[LSF/MM TOPIC] irq affinity handling for high CPU count machines

[Kashyap Desai]

> > > > Today I am looking at one megaraid_sas related issue, and found
> > > > pci_alloc_irq_vectors(PCI_IRQ_AFFINITY) is used in the driver, so
> > > > looks each reply queue has been handled by more than one CPU if
> > > > there are more CPUs than MSIx vectors in the system, which is done
> > > > by generic irq affinity code, please see kernel/irq/affinity.c.
> >
> > Yes. That is a problematic area. If CPU and MSI-x(reply queue) is 1:1
> > mapped, we don't have any issue.
>
> I guess the problematic area is similar with the following link:
>
> 	https://marc.info/?l=linux-kernel&m=151748144730409&w=2

Hi Ming,

Above mentioned link is different discussion and looks like a generic
issue. megaraid_sas/mpt3sas will have same symptoms if irq affinity has
only offline CPUs.
Just for info - "In such condition, we can ask users to disable affinity
hit via module parameter " smp_affinity_enable".

>
> otherwise could you explain a bit about the area?

Please check below post for more details.

https://marc.info/?l=linux-scsi&m=151601833418346&w=2