Kernel-3.10.0-957.el7_kernel-per-CPU-kthreads

REDUCING OS JITTER DUE TO PER-CPU KTHREADS

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.

REFERENCES

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

o Documentation/cgroups: Using cgroups to bind tasks to sets of CPUs.

o man taskset: Using the taskset command to bind tasks to sets
of CPUs.

o man sched_setaffinity: Using the sched_setaffinity() system
call to bind tasks to sets of CPUs.

o /sys/devices/system/cpu/cpuN/online: Control CPU N’s hotplug state,
writing “0” to offline and “1” to online.

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

    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

KTHREADS

Name: ehca_comp/%u
Purpose: Periodically process Infiniband-related work.
To reduce its OS jitter, do any of the following:

  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.

Name: irq/%d-%s
Purpose: Handle threaded interrupts.
To reduce its OS jitter, do the following:

  1. Use irq affinity to force the irq threads to execute on
    some other CPU.

Name: kcmtpd_ctr_%d
Purpose: Handle Bluetooth work.
To reduce its OS jitter, do one of the following:

  1. Don’t use Bluetooth, in which case these kthreads won’t be
    created in the first place.
  2. Use irq affinity to force Bluetooth-related interrupts to
    occur on some other CPU and furthermore initiate all
    Bluetooth activity on some other CPU.

Name: ksoftirqd/%u
Purpose: Execute softirq handlers when threaded or when under heavy load.
To reduce its OS jitter, each softirq vector must be handled
separately as follows:
TIMER_SOFTIRQ: Do all of the following:

  1. To the extent possible, keep the CPU out of the kernel when it
    is non-idle, for example, by avoiding system calls and by forcing
    both kernel threads and interrupts to execute elsewhere.
  2. Build with CONFIG_HOTPLUG_CPU=y. After boot completes, force
    the CPU offline, then bring it back online. This forces
    recurring timers to migrate elsewhere. If you are concerned
    with multiple CPUs, force them all offline before bringing the
    first one back online. Once you have onlined the CPUs in question,
    do not offline any other CPUs, because doing so could force the
    timer back onto one of the CPUs in question.
    NET_TX_SOFTIRQ and NET_RX_SOFTIRQ: Do all of the following:
  3. Force networking interrupts onto other CPUs.
  4. Initiate any network I/O on other CPUs.
  5. Once your application has started, prevent CPU-hotplug operations
    from being initiated from tasks that might run on the CPU to
    be de-jittered. (It is OK to force this CPU offline and then
    bring it back online before you start your application.)
    BLOCK_SOFTIRQ: Do all of the following:
  6. Force block-device interrupts onto some other CPU.
  7. Initiate any block I/O on other CPUs.
  8. Once your application has started, prevent CPU-hotplug operations
    from being initiated from tasks that might run on the CPU to
    be de-jittered. (It is OK to force this CPU offline and then
    bring it back online before you start your application.)
    IRQ_POLL_SOFTIRQ: Do all of the following:
  9. Force block-device interrupts onto some other CPU.
  10. Initiate any block I/O and block-I/O polling on other CPUs.
  11. Once your application has started, prevent CPU-hotplug operations
    from being initiated from tasks that might run on the CPU to
    be de-jittered. (It is OK to force this CPU offline and then
    bring it back online before you start your application.)
    TASKLET_SOFTIRQ: Do one or more of the following:
  12. Avoid use of drivers that use tasklets. (Such drivers will contain
    calls to things like tasklet_schedule().)
  13. Convert all drivers that you must use from tasklets to workqueues.
  14. Force interrupts for drivers using tasklets onto other CPUs,
    and also do I/O involving these drivers on other CPUs.
    SCHED_SOFTIRQ: Do all of the following:
  15. Avoid sending scheduler IPIs to the CPU to be de-jittered,
    for example, ensure that at most one runnable kthread is present
    on that CPU. If a thread that expects to run on the de-jittered
    CPU awakens, the scheduler will send an IPI that can result in
    a subsequent SCHED_SOFTIRQ.
  16. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
    CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU
    to be de-jittered is marked as an adaptive-ticks CPU using the
    “nohz_full=” boot parameter. This reduces the number of
    scheduler-clock interrupts that the de-jittered CPU receives,
    minimizing its chances of being selected to do the load balancing
    work that runs in SCHED_SOFTIRQ context.
  17. To the extent possible, keep the CPU out of the kernel when it
    is non-idle, for example, by avoiding system calls and by
    forcing both kernel threads and interrupts to execute elsewhere.
    This further reduces the number of scheduler-clock interrupts
    received by the de-jittered CPU.
    HRTIMER_SOFTIRQ: Do all of the following:
  18. To the extent possible, keep the CPU out of the kernel when it
    is non-idle. For example, avoid system calls and force both
    kernel threads and interrupts to execute elsewhere.
  19. Build with CONFIG_HOTPLUG_CPU=y. Once boot completes, force the
    CPU offline, then bring it back online. This forces recurring
    timers to migrate elsewhere. If you are concerned with multiple
    CPUs, force them all offline before bringing the first one
    back online. Once you have onlined the CPUs in question, do not
    offline any other CPUs, because doing so could force the timer
    back onto one of the CPUs in question.
    RCU_SOFTIRQ: Do at least one of the following:
  20. Offload callbacks and keep the CPU in either dyntick-idle or
    adaptive-ticks state by doing all of the following:
    a. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
     CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU
     to be de-jittered is marked as an adaptive-ticks CPU using
     the "nohz_full=" boot parameter.  Bind the rcuo kthreads
     to housekeeping CPUs, which can tolerate OS jitter.
    
