Re: [PATCH 0/9] Support using MSI interrupts in ntb_transport

[Dave Jiang]

On 1/31/2019 4:41 PM, Logan Gunthorpe wrote:
> On 2019-01-31 3:46 p.m., Dave Jiang wrote:
> > I believe irqbalance writes to the file /proc/irq/N/smp_affinity. So
> > maybe take a look at the code that starts from there and see if it would
> > have any impact on your stuff.
> Ok, well on my system I can write to the smp_affinity all day and the
> MSI interrupts still work fine.

Maybe your code is ok then. If the stats show up in /proc/interrupts 
then you can see it moving to different cores.

> The MSI code is a bit difficult to trace and audit with all the
> different chips and the parent chips which I don't have a good
> understanding of. But I can definitely see that it could be possible for
> some chips to change the address as smp_affinitiy will eventually
> sometimes call msi_domain_set_affinity() which does seem to recompose
> the message and write it back to the chip.
>
> So, I could relatively easily add a callback to msi_desc to catch this
> and resend the MSI address/data. However, I'm not sure how this is ever
> done atomically. It seems like there would be a race while the device
> updates its address where old interrupts could be triggered. This race
> would be much longer for us when sending this information over the NTB
> link. Though, I guess if the only change is that it encodes CPU
> information in the address then that would not be an issue. However, I'm
> not sure I can say that for certain without a comprehensive
> understanding of all the IRQ chips.
>
> Any thoughts on this?

Yeah I'm not sure what to do about it either as I'm not super familiar 
with that area either. Just making note of what I encountered. And you 
are right, the updated info has to go over NTB for the other side to 
write to the updated place. So there's a lot of latency involved.



> Logan