sys kernel debug tracing trace - MarekBykowski/readme GitHub Wiki
https://lwn.net/Articles/290277/
irqs-off: 'd' interrupts are disabled. '.' otherwise.
need-resched: 'N' task need_resched is set, '.' otherwise.
hardirq/softirq: 'H' - hard irq occurred inside a softirq. 'h' - hard irq is running 's' - soft irq is running '.' - normal context.
preempt-depth: The level of preempt_disabled