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0001 REDUCING OS JITTER DUE TO PER-CPU KTHREADS
0002 
0003 This document lists per-CPU kthreads in the Linux kernel and presents
0004 options to control their OS jitter.  Note that non-per-CPU kthreads are
0005 not listed here.  To reduce OS jitter from non-per-CPU kthreads, bind
0006 them to a "housekeeping" CPU dedicated to such work.
0007 
0008 
0009 REFERENCES
0010 
0011 o       Documentation/IRQ-affinity.txt:  Binding interrupts to sets of CPUs.
0012 
0013 o       Documentation/cgroup-v1:  Using cgroups to bind tasks to sets of CPUs.
0014 
0015 o       man taskset:  Using the taskset command to bind tasks to sets
0016         of CPUs.
0017 
0018 o       man sched_setaffinity:  Using the sched_setaffinity() system
0019         call to bind tasks to sets of CPUs.
0020 
0021 o       /sys/devices/system/cpu/cpuN/online:  Control CPU N's hotplug state,
0022         writing "0" to offline and "1" to online.
0023 
0024 o       In order to locate kernel-generated OS jitter on CPU N:
0025 
0026                 cd /sys/kernel/debug/tracing
0027                 echo 1 > max_graph_depth # Increase the "1" for more detail
0028                 echo function_graph > current_tracer
0029                 # run workload
0030                 cat per_cpu/cpuN/trace
0031 
0032 
0033 KTHREADS
0034 
0035 Name: ehca_comp/%u
0036 Purpose: Periodically process Infiniband-related work.
0037 To reduce its OS jitter, do any of the following:
0038 1.      Don't use eHCA Infiniband hardware, instead choosing hardware
0039         that does not require per-CPU kthreads.  This will prevent these
0040         kthreads from being created in the first place.  (This will
0041         work for most people, as this hardware, though important, is
0042         relatively old and is produced in relatively low unit volumes.)
0043 2.      Do all eHCA-Infiniband-related work on other CPUs, including
0044         interrupts.
0045 3.      Rework the eHCA driver so that its per-CPU kthreads are
0046         provisioned only on selected CPUs.
0047 
0048 
0049 Name: irq/%d-%s
0050 Purpose: Handle threaded interrupts.
0051 To reduce its OS jitter, do the following:
0052 1.      Use irq affinity to force the irq threads to execute on
0053         some other CPU.
0054 
0055 Name: kcmtpd_ctr_%d
0056 Purpose: Handle Bluetooth work.
0057 To reduce its OS jitter, do one of the following:
0058 1.      Don't use Bluetooth, in which case these kthreads won't be
0059         created in the first place.
0060 2.      Use irq affinity to force Bluetooth-related interrupts to
0061         occur on some other CPU and furthermore initiate all
0062         Bluetooth activity on some other CPU.
0063 
0064 Name: ksoftirqd/%u
0065 Purpose: Execute softirq handlers when threaded or when under heavy load.
0066 To reduce its OS jitter, each softirq vector must be handled
0067 separately as follows:
0068 TIMER_SOFTIRQ:  Do all of the following:
0069 1.      To the extent possible, keep the CPU out of the kernel when it
0070         is non-idle, for example, by avoiding system calls and by forcing
0071         both kernel threads and interrupts to execute elsewhere.
0072 2.      Build with CONFIG_HOTPLUG_CPU=y.  After boot completes, force
0073         the CPU offline, then bring it back online.  This forces
0074         recurring timers to migrate elsewhere.  If you are concerned
0075         with multiple CPUs, force them all offline before bringing the
0076         first one back online.  Once you have onlined the CPUs in question,
0077         do not offline any other CPUs, because doing so could force the
0078         timer back onto one of the CPUs in question.
0079 NET_TX_SOFTIRQ and NET_RX_SOFTIRQ:  Do all of the following:
0080 1.      Force networking interrupts onto other CPUs.
0081 2.      Initiate any network I/O on other CPUs.
0082 3.      Once your application has started, prevent CPU-hotplug operations
0083         from being initiated from tasks that might run on the CPU to
0084         be de-jittered.  (It is OK to force this CPU offline and then
0085         bring it back online before you start your application.)
0086 BLOCK_SOFTIRQ:  Do all of the following:
0087 1.      Force block-device interrupts onto some other CPU.
0088 2.      Initiate any block I/O on other CPUs.
0089 3.      Once your application has started, prevent CPU-hotplug operations
0090         from being initiated from tasks that might run on the CPU to
0091         be de-jittered.  (It is OK to force this CPU offline and then
0092         bring it back online before you start your application.)
