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202 lines
9.5 KiB
202 lines
9.5 KiB
REDUCING OS JITTER DUE TO PER-CPU KTHREADS
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This document lists per-CPU kthreads in the Linux kernel and presents
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options to control their OS jitter. Note that non-per-CPU kthreads are
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not listed here. To reduce OS jitter from non-per-CPU kthreads, bind
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them to a "housekeeping" CPU dedicated to such work.
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REFERENCES
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o Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
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o Documentation/cgroups: Using cgroups to bind tasks to sets of CPUs.
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o man taskset: Using the taskset command to bind tasks to sets
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of CPUs.
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o man sched_setaffinity: Using the sched_setaffinity() system
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call to bind tasks to sets of CPUs.
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o /sys/devices/system/cpu/cpuN/online: Control CPU N's hotplug state,
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writing "0" to offline and "1" to online.
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o In order to locate kernel-generated OS jitter on CPU N:
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cd /sys/kernel/debug/tracing
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echo 1 > max_graph_depth # Increase the "1" for more detail
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echo function_graph > current_tracer
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# run workload
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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
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that does not require per-CPU kthreads. This will prevent these
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kthreads from being created in the first place. (This will
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work for most people, as this hardware, though important, is
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relatively old and is produced in relatively low unit volumes.)
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2. Do all eHCA-Infiniband-related work on other CPUs, including
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interrupts.
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3. Rework the eHCA driver so that its per-CPU kthreads are
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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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1. Use irq affinity to force the irq threads to execute on
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some other CPU.
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Name: kcmtpd_ctr_%d
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Purpose: Handle Bluetooth work.
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To reduce its OS jitter, do one of the following:
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1. Don't use Bluetooth, in which case these kthreads won't be
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created in the first place.
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2. Use irq affinity to force Bluetooth-related interrupts to
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occur on some other CPU and furthermore initiate all
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Bluetooth activity on some other CPU.
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Name: ksoftirqd/%u
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Purpose: Execute softirq handlers when threaded or when under heavy load.
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To reduce its OS jitter, each softirq vector must be handled
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separately as follows:
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TIMER_SOFTIRQ: Do all of the following:
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1. To the extent possible, keep the CPU out of the kernel when it
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is non-idle, for example, by avoiding system calls and by forcing
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both kernel threads and interrupts to execute elsewhere.
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2. Build with CONFIG_HOTPLUG_CPU=y. After boot completes, force
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the CPU offline, then bring it back online. This forces
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recurring timers to migrate elsewhere. If you are concerned
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with multiple CPUs, force them all offline before bringing the
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first one back online. Once you have onlined the CPUs in question,
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do not offline any other CPUs, because doing so could force the
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timer back onto one of the CPUs in question.
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NET_TX_SOFTIRQ and NET_RX_SOFTIRQ: Do all of the following:
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1. Force networking interrupts onto other CPUs.
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2. Initiate any network I/O on other CPUs.
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3. Once your application has started, prevent CPU-hotplug operations
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from being initiated from tasks that might run on the CPU to
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be de-jittered. (It is OK to force this CPU offline and then
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bring it back online before you start your application.)
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BLOCK_SOFTIRQ: Do all of the following:
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1. Force block-device interrupts onto some other CPU.
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2. Initiate any block I/O on other CPUs.
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3. Once your application has started, prevent CPU-hotplug operations
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from being initiated from tasks that might run on the CPU to
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be de-jittered. (It is OK to force this CPU offline and then
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bring it back online before you start your application.)
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BLOCK_IOPOLL_SOFTIRQ: Do all of the following:
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1. Force block-device interrupts onto some other CPU.
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2. Initiate any block I/O and block-I/O polling on other CPUs.
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3. Once your application has started, prevent CPU-hotplug operations
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from being initiated from tasks that might run on the CPU to
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be de-jittered. (It is OK to force this CPU offline and then
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bring it back online before you start your application.)
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TASKLET_SOFTIRQ: Do one or more of the following:
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1. Avoid use of drivers that use tasklets. (Such drivers will contain
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calls to things like tasklet_schedule().)
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2. Convert all drivers that you must use from tasklets to workqueues.
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3. Force interrupts for drivers using tasklets onto other CPUs,
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and also do I/O involving these drivers on other CPUs.
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SCHED_SOFTIRQ: Do all of the following:
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1. Avoid sending scheduler IPIs to the CPU to be de-jittered,
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for example, ensure that at most one runnable kthread is present
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on that CPU. If a thread that expects to run on the de-jittered
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CPU awakens, the scheduler will send an IPI that can result in
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a subsequent SCHED_SOFTIRQ.
