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

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Message-Id: <36A821E7-7F37-42AF-9A05-7205FCBF89EE@kernel.crashing.org>
Date:	Fri, 24 Oct 2008 10:39:05 -0500
From:	Kumar Gala <galak@...nel.crashing.org>
To:	Chris Snook <csnook@...hat.com>
Cc:	maxk@...lcomm.com, LinuxPPC-dev list <linuxppc-dev@...abs.org>,
	linux-kernel Kernel <linux-kernel@...r.kernel.org>,
	tglx@...utronix.de
Subject: Re: default IRQ affinity change in v2.6.27 (breaking several SMP PPC based systems)


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.

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.

- k
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