This option measures the time spent in irqs-off critical
sections, with microsecond accuracy.

The default measurement method is a maximum search, which is
disabled by default and can be runtime (re-)started
via:

echo 0 > /sys/kernel/debug/tracing/tracing_max_latency

(Note that kernel size and overhead increases with this option
enabled. This option and the preempt-off timing option can be
used together or separately.)

source code:

kernel/trace/trace_irqsoff.c