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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	REDUCING OS JITTER DUE TO PER-CPU KTHREADS
2	
3	This document lists per-CPU kthreads in the Linux kernel and presents
4	options to control their OS jitter.  Note that non-per-CPU kthreads are
5	not listed here.  To reduce OS jitter from non-per-CPU kthreads, bind
6	them to a "housekeeping" CPU dedicated to such work.
7	
8	
9	REFERENCES
10	
11	o	Documentation/IRQ-affinity.txt:  Binding interrupts to sets of CPUs.
12	
13	o	Documentation/cgroups:  Using cgroups to bind tasks to sets of CPUs.
14	
15	o	man taskset:  Using the taskset command to bind tasks to sets
16		of CPUs.
17	
18	o	man sched_setaffinity:  Using the sched_setaffinity() system
19		call to bind tasks to sets of CPUs.
20	
21	o	/sys/devices/system/cpu/cpuN/online:  Control CPU N's hotplug state,
22		writing "0" to offline and "1" to online.
23	
24	o	In order to locate kernel-generated OS jitter on CPU N:
25	
26			cd /sys/kernel/debug/tracing
27			echo 1 > max_graph_depth # Increase the "1" for more detail
28			echo function_graph > current_tracer
29			# run workload
30			cat per_cpu/cpuN/trace
31	
32	
33	KTHREADS
34	
35	Name: ehca_comp/%u
36	Purpose: Periodically process Infiniband-related work.
37	To reduce its OS jitter, do any of the following:
38	1.	Don't use eHCA Infiniband hardware, instead choosing hardware
39		that does not require per-CPU kthreads.  This will prevent these
40		kthreads from being created in the first place.  (This will
41		work for most people, as this hardware, though important, is
42		relatively old and is produced in relatively low unit volumes.)
43	2.	Do all eHCA-Infiniband-related work on other CPUs, including
44		interrupts.
45	3.	Rework the eHCA driver so that its per-CPU kthreads are
46		provisioned only on selected CPUs.
47	
48	
49	Name: irq/%d-%s
50	Purpose: Handle threaded interrupts.
51	To reduce its OS jitter, do the following:
52	1.	Use irq affinity to force the irq threads to execute on
53		some other CPU.
54	
55	Name: kcmtpd_ctr_%d
56	Purpose: Handle Bluetooth work.
57	To reduce its OS jitter, do one of the following:
58	1.	Don't use Bluetooth, in which case these kthreads won't be
59		created in the first place.
60	2.	Use irq affinity to force Bluetooth-related interrupts to
61		occur on some other CPU and furthermore initiate all
62		Bluetooth activity on some other CPU.
63	
64	Name: ksoftirqd/%u
65	Purpose: Execute softirq handlers when threaded or when under heavy load.
66	To reduce its OS jitter, each softirq vector must be handled
67	separately as follows:
68	TIMER_SOFTIRQ:  Do all of the following:
69	1.	To the extent possible, keep the CPU out of the kernel when it
70		is non-idle, for example, by avoiding system calls and by forcing
71		both kernel threads and interrupts to execute elsewhere.
72	2.	Build with CONFIG_HOTPLUG_CPU=y.  After boot completes, force
73		the CPU offline, then bring it back online.  This forces
74		recurring timers to migrate elsewhere.	If you are concerned
75		with multiple CPUs, force them all offline before bringing the
76		first one back online.  Once you have onlined the CPUs in question,
77		do not offline any other CPUs, because doing so could force the
78		timer back onto one of the CPUs in question.
79	NET_TX_SOFTIRQ and NET_RX_SOFTIRQ:  Do all of the following:
80	1.	Force networking interrupts onto other CPUs.
81	2.	Initiate any network I/O on other CPUs.
82	3.	Once your application has started, prevent CPU-hotplug operations
83		from being initiated from tasks that might run on the CPU to
84		be de-jittered.  (It is OK to force this CPU offline and then
85		bring it back online before you start your application.)
86	BLOCK_SOFTIRQ:  Do all of the following:
87	1.	Force block-device interrupts onto some other CPU.
88	2.	Initiate any block I/O on other CPUs.
89	3.	Once your application has started, prevent CPU-hotplug operations
90		from being initiated from tasks that might run on the CPU to
91		be de-jittered.  (It is OK to force this CPU offline and then
92		bring it back online before you start your application.)
93	BLOCK_IOPOLL_SOFTIRQ:  Do all of the following:
94	1.	Force block-device interrupts onto some other CPU.
