IRQ migration on CPU offline path

[Will Deacon <will.deacon@arm.com>]

Hi Thomas,

I've been looking at the IRQ migration code on x86 (fixup_irqs) for the CPU
hotplug path in order to try and fix a bug we have on ARM with the
desc->affinity mask not getting updated. Compared to irq_set_affinity, the code
is pretty whacky (I guess because it's called via stop_machine) so I wondered if
you could help me understand a few points:

(1) Affinity notifiers - we seem to ignore these and I guess they don't expect
    to be called from this context. It could lead to the cpu_rmap stuff being
    bogus, but that's not used for anything much. Do we just depend on people
    having hotplug notifiers to deal with this?

(2) Threaded handlers - I can't see where we update irqaction->thread_flags with
    IRQTF_AFFINITY for handlers that are migrated by the scheduler when a CPU
    goes down. Is this required?

(3) On x86, we rely on the irq_chip updating the desc->affinity mask in
    ->irq_set_affinity. It seems like we could use IRQ_SET_MASK_OK{_NOCOPY} for
    this and, in the case of the ioapic, return IRQ_SET_MASK_OK_NOCOPY (removing
    a redundant copy from the usual affinity path).

Of course, I could just be completely confused, which is why I haven't started
hacking code just yet :)

Cheers and sorry for the barrage of questions!,

Will