kthread: Document ways of 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

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.