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+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:
+1.      Force networking interrupts onto other CPUs.
+2.      Initiate any network I/O on other CPUs.
+3.      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:
+1.      Force block-device interrupts onto some other CPU.
+2.      Initiate any block I/O on other CPUs.
+3.      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_IOPOLL_SOFTIRQ:  Do all of the following:
+1.      Force block-device interrupts onto some other CPU.
+2.      Initiate any block I/O and block-I/O polling on other CPUs.
+3.      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:
+1.      Avoid use of drivers that use tasklets.  (Such drivers will contain
+        calls to things like tasklet_schedule().)
+2.      Convert all drivers that you must use from tasklets to workqueues.
+3.      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:
+1.      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.
+2.      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.
+3.      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:
+1.      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.
+2.      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:
+1.      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.
+2.      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.