The ixl.4 manual page has documented that the threshold falsely detects
interrupt storms on 40Gbit NICs as long ago as 2015, and we have seen
similar false positives with the ioat(4) DMA device (which can push GB/s).
For example, synthetic load can be generated with tools/tools/ioat
'ioatcontrol 0 200 8192 1 1000' (allocate 200x8kB buffers, generate an
interrupt for each one, and do this for 1000 milliseconds). With
storm-detection disabled, the Broadwell-EP version of this device is capable
of generating ~350k real interrupts per second.
The following historical context comes from jhb@: Originally, the threshold
worked around incorrect routing of PCI INTx interrupts on single-CPU systems
which would end up in a hard hang during boot. Since the threshold was
added, our PCI interrupt routing was improved, most PCI interrupts use
edge-triggered MSI instead of level-triggered INTx, and typical systems have
multiple CPUs available to service interrupts.
On the off chance that the threshold is useful in the future, it remains
available as a tunable and sysctl.
Reviewed by: jhb
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20401
5d0e829978