1==========================================
2Reducing OS jitter due to per-cpu kthreads
3==========================================
4
5This document lists per-CPU kthreads in the Linux kernel and presents
6options to control their OS jitter.  Note that non-per-CPU kthreads are
7not listed here.  To reduce OS jitter from non-per-CPU kthreads, bind
8them to a "housekeeping" CPU dedicated to such work.
9
10References
11==========
12
13-	Documentation/core-api/irq/irq-affinity.rst:  Binding interrupts to sets of CPUs.
14
15-	Documentation/admin-guide/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.
16
17-	man taskset:  Using the taskset command to bind tasks to sets
18	of CPUs.
19
20-	man sched_setaffinity:  Using the sched_setaffinity() system
21	call to bind tasks to sets of CPUs.
22
23-	/sys/devices/system/cpu/cpuN/online:  Control CPU N's hotplug state,
24	writing "0" to offline and "1" to online.
25
26-	In order to locate kernel-generated OS jitter on CPU N:
27
28		cd /sys/kernel/debug/tracing
29		echo 1 > max_graph_depth # Increase the "1" for more detail
30		echo function_graph > current_tracer
31		# run workload
32		cat per_cpu/cpuN/trace
33
34kthreads
35========
36
37Name:
38  ehca_comp/%u
39
40Purpose:
41  Periodically process Infiniband-related work.
42
43To reduce its OS jitter, do any of the following:
44
451.	Don't use eHCA Infiniband hardware, instead choosing hardware
46	that does not require per-CPU kthreads.  This will prevent these
47	kthreads from being created in the first place.  (This will
48	work for most people, as this hardware, though important, is
49	relatively old and is produced in relatively low unit volumes.)
502.	Do all eHCA-Infiniband-related work on other CPUs, including
51	interrupts.
523.	Rework the eHCA driver so that its per-CPU kthreads are
53	provisioned only on selected CPUs.
54
55
56Name:
57  irq/%d-%s
58
59Purpose:
60  Handle threaded interrupts.
61
62To reduce its OS jitter, do the following:
63
641.	Use irq affinity to force the irq threads to execute on
65	some other CPU.
66
67Name:
68  kcmtpd_ctr_%d
69
70Purpose:
71  Handle Bluetooth work.
72
73To reduce its OS jitter, do one of the following:
74
751.	Don't use Bluetooth, in which case these kthreads won't be
76	created in the first place.
772.	Use irq affinity to force Bluetooth-related interrupts to
78	occur on some other CPU and furthermore initiate all
79	Bluetooth activity on some other CPU.
80
81Name:
82  ksoftirqd/%u
83
84Purpose:
85  Execute softirq handlers when threaded or when under heavy load.
86
87To reduce its OS jitter, each softirq vector must be handled
88separately as follows:
89
90TIMER_SOFTIRQ
91-------------
92
93Do all of the following:
94
951.	To the extent possible, keep the CPU out of the kernel when it
96	is non-idle, for example, by avoiding system calls and by forcing
97	both kernel threads and interrupts to execute elsewhere.
982.	Build with CONFIG_HOTPLUG_CPU=y.  After boot completes, force
99	the CPU offline, then bring it back online.  This forces
100	recurring timers to migrate elsewhere.	If you are concerned
101	with multiple CPUs, force them all offline before bringing the
102	first one back online.  Once you have onlined the CPUs in question,
103	do not offline any other CPUs, because doing so could force the
104	timer back onto one of the CPUs in question.
105
106NET_TX_SOFTIRQ and NET_RX_SOFTIRQ
107---------------------------------
108
109Do all of the following:
110
1111.	Force networking interrupts onto other CPUs.
1122.	Initiate any network I/O on other CPUs.
1133.	Once your application has started, prevent CPU-hotplug operations
114	from being initiated from tasks that might run on the CPU to
115	be de-jittered.  (It is OK to force this CPU offline and then
116	bring it back online before you start your application.)
117
118BLOCK_SOFTIRQ
119-------------
120
121Do all of the following:
122
1231.	Force block-device interrupts onto some other CPU.
1242.	Initiate any block I/O on other CPUs.
1253.	Once your application has started, prevent CPU-hotplug operations
126	from being initiated from tasks that might run on the CPU to
127	be de-jittered.  (It is OK to force this CPU offline and then
128	bring it back online before you start your application.)
