xref: /kernel/linux/linux-6.6/Documentation/admin-guide/kernel-per-CPU-kthreads.rst

==========================================
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
==========

-	Documentation/core-api/irq/irq-affinity.rst:  Binding interrupts to sets of CPUs.

-	Documentation/admin-guide/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.

-	man taskset:  Using the taskset command to bind tasks to sets
	of CPUs.

-	man sched_setaffinity:  Using the sched_setaffinity() system
	call to bind tasks to sets of CPUs.

-	/sys/devices/system/cpu/cpuN/online:  Control CPU N's hotplug state,
	writing "0" to offline and "1" to online.

-	In order to locate kernel-generated OS jitter on CPU N:

		cd /sys/kernel/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.)

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:

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.	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.  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.	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
		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.
	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:
  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
	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
	sprinkling WQ_SYSFS across all the workqueues.	The reason for
	caution is that it is easy to add WQ_SYSFS, but because sysfs is
	part of the formal user/kernel API, it can be nearly impossible
	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()
		function.
	b.	Avoid using oprofile, thus avoiding OS jitter from
		wq_sync_buffer().
	c.	Limit your CPU frequency so that a CPU-frequency
		governor is not required, possibly enlisting the aid of
		special heatsinks or other cooling technologies.  If done
		correctly, and if you CPU architecture permits, you should
		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
		commit prevents OS jitter due to vmstat_update() on
		CONFIG_SMP=y systems.  Before v3.18, is not possible
		to entirely get rid of the OS jitter, but you can
		decrease its frequency by writing a large value to
		/proc/sys/vm/stat_interval.  The default value is HZ,
		for an interval of one second.	Of course, larger values
		will make your virtual-memory statistics update more
		slowly.  Of course, you can also run your workload at
		a real-time priority, thus preempting vmstat_update(),
		but if your workload is CPU-bound, this is a bad idea.
		However, there is an RFC patch from Christoph Lameter
		(based on an earlier one from Gilad Ben-Yossef) that
		reduces or even eliminates vmstat overhead for some
		workloads at https://lore.kernel.org/r/00000140e9dfd6bd-40db3d4f-c1be-434f-8132-7820f81bb586-000000@email.amazonses.com.
	e.	If running on high-end powerpc servers, build with
		CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
		daemon from running on each CPU every second or so.
		(This will require editing Kconfig files and will defeat
		this platform's RAS functionality.)  This avoids jitter
		due to the rtas_event_scan() function.
		WARNING:  Please check your CPU specifications to
		make sure that this is safe on your particular system.
	f.	If running on Cell Processor, build your kernel with
		CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
		spu_gov_work().
		WARNING:  Please check your CPU specifications to
		make sure that this is safe on your particular system.
	g.	If running on PowerMAC, build your kernel with
		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.

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 boot with the rcu_nocbs=
	boot parameter offloading RCU callbacks from all 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:
  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
	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.