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Linux/Documentation/kernel-per-CPU-kthreads.txt

  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/cgroup-v1:  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 IRQ_POLL_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.      As of v3.18, Christoph Lameter's on-demand vmstat workers
194                 commit prevents OS jitter due to vmstat_update() on
195                 CONFIG_SMP=y systems.  Before v3.18, is not possible
196                 to entirely get rid of the OS jitter, but you can
197                 decrease its frequency by writing a large value to
198                 /proc/sys/vm/stat_interval.  The default value is HZ,
199                 for an interval of one second.  Of course, larger values
200                 will make your virtual-memory statistics update more
201                 slowly.  Of course, you can also run your workload at
202                 a real-time priority, thus preempting vmstat_update(),
203                 but if your workload is CPU-bound, this is a bad idea.
204                 However, there is an RFC patch from Christoph Lameter
205                 (based on an earlier one from Gilad Ben-Yossef) that
206                 reduces or even eliminates vmstat overhead for some
207                 workloads at https://lkml.org/lkml/2013/9/4/379.
208         e.      Boot with "elevator=noop" to avoid workqueue use by
209                 the block layer.
210         f.      If running on high-end powerpc servers, build with
211                 CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
212                 daemon from running on each CPU every second or so.
213                 (This will require editing Kconfig files and will defeat
214                 this platform's RAS functionality.)  This avoids jitter
215                 due to the rtas_event_scan() function.
216                 WARNING:  Please check your CPU specifications to
217                 make sure that this is safe on your particular system.
218         g.      If running on Cell Processor, build your kernel with
219                 CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
220                 spu_gov_work().
221                 WARNING:  Please check your CPU specifications to
222                 make sure that this is safe on your particular system.
223         h.      If running on PowerMAC, build your kernel with
224                 CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
225                 avoiding OS jitter from rackmeter_do_timer().
226 
227 Name: rcuc/%u
228 Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
229 To reduce its OS jitter, do at least one of the following:
230 1.      Build the kernel with CONFIG_PREEMPT=n.  This prevents these
231         kthreads from being created in the first place, and also obviates
232         the need for RCU priority boosting.  This approach is feasible
233         for workloads that do not require high degrees of responsiveness.
234 2.      Build the kernel with CONFIG_RCU_BOOST=n.  This prevents these
235         kthreads from being created in the first place.  This approach
236         is feasible only if your workload never requires RCU priority
237         boosting, for example, if you ensure frequent idle time on all
238         CPUs that might execute within the kernel.
239 3.      Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
240         which offloads all RCU callbacks to kthreads that can be moved
241         off of CPUs susceptible to OS jitter.  This approach prevents the
242         rcuc/%u kthreads from having any work to do, so that they are
243         never awakened.
244 4.      Ensure that the CPU never enters the kernel, and, in particular,
245         avoid initiating any CPU hotplug operations on this CPU.  This is
246         another way of preventing any callbacks from being queued on the
247         CPU, again preventing the rcuc/%u kthreads from having any work
248         to do.
249 
250 Name: rcuob/%d, rcuop/%d, and rcuos/%d
251 Purpose: Offload RCU callbacks from the corresponding CPU.
252 To reduce its OS jitter, do at least one of the following:
253 1.      Use affinity, cgroups, or other mechanism to force these kthreads
254         to execute on some other CPU.
255 2.      Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
256         kthreads from being created in the first place.  However, please
257         note that this will not eliminate OS jitter, but will instead
258         shift it to RCU_SOFTIRQ.
259 
260 Name: watchdog/%u
261 Purpose: Detect software lockups on each CPU.
262 To reduce its OS jitter, do at least one of the following:
263 1.      Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
264         kthreads from being created in the first place.
265 2.      Boot with "nosoftlockup=0", which will also prevent these kthreads
266         from being created.  Other related watchdog and softlockup boot
267         parameters may be found in Documentation/admin-guide/kernel-parameters.rst
268         and Documentation/watchdog/watchdog-parameters.txt.
269 3.      Echo a zero to /proc/sys/kernel/watchdog to disable the
270         watchdog timer.
271 4.      Echo a large number of /proc/sys/kernel/watchdog_thresh in
272         order to reduce the frequency of OS jitter due to the watchdog
273         timer down to a level that is acceptable for your workload.

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