129
130IRQ_POLL_SOFTIRQ
131----------------
132
133Do all of the following:
134
1351.	Force block-device interrupts onto some other CPU.
1362.	Initiate any block I/O and block-I/O polling on other CPUs.
1373.	Once your application has started, prevent CPU-hotplug operations
138	from being initiated from tasks that might run on the CPU to
139	be de-jittered.  (It is OK to force this CPU offline and then
140	bring it back online before you start your application.)
141
142TASKLET_SOFTIRQ
143---------------
144
145Do one or more of the following:
146
1471.	Avoid use of drivers that use tasklets.  (Such drivers will contain
148	calls to things like tasklet_schedule().)
1492.	Convert all drivers that you must use from tasklets to workqueues.
1503.	Force interrupts for drivers using tasklets onto other CPUs,
151	and also do I/O involving these drivers on other CPUs.
152
153SCHED_SOFTIRQ
154-------------
155
156Do all of the following:
157
1581.	Avoid sending scheduler IPIs to the CPU to be de-jittered,
159	for example, ensure that at most one runnable kthread is present
160	on that CPU.  If a thread that expects to run on the de-jittered
161	CPU awakens, the scheduler will send an IPI that can result in
162	a subsequent SCHED_SOFTIRQ.
1632.	CONFIG_NO_HZ_FULL=y and ensure that the CPU to be de-jittered
164	is marked as an adaptive-ticks CPU using the "nohz_full="
165	boot parameter.  This reduces the number of scheduler-clock
166	interrupts that the de-jittered CPU receives, minimizing its
167	chances of being selected to do the load balancing work that
168	runs in SCHED_SOFTIRQ context.
1693.	To the extent possible, keep the CPU out of the kernel when it
170	is non-idle, for example, by avoiding system calls and by
171	forcing both kernel threads and interrupts to execute elsewhere.
172	This further reduces the number of scheduler-clock interrupts
173	received by the de-jittered CPU.
174
175HRTIMER_SOFTIRQ
176---------------
177
178Do all of the following:
179
1801.	To the extent possible, keep the CPU out of the kernel when it
181	is non-idle.  For example, avoid system calls and force both
182	kernel threads and interrupts to execute elsewhere.
1832.	Build with CONFIG_HOTPLUG_CPU=y.  Once boot completes, force the
184	CPU offline, then bring it back online.  This forces recurring
185	timers to migrate elsewhere.  If you are concerned with multiple
186	CPUs, force them all offline before bringing the first one
187	back online.  Once you have onlined the CPUs in question, do not
188	offline any other CPUs, because doing so could force the timer
189	back onto one of the CPUs in question.
190
191RCU_SOFTIRQ
192-----------
193
194Do at least one of the following:
195
1961.	Offload callbacks and keep the CPU in either dyntick-idle or
197	adaptive-ticks state by doing all of the following:
198
199	a.	CONFIG_NO_HZ_FULL=y and ensure that the CPU to be
200		de-jittered is marked as an adaptive-ticks CPU using the
201		"nohz_full=" boot parameter.  Bind the rcuo kthreads to
202		housekeeping CPUs, which can tolerate OS jitter.
203	b.	To the extent possible, keep the CPU out of the kernel
204		when it is non-idle, for example, by avoiding system
205		calls and by forcing both kernel threads and interrupts
206		to execute elsewhere.
207
2082.	Enable RCU to do its processing remotely via dyntick-idle by
209	doing all of the following:
210
211	a.	Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
212	b.	Ensure that the CPU goes idle frequently, allowing other
213		CPUs to detect that it has passed through an RCU quiescent
214		state.	If the kernel is built with CONFIG_NO_HZ_FULL=y,
215		userspace execution also allows other CPUs to detect that
216		the CPU in question has passed through a quiescent state.
217	c.	To the extent possible, keep the CPU out of the kernel
218		when it is non-idle, for example, by avoiding system
219		calls and by forcing both kernel threads and interrupts
220		to execute elsewhere.
221
222Name:
223  kworker/%u:%d%s (cpu, id, priority)
224
225Purpose:
226  Execute workqueue requests
227
228To reduce its OS jitter, do any of the following:
229
2301.	Run your workload at a real-time priority, which will allow
231	preempting the kworker daemons.
2322.	A given workqueue can be made visible in the sysfs filesystem
233	by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
234	Such a workqueue can be confined to a given subset of the
235	CPUs using the ``/sys/devices/virtual/workqueue/*/cpumask`` sysfs
236	files.	The set of WQ_SYSFS workqueues can be displayed using
237	"ls /sys/devices/virtual/workqueue".  That said, the workqueues
238	maintainer would like to caution people against indiscriminately
239	sprinkling WQ_SYSFS across all the workqueues.	The reason for
240	caution is that it is easy to add WQ_SYSFS, but because sysfs is
241	part of the formal user/kernel API, it can be nearly impossible
242	to remove it, even if its addition was a mistake.
