-REDUCING OS JITTER DUE TO 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
+==========
-REFERENCES
+- Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
-o Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
+- Documentation/cgroup-v1: Using cgroups to bind tasks to sets of CPUs.
-o Documentation/cgroup-v1: Using cgroups to bind tasks to sets of CPUs.
-
-o man taskset: Using the taskset command to bind tasks to sets
+- man taskset: Using the taskset command to bind tasks to sets
of CPUs.
-o man sched_setaffinity: Using the sched_setaffinity() system
+- 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,
+- /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:
+- 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
# run workload
cat per_cpu/cpuN/trace
+kthreads
+========
+
+Name:
+ ehca_comp/%u
-KTHREADS
+Purpose:
+ Periodically process Infiniband-related work.
-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
provisioned only on selected CPUs.
-Name: irq/%d-%s
-Purpose: Handle threaded interrupts.
+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.
+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.
+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:
+
+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.
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:
+
+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:
+
+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.)
-IRQ_POLL_SOFTIRQ: Do all of the following:
+
+IRQ_POLL_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:
+
+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:
+
+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
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:
+
+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.
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:
+
+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. CONFIG_NO_HZ_FULL=y and 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
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
calls and by forcing both kernel threads and interrupts
to execute elsewhere.
-Name: kworker/%u:%d%s (cpu, id, priority)
-Purpose: Execute workqueue requests
+Name:
+ kworker/%u:%d%s (cpu, id, priority)
+
+Purpose:
+ Execute workqueue requests
+
To reduce its OS jitter, do any of the following:
+
1. Run your workload at a real-time priority, which will allow
preempting the kworker daemons.
2. A given workqueue can be made visible in the sysfs filesystem
by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
Such a workqueue can be confined to a given subset of the
- CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
+ CPUs using the ``/sys/devices/virtual/workqueue/*/cpumask`` sysfs
files. The set of WQ_SYSFS workqueues can be displayed using
"ls sys/devices/virtual/workqueue". That said, the workqueues
maintainer would like to caution people against indiscriminately
to remove it, even if its addition was a mistake.
3. Do any of the following needed to avoid jitter that your
application cannot tolerate:
+
a. Build your kernel with CONFIG_SLUB=y rather than
CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
use of each CPU's workqueues to run its cache_reap()
be able to build your kernel with CONFIG_CPU_FREQ=n to
avoid the CPU-frequency governor periodically running
on each CPU, including cs_dbs_timer() and od_dbs_timer().
+
WARNING: Please check your CPU specifications to
make sure that this is safe on your particular system.
d. As of v3.18, Christoph Lameter's on-demand vmstat workers
CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
avoiding OS jitter from rackmeter_do_timer().
-Name: rcuc/%u
-Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
+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
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.
+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_CPU=n, which will prevent these
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.
+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. Boot with "nosoftlockup=0", which will also prevent these kthreads
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