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SubjectRe: default IRQ affinity change in v2.6.27 (breaking several SMP PPC based systems)
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On Oct 24, 2008, at 11:09 AM, Chris Snook wrote:

> Kumar Gala wrote:
>> On Oct 24, 2008, at 10:17 AM, Chris Snook wrote:
>>> Kumar Gala wrote:
>>>> It appears the default IRQ affinity changes from being just cpu 0
>>>> to all cpu's. This breaks several PPC SMP systems in which only
>>>> a single processor is allowed to be selected as the destination
>>>> of the IRQ.
>>>> What is the right answer in fixing this? Should we:
>>>> cpumask_t irq_default_affinity = 1;
>>>> instead of
>>>> cpumask_t irq_default_affinity = CPU_MASK_ALL?
>>>
>>> On those systems, perhaps, but not universally. There's plenty of
>>> hardware where the physical topology of the machine is abstracted
>>> away from the OS, and you need to leave the mask wide open and let
>>> the APIC figure out where to map the IRQs. Ideally, we should
>>> probably make this decision based on the APIC, but if there's no
>>> PPC hardware that uses this technique, then it would suffice to
>>> make this arch-specific.
>> What did those systems do before this patch? Its one thing to
>> expose a mask in the ability to change the default mask in /proc/
>> irq/default_smp_affinity. Its another (and a regression in my
>> opinion) to change the mask value itself.
>
> Before the patch they took an extremely long time to boot if they
> had storage attached to each node of a multi-chassis system,
> performed poorly unless special irqbalance hackery or manual
> assignment was used, and imposed artificial restrictions on the
> granularity of hardware partitioning to ensure that CPU 0 would
> always be a CPU that could service all interrupts necessary to boot
> the OS.
>
>> As for making it ARCH specific, that doesn't really help since not
>> all PPC hw has the limitation I spoke of. Not even all MPIC (in
>> our cases) have the limitation.
>
> What did those systems do before this patch? :)
>
> Making it arch-specific is an extremely simple way to solve your
> problem without making trouble for the people who wanted this patch
> in the first place. If PPC needs further refinement to handle
> particular *PICs, you can implement that without touching any arch-
> generic code.


So why not just have x86 startup code set irq_default_affinity =
CPU_MASK_ALL than?

- k


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