2433.	Do any of the following needed to avoid jitter that your
244	application cannot tolerate:
245
246	a.	Build your kernel with CONFIG_SLUB=y rather than
247		CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
248		use of each CPU's workqueues to run its cache_reap()
249		function.
250	b.	Avoid using oprofile, thus avoiding OS jitter from
251		wq_sync_buffer().
252	c.	Limit your CPU frequency so that a CPU-frequency
253		governor is not required, possibly enlisting the aid of
254		special heatsinks or other cooling technologies.  If done
255		correctly, and if you CPU architecture permits, you should
256		be able to build your kernel with CONFIG_CPU_FREQ=n to
257		avoid the CPU-frequency governor periodically running
258		on each CPU, including cs_dbs_timer() and od_dbs_timer().
259
260		WARNING:  Please check your CPU specifications to
261		make sure that this is safe on your particular system.
262	d.	As of v3.18, Christoph Lameter's on-demand vmstat workers
263		commit prevents OS jitter due to vmstat_update() on
264		CONFIG_SMP=y systems.  Before v3.18, is not possible
265		to entirely get rid of the OS jitter, but you can
266		decrease its frequency by writing a large value to
267		/proc/sys/vm/stat_interval.  The default value is HZ,
268		for an interval of one second.	Of course, larger values
269		will make your virtual-memory statistics update more
270		slowly.  Of course, you can also run your workload at
271		a real-time priority, thus preempting vmstat_update(),
272		but if your workload is CPU-bound, this is a bad idea.
273		However, there is an RFC patch from Christoph Lameter
274		(based on an earlier one from Gilad Ben-Yossef) that
275		reduces or even eliminates vmstat overhead for some
276		workloads at https://lore.kernel.org/r/00000140e9dfd6bd-40db3d4f-c1be-434f-8132-7820f81bb586-000000@email.amazonses.com.
277	e.	If running on high-end powerpc servers, build with
278		CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
279		daemon from running on each CPU every second or so.
280		(This will require editing Kconfig files and will defeat
281		this platform's RAS functionality.)  This avoids jitter
282		due to the rtas_event_scan() function.
283		WARNING:  Please check your CPU specifications to
284		make sure that this is safe on your particular system.
285	f.	If running on Cell Processor, build your kernel with
286		CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
287		spu_gov_work().
288		WARNING:  Please check your CPU specifications to
289		make sure that this is safe on your particular system.
290	g.	If running on PowerMAC, build your kernel with
291		CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
292		avoiding OS jitter from rackmeter_do_timer().
293
294Name:
295  rcuc/%u
296
297Purpose:
298  Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
299
300To reduce its OS jitter, do at least one of the following:
301
3021.	Build the kernel with CONFIG_PREEMPT=n.  This prevents these
303	kthreads from being created in the first place, and also obviates
304	the need for RCU priority boosting.  This approach is feasible
305	for workloads that do not require high degrees of responsiveness.
3062.	Build the kernel with CONFIG_RCU_BOOST=n.  This prevents these
307	kthreads from being created in the first place.  This approach
308	is feasible only if your workload never requires RCU priority
309	boosting, for example, if you ensure frequent idle time on all
310	CPUs that might execute within the kernel.
3113.	Build with CONFIG_RCU_NOCB_CPU=y and boot with the rcu_nocbs=
312	boot parameter offloading RCU callbacks from all CPUs susceptible
313	to OS jitter.  This approach prevents the rcuc/%u kthreads from
314	having any work to do, so that they are never awakened.
3154.	Ensure that the CPU never enters the kernel, and, in particular,
316	avoid initiating any CPU hotplug operations on this CPU.  This is
317	another way of preventing any callbacks from being queued on the
318	CPU, again preventing the rcuc/%u kthreads from having any work
319	to do.
320
321Name:
322  rcuop/%d and rcuos/%d
323
324Purpose:
325  Offload RCU callbacks from the corresponding CPU.
326
327To reduce its OS jitter, do at least one of the following:
328
3291.	Use affinity, cgroups, or other mechanism to force these kthreads
330	to execute on some other CPU.
3312.	Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
332	kthreads from being created in the first place.  However, please
333	note that this will not eliminate OS jitter, but will instead
334	shift it to RCU_SOFTIRQ.
335