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2. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
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CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU
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to be de-jittered is marked as an adaptive-ticks CPU using the
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"nohz_full=" boot parameter. This reduces the number of
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scheduler-clock interrupts that the de-jittered CPU receives,
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minimizing its chances of being selected to do the load balancing
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work that runs in SCHED_SOFTIRQ context.
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3. To the extent possible, keep the CPU out of the kernel when it
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is non-idle, for example, by avoiding system calls and by
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forcing both kernel threads and interrupts to execute elsewhere.
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This further reduces the number of scheduler-clock interrupts
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received by the de-jittered CPU.
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HRTIMER_SOFTIRQ: Do all of the following:
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1. To the extent possible, keep the CPU out of the kernel when it
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is non-idle. For example, avoid system calls and force both
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kernel threads and interrupts to execute elsewhere.
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2. Build with CONFIG_HOTPLUG_CPU=y. Once boot completes, force the
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CPU offline, then bring it back online. This forces recurring
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timers to migrate elsewhere. If you are concerned with multiple
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CPUs, force them all offline before bringing the first one
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back online. Once you have onlined the CPUs in question, do not
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offline any other CPUs, because doing so could force the timer
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back onto one of the CPUs in question.
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RCU_SOFTIRQ: Do at least one of the following:
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1. Offload callbacks and keep the CPU in either dyntick-idle or
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adaptive-ticks state by doing all of the following:
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a. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
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CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU
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to be de-jittered is marked as an adaptive-ticks CPU using
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the "nohz_full=" boot parameter. Bind the rcuo kthreads
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to housekeeping CPUs, which can tolerate OS jitter.
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b. To the extent possible, keep the CPU out of the kernel
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when it is non-idle, for example, by avoiding system
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calls and by forcing both kernel threads and interrupts
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to execute elsewhere.
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2. Enable RCU to do its processing remotely via dyntick-idle by
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doing all of the following:
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a. Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
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b. Ensure that the CPU goes idle frequently, allowing other
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CPUs to detect that it has passed through an RCU quiescent
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state. If the kernel is built with CONFIG_NO_HZ_FULL=y,
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userspace execution also allows other CPUs to detect that
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the CPU in question has passed through a quiescent state.
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c. To the extent possible, keep the CPU out of the kernel
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when it is non-idle, for example, by avoiding system
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calls and by forcing both kernel threads and interrupts
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to execute elsewhere.
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Name: rcuc/%u
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Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
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To reduce its OS jitter, do at least one of the following:
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1. Build the kernel with CONFIG_PREEMPT=n. This prevents these
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kthreads from being created in the first place, and also obviates
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the need for RCU priority boosting. This approach is feasible
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for workloads that do not require high degrees of responsiveness.
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2. Build the kernel with CONFIG_RCU_BOOST=n. This prevents these
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kthreads from being created in the first place. This approach
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is feasible only if your workload never requires RCU priority
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boosting, for example, if you ensure frequent idle time on all
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CPUs that might execute within the kernel.
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3. Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
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which offloads all RCU callbacks to kthreads that can be moved
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off of CPUs susceptible to OS jitter. This approach prevents the
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rcuc/%u kthreads from having any work to do, so that they are
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never awakened.
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4. Ensure that the CPU never enters the kernel, and, in particular,
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avoid initiating any CPU hotplug operations on this CPU. This is
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another way of preventing any callbacks from being queued on the
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CPU, again preventing the rcuc/%u kthreads from having any work
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to do.
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Name: rcuob/%d, rcuop/%d, and rcuos/%d
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Purpose: Offload RCU callbacks from the corresponding CPU.
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To reduce its OS jitter, do at least one of the following:
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1. Use affinity, cgroups, or other mechanism to force these kthreads
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to execute on some other CPU.
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2. Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
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kthreads from being created in the first place. However, please
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note that this will not eliminate OS jitter, but will instead
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shift it to RCU_SOFTIRQ.
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Name: watchdog/%u
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Purpose: Detect software lockups on each CPU.
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To reduce its OS jitter, do at least one of the following:
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1. Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
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kthreads from being created in the first place.
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2. Echo a zero to /proc/sys/kernel/watchdog to disable the
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watchdog timer.
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3. Echo a large number of /proc/sys/kernel/watchdog_thresh in
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order to reduce the frequency of OS jitter due to the watchdog
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timer down to a level that is acceptable for your workload.
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