Re: default IRQ affinity change in v2.6.27 (breaking several SMP PPC based systems)

On Oct 24, 2008, at 11:09 AM, Chris Snook wrote:

Kumar Gala wrote:

quoted

On Oct 24, 2008, at 10:17 AM, Chris Snook wrote:

quoted

Kumar Gala wrote:

quoted

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.

quoted

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