Re: [PATCH] x86: Add check for number of available vectors before CPUdown [v2]

[rui wang]

On 12/20/13, Prarit Bhargava <prarit@xxxxxxxxxx> wrote:
>
>
> On 12/19/2013 01:05 PM, Tony Luck wrote:
>> On Wed, Dec 18, 2013 at 11:50 AM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
>>> Looks good to me.
>>
>> Though now I've been confused by an offline question about affinity.
>
> Heh :)  I'm pursuing it now.  Rui has asked a pretty good question that I
> don't
> know the answer to off the top of my head.  I'm still looking at the code.
>
>>
>> Suppose we have some interrupt that has affinity to multiple cpus. E.g.
>> (real example from one of my machines):
>>
>> # cat /proc/irq/94/smp_affinity_list
>> 26,54
>>
>> Now If I want to take either cpu26 or cpu54 offline - I'm guessing that I
>> don't
>> really need to find a new home for vector 94 - because the other one of
>> that
>> pair already has that set up.  But your check_vectors code doesn't look
>> like
>> it accounts for that - if we take cpu26 offline - it would see that
>> cpu54 doesn't
>> have 94 free - but doesn't check that it is for the same interrupt.
>>
>> But I may be mixing "vectors" and "irqs" here.
>
> Yep.  The question really is this: is the irq mapped to a single vector or
> multiple vectors. (I think)
>

The vector number for an irq is programmed in the LSB of the IOAPIC
IRTE (or MSI data register in the case of MSI/MSIx). So there can be
only one vector number (although multiple CPUs can be specified
through DM). An MSI-capable device can dynamically change the lower
few bits in the LSB to signal multiple interrupts with a contiguous
range of vectors in powers of 2,but each of these vectors is treated
as a separate IRQ. i.e. each of them has a separate irq desc, or a
separate line in the /proc/interrupt file. This patch shows the MSI
irq allocation in detail:
http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?id=51906e779f2b13b38f8153774c4c7163d412ffd9

Thanks
Rui


> P.
>
>>
>> -Tony
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