95	2.	Initiate any block I/O and block-I/O polling on other CPUs.
96	3.	Once your application has started, prevent CPU-hotplug operations
97		from being initiated from tasks that might run on the CPU to
98		be de-jittered.  (It is OK to force this CPU offline and then
99		bring it back online before you start your application.)
100	TASKLET_SOFTIRQ: Do one or more of the following:
101	1.	Avoid use of drivers that use tasklets.  (Such drivers will contain
102		calls to things like tasklet_schedule().)
103	2.	Convert all drivers that you must use from tasklets to workqueues.
104	3.	Force interrupts for drivers using tasklets onto other CPUs,
105		and also do I/O involving these drivers on other CPUs.
106	SCHED_SOFTIRQ: Do all of the following:
107	1.	Avoid sending scheduler IPIs to the CPU to be de-jittered,
108		for example, ensure that at most one runnable kthread is present
109		on that CPU.  If a thread that expects to run on the de-jittered
110		CPU awakens, the scheduler will send an IPI that can result in
111		a subsequent SCHED_SOFTIRQ.
112	2.	Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
113		CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU
114		to be de-jittered is marked as an adaptive-ticks CPU using the
115		"nohz_full=" boot parameter.  This reduces the number of
116		scheduler-clock interrupts that the de-jittered CPU receives,
117		minimizing its chances of being selected to do the load balancing
118		work that runs in SCHED_SOFTIRQ context.
119	3.	To the extent possible, keep the CPU out of the kernel when it
120		is non-idle, for example, by avoiding system calls and by
121		forcing both kernel threads and interrupts to execute elsewhere.
122		This further reduces the number of scheduler-clock interrupts
123		received by the de-jittered CPU.
124	HRTIMER_SOFTIRQ:  Do all of the following:
125	1.	To the extent possible, keep the CPU out of the kernel when it
126		is non-idle.  For example, avoid system calls and force both
127		kernel threads and interrupts to execute elsewhere.
128	2.	Build with CONFIG_HOTPLUG_CPU=y.  Once boot completes, force the
129		CPU offline, then bring it back online.  This forces recurring
130		timers to migrate elsewhere.  If you are concerned with multiple
131		CPUs, force them all offline before bringing the first one
132		back online.  Once you have onlined the CPUs in question, do not
133		offline any other CPUs, because doing so could force the timer
134		back onto one of the CPUs in question.
135	RCU_SOFTIRQ:  Do at least one of the following:
136	1.	Offload callbacks and keep the CPU in either dyntick-idle or
137		adaptive-ticks state by doing all of the following:
138		a.	Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
139			CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU
140			to be de-jittered is marked as an adaptive-ticks CPU using
141			the "nohz_full=" boot parameter.  Bind the rcuo kthreads
142			to housekeeping CPUs, which can tolerate OS jitter.
143		b.	To the extent possible, keep the CPU out of the kernel
144			when it is non-idle, for example, by avoiding system
145			calls and by forcing both kernel threads and interrupts
146			to execute elsewhere.
147	2.	Enable RCU to do its processing remotely via dyntick-idle by
148		doing all of the following:
149		a.	Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
150		b.	Ensure that the CPU goes idle frequently, allowing other
151			CPUs to detect that it has passed through an RCU quiescent
152			state.	If the kernel is built with CONFIG_NO_HZ_FULL=y,
153			userspace execution also allows other CPUs to detect that
154			the CPU in question has passed through a quiescent state.
155		c.	To the extent possible, keep the CPU out of the kernel
156			when it is non-idle, for example, by avoiding system
157			calls and by forcing both kernel threads and interrupts
158			to execute elsewhere.
159	
160	Name: kworker/%u:%d%s (cpu, id, priority)
161	Purpose: Execute workqueue requests
162	To reduce its OS jitter, do any of the following:
163	1.	Run your workload at a real-time priority, which will allow
164		preempting the kworker daemons.
165	2.	A given workqueue can be made visible in the sysfs filesystem
166		by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
167		Such a workqueue can be confined to a given subset of the
168		CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
169		files.	The set of WQ_SYSFS workqueues can be displayed using
170		"ls sys/devices/virtual/workqueue".  That said, the workqueues
171		maintainer would like to caution people against indiscriminately
172		sprinkling WQ_SYSFS across all the workqueues.	The reason for
173		caution is that it is easy to add WQ_SYSFS, but because sysfs is
174		part of the formal user/kernel API, it can be nearly impossible
175		to remove it, even if its addition was a mistake.
