Splitting ksoftirqd was somewhat useful.  Seems somebody is firing off
tasklets heftily, at least that's always current when the throttle kicks
in and saves the day.

(tip-rt13 UV100 box)

[   40.701221] ioatdma 0000:00:16.7: enabling device (0000 -> 0002)
[   40.701239] ioatdma 0000:00:16.7: PCI INT D -> GSI 46 (level, low) -> IRQ 46
[   40.701281] ioatdma 0000:00:16.7: setting latency timer to 64
[   40.701696] ioatdma 0000:00:16.7: irq 128 for MSI/MSI-X
[   40.951009] RT: throttling CPU25
[   40.951013] RT: Danger!  (sirq-tasklet/25) is potential runaway.
[   40.951016] sched: RT throttling activated
[   40.951019] RT: throttling CPU26
[   40.951026] RT: Danger!  (sirq-tasklet/26) is potential runaway.
[   40.960010] RT: throttling CPU13
[   40.960013] RT: Danger!  (sirq-tasklet/13) is potential runaway.
[   41.000014] RT: throttling CPU13
[   41.000014] RT: throttling CPU25
[   41.000014] RT: throttling CPU26
[   41.656925] Adding 1959924k swap on /dev/sda2.  Priority:-1 extents:1 across:1959924k 
[   41.950011] RT: throttling CPU25
[   41.950013] RT: Danger!  (sirq-tasklet/25) is potential runaway.
[   41.950024] RT: throttling CPU30
[   41.950031] RT: Danger!  (sirq-tasklet/30) is potential runaway.
[   41.951010] RT: throttling CPU21
[   41.951014] RT: Danger!  (sirq-tasklet/21) is potential runaway.
[   42.000014] RT: throttling CPU13