On 11/08/2016 03:56 PM, Christoph Hellwig wrote:

On Tue, Nov 08, 2016 at 08:47:21AM +0100, Hannes Reinecke wrote:

quoted

Add a reverse-mapping function to return the interrupt vector for
any CPU if interrupt affinity is enabled.

What's the use case of it?

Also as-is this won't work due to the non-affinity vectors that
have the affinity set to all cpus.  It will get even worse if we have
to support things like virtio_net that have multiple interrupts per
CPU due to the send and receive virtqueues.

The use-case here is that one needs to feed the MSI-X index into the 
driver command structure. While we can extract that number trivially 
with scsi-mq, but for scsi-sq we don't have such means.

So if we start assigning interrupt affinity per default we need to 
figure out the msi-x index from a given SCSI command.
Currently most of these drivers keep an internal CPU map which I'd love 
to get rid of.
Hence this patch.

And before you complain: Yes, this patch is wrong; it returns the vector 
and not the index (which is what I'm after).
I found that on my test machine :-(

The main impetus of this RFC is to figure out if such a function would 
have a chance of getting upstream, or if I have to continue use cpumaps 
in the drivers.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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