176	3.	Do any of the following needed to avoid jitter that your
177		application cannot tolerate:
178		a.	Build your kernel with CONFIG_SLUB=y rather than
179			CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
180			use of each CPU's workqueues to run its cache_reap()
181			function.
182		b.	Avoid using oprofile, thus avoiding OS jitter from
183			wq_sync_buffer().
184		c.	Limit your CPU frequency so that a CPU-frequency
185			governor is not required, possibly enlisting the aid of
186			special heatsinks or other cooling technologies.  If done
187			correctly, and if you CPU architecture permits, you should
188			be able to build your kernel with CONFIG_CPU_FREQ=n to
189			avoid the CPU-frequency governor periodically running
190			on each CPU, including cs_dbs_timer() and od_dbs_timer().
191			WARNING:  Please check your CPU specifications to
192			make sure that this is safe on your particular system.
193		d.	It is not possible to entirely get rid of OS jitter
194			from vmstat_update() on CONFIG_SMP=y systems, but you
195			can decrease its frequency by writing a large value
196			to /proc/sys/vm/stat_interval.	The default value is
197			HZ, for an interval of one second.  Of course, larger
198			values will make your virtual-memory statistics update
199			more slowly.  Of course, you can also run your workload
200			at a real-time priority, thus preempting vmstat_update(),
201			but if your workload is CPU-bound, this is a bad idea.
202			However, there is an RFC patch from Christoph Lameter
203			(based on an earlier one from Gilad Ben-Yossef) that
204			reduces or even eliminates vmstat overhead for some
205			workloads at https://lkml.org/lkml/2013/9/4/379.
206		e.	If running on high-end powerpc servers, build with
207			CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
208			daemon from running on each CPU every second or so.
209			(This will require editing Kconfig files and will defeat
210			this platform's RAS functionality.)  This avoids jitter
211			due to the rtas_event_scan() function.
212			WARNING:  Please check your CPU specifications to
213			make sure that this is safe on your particular system.
214		f.	If running on Cell Processor, build your kernel with
215			CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
216			spu_gov_work().
217			WARNING:  Please check your CPU specifications to
218			make sure that this is safe on your particular system.
219		g.	If running on PowerMAC, build your kernel with
220			CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
221			avoiding OS jitter from rackmeter_do_timer().
222	
223	Name: rcuc/%u
224	Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
225	To reduce its OS jitter, do at least one of the following:
226	1.	Build the kernel with CONFIG_PREEMPT=n.  This prevents these
227		kthreads from being created in the first place, and also obviates
228		the need for RCU priority boosting.  This approach is feasible
229		for workloads that do not require high degrees of responsiveness.
230	2.	Build the kernel with CONFIG_RCU_BOOST=n.  This prevents these
231		kthreads from being created in the first place.  This approach
232		is feasible only if your workload never requires RCU priority
233		boosting, for example, if you ensure frequent idle time on all
234		CPUs that might execute within the kernel.
235	3.	Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
236		which offloads all RCU callbacks to kthreads that can be moved
237		off of CPUs susceptible to OS jitter.  This approach prevents the
238		rcuc/%u kthreads from having any work to do, so that they are
239		never awakened.
240	4.	Ensure that the CPU never enters the kernel, and, in particular,
241		avoid initiating any CPU hotplug operations on this CPU.  This is
242		another way of preventing any callbacks from being queued on the
243		CPU, again preventing the rcuc/%u kthreads from having any work
244		to do.
245	
246	Name: rcuob/%d, rcuop/%d, and rcuos/%d
247	Purpose: Offload RCU callbacks from the corresponding CPU.
248	To reduce its OS jitter, do at least one of the following:
249	1.	Use affinity, cgroups, or other mechanism to force these kthreads
250		to execute on some other CPU.
251	2.	Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
252		kthreads from being created in the first place.  However, please
253		note that this will not eliminate OS jitter, but will instead
254		shift it to RCU_SOFTIRQ.
255	
256	Name: watchdog/%u
257	Purpose: Detect software lockups on each CPU.
258	To reduce its OS jitter, do at least one of the following:
259	1.	Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
260		kthreads from being created in the first place.
261	2.	Echo a zero to /proc/sys/kernel/watchdog to disable the
262		watchdog timer.
263	3.	Echo a large number of /proc/sys/kernel/watchdog_thresh in
264		order to reduce the frequency of OS jitter due to the watchdog
265		timer down to a level that is acceptable for your workload.
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