The implementation of __cpu_disable() and __cpu_die() may be a bit more
> tricky, however. In __cpu_disable() we need to save the settings of the local
> APIC of each CPU (on x86-64, at least) etc. In __cpu_die() we should give the
> "frozen" CPU some "neutral" code to execute, it seems (or "hlt" it?).
>
> I've looked at the ia64 implementation of these functions, but I haven't fully
> understood it yet.

It would be good (32 bit) if you handled MTRRs as well. Putting their
save/restore code in the driver model is racy (SMP calls). I've worked
around that in Suspend2 by removing the driver model support and making
the save/restore calls from the lowlevel code. I think this should be
considered the right thing to do.

By the way, I'm keen to test this with Suspend2 as well. I would have
done so before now, but I didn't realise Zwane had the support done.

Regards,

Nigel
--
Nigel Cunningham
Software Engineer, Canberra, Australia
http://www.cyclades.com
Bus: +61 (2) 6291 9554; Hme: +61 (2) 6292 8028; Mob: +61 (417) 100 574

Maintainer of Suspend2 Kernel Patches http://suspend2.net

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