Re: [PATCH] x86: Document the hotplug code is incompatible with x86 irq handling

Pavel Machek
2007-06-08T15:11:48Z

Hi!

> I just realized that except for doing the code review and noticing
> that the current cpu hotplug code is fundamentally incompatible
> with x86 I haven't done anything about it. So here is my patch
> to document what is wrong.
>
> The current cpu hotplug code requires irqs to be migrated from a cpu
> outside of irq context. On x86 ioapics simply do not support this,
> making the code unfixable without major redesign of the generic cpu
> hotplug code.
>
> So this patch makes CPU_HOTPLUG on x86 depend on CONFIG_BROKEN
> and adds a WARN_ON so people that do enable it are not in doubt about
> which part of the code is broken, even if it does work for them.

> --- a/arch/i386/kernel/irq.c
> +++ b/arch/i386/kernel/irq.c
> @@ -312,6 +312,19 @@ void fixup_irqs(cpumask_t map)
> unsigned int irq;
> static int warned;
>
> + /*
> + * Function is so wrong at so many levels.
> + * - We migrate irqs that are directed at the cpu we are
> + * removing.

Is this about irq pinning?

> + * - We cannot safely migrate ioapic irqs on x86 except in
> + * side of irq context.

'inside'?

Can you be more specific for this one?

> + * Since someone probably finds this useful just warn very
> + * loudly until cpu hotplug is redesigned.
> + */
> + WARN_ON(1);

Ugh, no, this does not warn anyone. This will just make people ask me
why they see stack trace while suspending... and we are not interested
in the stack trace, anyway.

printk(KERN_WARNING)?

> index 5ce9443..a61c4f2 100644
> --- a/arch/x86_64/Kconfig
> +++ b/arch/x86_64/Kconfig
> @@ -429,7 +429,7 @@ config NR_CPUS
>
> config HOTPLUG_CPU
> bool "Support for suspend on SMP and hot-pluggable CPUs (EXPERIMENTAL)"
> - depends on SMP && HOTPLUG && EXPERIMENTAL
> + depends on SMP && HOTPLUG && EXPERIMENTAL && BROKEN
> help

Great, this will force everyone and their dog to enable broken, making
broken useless. Please don't.
Pavel
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