linux-imx

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Reducing OS jitter due to per-cpu kthreads

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This document lists per-CPU kthreads in the Linux kernel and presents

options to control their OS jitter.  Note that non-per-CPU kthreads are

not listed here.  To reduce OS jitter from non-per-CPU kthreads, bind

them to a "housekeeping" CPU dedicated to such work.

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References

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-   Documentation/IRQ-affinity.txt:  Binding interrupts to sets of CPUs.

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-   Documentation/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.

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-   man taskset:  Using the taskset command to bind tasks to sets

    of CPUs.

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-   man sched_setaffinity:  Using the sched_setaffinity() system

    call to bind tasks to sets of CPUs.

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-   /sys/devices/system/cpu/cpuN/online:  Control CPU N's hotplug state,

    writing "0" to offline and "1" to online.

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-   In order to locate kernel-generated OS jitter on CPU N:

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        cd /sys/kernel/debug/tracing

        echo 1 > max_graph_depth # Increase the "1" for more detail

        echo function_graph > current_tracer

        # run workload

        cat per_cpu/cpuN/trace

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kthreads

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Name:

  ehca_comp/%u

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Purpose:

  Periodically process Infiniband-related work.

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To reduce its OS jitter, do any of the following:

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1.  Don't use eHCA Infiniband hardware, instead choosing hardware

    that does not require per-CPU kthreads.  This will prevent these

    kthreads from being created in the first place.  (This will

    work for most people, as this hardware, though important, is

    relatively old and is produced in relatively low unit volumes.)

2.  Do all eHCA-Infiniband-related work on other CPUs, including

    interrupts.

3.  Rework the eHCA driver so that its per-CPU kthreads are

    provisioned only on selected CPUs.

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Name:

  irq/%d-%s

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Purpose:

  Handle threaded interrupts.

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To reduce its OS jitter, do the following:

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