==========================================Reducing OS jitter due to per-cpu kthreads========================================== This document lists per-CPU kthreads in the Linux kernel and presentsoptions to control their OS jitter.  Note that non-per-CPU kthreads arenot listed here.  To reduce OS jitter from non-per-CPU kthreads, bindthem 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 handledseparately 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.	Avoid using oprofile, thus avoiding OS jitter from		wq_sync_buffer().	b.	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.	c.	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.	d.	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.	e.	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, rcuos/%d, and rcuog/%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.