SLE RT Hardware Testing #

The first task you need to do when evaluating hardware for real-time is described in the following procedure:

If you encounter a significant difference between latencies measured by both methods there might be a software bug involved. Such latencies are a significant part caused by the operating system, not the hardware. In such a case, contact SUSE for support.

The cyclictest command is part of the real-time kernel test suite contained in the rt-tests RPM package. This package is distributed as part of SUSE Linux Enterprise Real Time.

Make sure to be root or use sudo to run cyclictest.

When cyclictest is running, it shows the number of cycles performed and the minimal, actual, average and maximum latency measured. To change the behaviour of cyclictest, the following list gives you an overview of the available options:

The hardware latency detector (hwlat_detector.ko) is installed with the real-time kernel (the kernel-rt package). It is a special kernel module that is used to detect large system latencies. It is caused by the behavior of underlying hardware or firmware, independent of Linux itself (system management interrupts, SMIs).

The hardware latency detector module works by making all CPUs busy for a configurable amounts of time. This is done by calling stop_machine(), polling the CPU Time Stamp Counter (TSC) for some period, and then looking for gaps in the TSC data. These gaps indicate times when the polling was disturbed by System Management Interrupts (SMI).

To start using hardware latency detector, use as root:

modprobe hwlat_detector enabled=1 threshold=10

The threshold value is the minimum latency value to be considered in microseconds. Use the value that is relevant for your particular situation.

After the module is loaded, the /sys/kernel/debug/hwlat_detector/ directory is created. The directory contains several files:

After running the hardware latency detector for a significant amount of time, check the values of count and max. For a detailed analysis, you can use the output from sample.

For more information on cyclictest, see man 8 cyclictest.

For more information on the hardware latency detector, see /usr/src/linux/Documentation/hwlat_detector.txt.

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