kernel-per-CPU-kthreads.txt: standardize document format
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Use title markups; - use "-" for bulletted lists; - Split Name/Purpose on two lines, in order to make visually easier to read (in text format), and to bold the title (on ReST output) - Add blank lines to split bulleted lists; - use sub-titles for the several kthread softirq types; - mark one literal var with asterisk as such, in order to avoid an error warning on Sphinx. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab
Jonathan Corbet
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1 changed files with 121 additions and 35 deletions
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@ -1,27 +1,29 @@
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REDUCING OS JITTER DUE TO PER-CPU KTHREADS
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==========================================
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Reducing OS jitter due to per-cpu kthreads
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==========================================
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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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==========
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REFERENCES
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- Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
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o 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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o Documentation/cgroup-v1: 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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- 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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- 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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- /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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- 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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@ -29,12 +31,17 @@ o In order to locate kernel-generated OS jitter on CPU N:
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# run workload
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cat per_cpu/cpuN/trace
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kthreads
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========
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KTHREADS
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Name:
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ehca_comp/%u
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Purpose:
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Periodically process Infiniband-related work.
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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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@ -46,26 +53,45 @@ To reduce its OS jitter, do any of the following:
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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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Name:
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irq/%d-%s
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Purpose:
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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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Name:
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kcmtpd_ctr_%d
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Purpose:
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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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Name:
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ksoftirqd/%u
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Purpose:
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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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TIMER_SOFTIRQ
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-------------
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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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@ -76,34 +102,59 @@ TIMER_SOFTIRQ: Do all of the following:
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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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NET_TX_SOFTIRQ and NET_RX_SOFTIRQ
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---------------------------------
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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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BLOCK_SOFTIRQ
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-------------
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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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IRQ_POLL_SOFTIRQ: Do all of the following:
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IRQ_POLL_SOFTIRQ
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----------------
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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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TASKLET_SOFTIRQ
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---------------
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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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SCHED_SOFTIRQ
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-------------
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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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@ -120,7 +171,12 @@ SCHED_SOFTIRQ: Do all of the following:
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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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HRTIMER_SOFTIRQ
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---------------
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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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@ -131,9 +187,15 @@ HRTIMER_SOFTIRQ: Do all of the following:
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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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RCU_SOFTIRQ
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-----------
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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. CONFIG_NO_HZ_FULL=y and ensure that the CPU to be
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de-jittered is marked as an adaptive-ticks CPU using the
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"nohz_full=" boot parameter. Bind the rcuo kthreads to
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@ -142,8 +204,10 @@ RCU_SOFTIRQ: Do at least one of the following:
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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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@ -155,15 +219,20 @@ RCU_SOFTIRQ: Do at least one of the following:
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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: kworker/%u:%d%s (cpu, id, priority)
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Purpose: Execute workqueue requests
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Name:
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kworker/%u:%d%s (cpu, id, priority)
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Purpose:
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Execute workqueue requests
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To reduce its OS jitter, do any of the following:
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1. Run your workload at a real-time priority, which will allow
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preempting the kworker daemons.
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2. A given workqueue can be made visible in the sysfs filesystem
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by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
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Such a workqueue can be confined to a given subset of the
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CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
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CPUs using the ``/sys/devices/virtual/workqueue/*/cpumask`` sysfs
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files. The set of WQ_SYSFS workqueues can be displayed using
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"ls sys/devices/virtual/workqueue". That said, the workqueues
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maintainer would like to caution people against indiscriminately
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@ -173,6 +242,7 @@ To reduce its OS jitter, do any of the following:
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to remove it, even if its addition was a mistake.
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3. Do any of the following needed to avoid jitter that your
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application cannot tolerate:
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a. Build your kernel with CONFIG_SLUB=y rather than
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CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
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use of each CPU's workqueues to run its cache_reap()
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@ -186,6 +256,7 @@ To reduce its OS jitter, do any of the following:
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be able to build your kernel with CONFIG_CPU_FREQ=n to
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avoid the CPU-frequency governor periodically running
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on each CPU, including cs_dbs_timer() and od_dbs_timer().
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WARNING: Please check your CPU specifications to
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make sure that this is safe on your particular system.
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d. As of v3.18, Christoph Lameter's on-demand vmstat workers
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@ -222,9 +293,14 @@ To reduce its OS jitter, do any of the following:
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CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
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avoiding OS jitter from rackmeter_do_timer().
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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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Name:
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rcuc/%u
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Purpose:
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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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@ -244,9 +320,14 @@ To reduce its OS jitter, do at least one of the following:
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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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Name:
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rcuob/%d, rcuop/%d, and rcuos/%d
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Purpose:
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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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@ -254,9 +335,14 @@ To reduce its OS jitter, do at least one of the following:
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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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Name:
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watchdog/%u
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Purpose:
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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. Boot with "nosoftlockup=0", which will also prevent these kthreads
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