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    Why do I see a lot of trace output in the /sys/kernel/trace file even though nop is set

    Posts: 5
    in LFX Mentorship: Linux Kernel

    # pwd
    /sys/kernel/tracing

    # cat current_tracer
    nop

    # echo sched_wakeup > /sys/kernel/tracing/set_event

    # cat trace
    < a lot of output related to sched_wakeup >

    Here, although I manually set sched_wakeup in the set_event file, I was expecting no traces to be captured because the current_tracer still has "nop". It's evident that, that isn't the case.

    Can someone please provide an answer or a link related to this? Thank you very much in advance.

    Best Answer

    • Posts: 6
      Answer ✓

      the "nop" tracer does not block trace_printk() calls and they will be traced unless you set tracing_on to 0. If you have a look at the commit where "nop" was implemented, you will find that one of the reasons to implement this tracer was exactly to get the trace_printk() logs without noise from other events.

    Answers

    • Posts: 5

      @picoteando said:
      the "nop" tracer does not block trace_printk() calls and they will be traced unless you set tracing_on to 0. If you have a look at the commit where "nop" was implemented, you will find that one of the reasons to implement this tracer was exactly to get the trace_printk() logs without noise from other events.

      That helps. Thank you very much.

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