    b. To the extent possible, keep the CPU out of the kernel
     when it is non-idle, for example, by avoiding system
     calls and by forcing both kernel threads and interrupts
     to execute elsewhere.
    
  21. Enable RCU to do its processing remotely via dyntick-idle by
    doing all of the following:
    a. Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
    b. Ensure that the CPU goes idle frequently, allowing other
     CPUs to detect that it has passed through an RCU quiescent
     state.    If the kernel is built with CONFIG_NO_HZ_FULL=y,
     userspace execution also allows other CPUs to detect that
     the CPU in question has passed through a quiescent state.
    
    c. To the extent possible, keep the CPU out of the kernel
     when it is non-idle, for example, by avoiding system
     calls and by forcing both kernel threads and interrupts
     to execute elsewhere.
    

Name: rcuc/%u
Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
To reduce its OS jitter, do at least one of the following:

  1. Build the kernel with CONFIG_PREEMPT=n. This prevents these
    kthreads from being created in the first place, and also obviates
    the need for RCU priority boosting. This approach is feasible
    for workloads that do not require high degrees of responsiveness.
  2. Build the kernel with CONFIG_RCU_BOOST=n. This prevents these
    kthreads from being created in the first place. This approach
    is feasible only if your workload never requires RCU priority
    boosting, for example, if you ensure frequent idle time on all
    CPUs that might execute within the kernel.
  3. Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
    which offloads all RCU callbacks to kthreads that can be moved
    off of CPUs susceptible to OS jitter. This approach prevents the
    rcuc/%u kthreads from having any work to do, so that they are
    never awakened.
  4. Ensure that the CPU never enters the kernel, and, in particular,
    avoid initiating any CPU hotplug operations on this CPU. This is
    another way of preventing any callbacks from being queued on the
    CPU, again preventing the rcuc/%u kthreads from having any work
    to do.

Name: rcuob/%d, rcuop/%d, and rcuos/%d
Purpose: Offload RCU callbacks from the corresponding CPU.
To reduce its OS jitter, do at least one of the following:

  1. Use affinity, cgroups, or other mechanism to force these kthreads
    to execute on some other CPU.
  2. Build with CONFIG_RCU_NOCB_CPUS=n, which will prevent these
    kthreads from being created in the first place. However, please
    note that this will not eliminate OS jitter, but will instead
    shift it to RCU_SOFTIRQ.

Name: watchdog/%u
Purpose: Detect software lockups on each CPU.
To reduce its OS jitter, do at least one of the following:

  1. Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
    kthreads from being created in the first place.
  2. Echo a zero to /proc/sys/kernel/watchdog to disable the
    watchdog timer.
  3. Echo a large number of /proc/sys/kernel/watchdog_thresh in
    order to reduce the frequency of OS jitter due to the watchdog
    timer down to a level that is acceptable for your workload.