kernel-per-CPU-kthreads.txt

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.