Subject: Re: [PATCH] irq: Add node_affinity CPU masks for smarter irqbalance hints

From: Arjan van de Ven <[email-masked]>
Date: Mon, 23 Nov 2009 22:07:38 -0800

Peter Zijlstra wrote:
> On Mon, 2009-11-23 at 01:36 -0800, Peter P Waskiewicz Jr wrote:
> 
>> This mechanism isn't going to be used by any internal kernel mechanism
>> for determining interrupt placement or operation.  It's purely something
>> that either a driver can modify, or external script (through /proc),
>> that irqbalance will make use of.  If irqbalance isn't running, or the
>> current version of irqbalance doesn't support reading node_affinity,
>> then it won't affect the system's operation.
>>
>> If irqbalance does support it, it'll read whatever the supplied mask is,
>> and then will try and balance interrupts within that mask.  It will bail
>> if the mask is invalid, or won't apply to the running system, just like
>> how putting a bogus mask into smp_affinity is ignored.
>>
>> If there's something I'm missing beyond this with the two suggestions
>> you've made (I looked into those two parameters and tried to draw
>> conclusions), please let me know.
> 
> I don't see the point in adding it, if the driver wants to set a node
> cpu mask it can already do that using the regular smp affinity settings.
> 
> Same for userspace.

the problem is that there is no way currently that the driver can communicate
"I allocated all my metadata on THIS numa node". irqbalance and sysadmins need
that to not make really stupid decisions.....