Ftrace has a number of plugins (or tracers) that it works with. First, you need to ensure that the irqsoff latency tracer (or plugin of Ftrace) is actually enabled within the kernel. You can check this in two different ways:
- Check the kernel config file (grep for CONFIG_IRQSOFF_TRACER within it).
- Check the available tracers (or plugins) via Ftrace infrastructure.
We'll go with the latter option here:
$ sudo cat /sys/kernel/debug/tracing/available_tracers
hwlat blk mmiotrace function_graph wakeup_dl wakeup_rt wakeup function nop
In the preceding output, the irqsoff tracer – the one we require – is missing! This is usually the case and implies that you will have to configure the kernel (turning it on) and (re)build your custom 5.4 kernel. (This will be provided as an exercise in the Questions section at the end of this chapter.) We also recommend that...