Chapter 25. Tracing latencies with trace-cmd

Prerequisites

• You have administrator privileges.

Procedure

• Install the trace-cmd utility.

  # dnf install trace-cmd

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Prerequisites

• You have administrator privileges.

Procedure

• Enter trace-cmd command

  where command is an ftrace option.

  Note

  See the trace-cmd(1) man page for a complete list of commands and options. Most of the individual commands also have their own man pages, trace-cmd-command.

Examples

• Enable and start recording functions executing within the kernel while myapp runs.

  # trace-cmd record -p function myapp

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  This records functions from all CPUs and all tasks, even those not related to myapp.

• Display the result.

  # trace-cmd report

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• Record only functions that start with sched while myapp runs.

  # trace-cmd record -p function -l 'sched*' myapp

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• Enable all the IRQ events.

  # trace-cmd start -e irq

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• Start the wakeup_rt tracer.

  # trace-cmd start -p wakeup_rt

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• Start the preemptirqsoff tracer, while disabling function tracing.

  # trace-cmd start -p preemptirqsoff -d

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  Note

  The version of trace-cmd in RHEL 8 turns off ftrace_enabled instead of using the function-trace option. You can enable ftrace again with trace-cmd start -p function.

• Restore the state in which the system was before trace-cmd started modifying it.

  # trace-cmd start -p nop

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  This is important if you want to use the debugfs file system after using trace-cmd, whether or not the system was restarted in the meantime.

• Trace a single trace point.

  # trace-cmd record -e sched_wakeup ls /bin

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• Stop tracing.

  # trace-cmd record stop

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