SBalance's statistic of new interrupts for each CPU is inherently flawed in
that it cannot track IRQ migration that occurs in between balance periods.
As a result, SBalance can observe a flawed number of new interrupts for a
CPU, which hurts its balancing decisions.
Furthermore, SBalance incorrectly assumes that IRQs are affined where
SBalance last placed them, which breaks SBalance entirely when the
assumption doesn't hold true.
As it turns out, it can be quite common to change an IRQ's affinity and
observe a successful return value despite the IRQ not actually moving. At
the very least this is observed on ARM's GICv3, and results in SBalance
never moving such an IRQ ever again because SBalance always thinks it has
zero new interrupts.
Since we can't trust irqchip drivers or hardware, gather IRQ statistics
directly in order to get the true number of new interrupts for each CPU and
the actual affinity of each IRQ based on the last CPU it fired upon.
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
dca8268454