Understanding SOFT IRQ 1

Hi

I am just getting started with LTTng and Trace Compass. I can capture
Kernel and UST events.

My purpose is to investigate why a periodic task, running on an
isolated core, has occasional blips i.e. it's period (duration)
extends.

I have inserted tracepoints to mark normal and extended durations. In
Trace Compass the blip tracepoint is preceded by a brown state that is
'SOFT IRQ 1'. This extends down all cores. It lasts 105us and is
likely to be the reason for the blip.

Please could someone explain to me what may be happening here and how
can I find out the reason for this SOFT IRQ?

Best regards
David