0093 IRQ_POLL_SOFTIRQ:  Do all of the following:
0094 1.      Force block-device interrupts onto some other CPU.
0095 2.      Initiate any block I/O and block-I/O polling on other CPUs.
0096 3.      Once your application has started, prevent CPU-hotplug operations
0097         from being initiated from tasks that might run on the CPU to
0098         be de-jittered.  (It is OK to force this CPU offline and then
0099         bring it back online before you start your application.)
0100 TASKLET_SOFTIRQ: Do one or more of the following:
0101 1.      Avoid use of drivers that use tasklets.  (Such drivers will contain
0102         calls to things like tasklet_schedule().)
0103 2.      Convert all drivers that you must use from tasklets to workqueues.
0104 3.      Force interrupts for drivers using tasklets onto other CPUs,
0105         and also do I/O involving these drivers on other CPUs.
0106 SCHED_SOFTIRQ: Do all of the following:
0107 1.      Avoid sending scheduler IPIs to the CPU to be de-jittered,
0108         for example, ensure that at most one runnable kthread is present
0109         on that CPU.  If a thread that expects to run on the de-jittered
0110         CPU awakens, the scheduler will send an IPI that can result in
0111         a subsequent SCHED_SOFTIRQ.
0112 2.      Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
0113         CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU
0114         to be de-jittered is marked as an adaptive-ticks CPU using the
0115         "nohz_full=" boot parameter.  This reduces the number of
0116         scheduler-clock interrupts that the de-jittered CPU receives,
0117         minimizing its chances of being selected to do the load balancing
0118         work that runs in SCHED_SOFTIRQ context.
0119 3.      To the extent possible, keep the CPU out of the kernel when it
0120         is non-idle, for example, by avoiding system calls and by
0121         forcing both kernel threads and interrupts to execute elsewhere.
0122         This further reduces the number of scheduler-clock interrupts
0123         received by the de-jittered CPU.
0124 HRTIMER_SOFTIRQ:  Do all of the following:
0125 1.      To the extent possible, keep the CPU out of the kernel when it
0126         is non-idle.  For example, avoid system calls and force both
0127         kernel threads and interrupts to execute elsewhere.
0128 2.      Build with CONFIG_HOTPLUG_CPU=y.  Once boot completes, force the
0129         CPU offline, then bring it back online.  This forces recurring
0130         timers to migrate elsewhere.  If you are concerned with multiple
0131         CPUs, force them all offline before bringing the first one
0132         back online.  Once you have onlined the CPUs in question, do not
0133         offline any other CPUs, because doing so could force the timer
0134         back onto one of the CPUs in question.
0135 RCU_SOFTIRQ:  Do at least one of the following:
0136 1.      Offload callbacks and keep the CPU in either dyntick-idle or
0137         adaptive-ticks state by doing all of the following:
0138         a.      Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
0139                 CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU
0140                 to be de-jittered is marked as an adaptive-ticks CPU using
0141                 the "nohz_full=" boot parameter.  Bind the rcuo kthreads
0142                 to housekeeping CPUs, which can tolerate OS jitter.
0143         b.      To the extent possible, keep the CPU out of the kernel
0144                 when it is non-idle, for example, by avoiding system
0145                 calls and by forcing both kernel threads and interrupts
0146                 to execute elsewhere.
0147 2.      Enable RCU to do its processing remotely via dyntick-idle by
0148         doing all of the following:
0149         a.      Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y.
0150         b.      Ensure that the CPU goes idle frequently, allowing other
0151                 CPUs to detect that it has passed through an RCU quiescent
0152                 state.  If the kernel is built with CONFIG_NO_HZ_FULL=y,
0153                 userspace execution also allows other CPUs to detect that
0154                 the CPU in question has passed through a quiescent state.
0155         c.      To the extent possible, keep the CPU out of the kernel
0156                 when it is non-idle, for example, by avoiding system
0157                 calls and by forcing both kernel threads and interrupts
0158                 to execute elsewhere.
0159 
0160 Name: kworker/%u:%d%s (cpu, id, priority)
0161 Purpose: Execute workqueue requests
0162 To reduce its OS jitter, do any of the following:
0163 1.      Run your workload at a real-time priority, which will allow
0164         preempting the kworker daemons.
0165 2.      A given workqueue can be made visible in the sysfs filesystem
0166         by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
0167         Such a workqueue can be confined to a given subset of the
0168         CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
0169         files.  The set of WQ_SYSFS workqueues can be displayed using
0170         "ls sys/devices/virtual/workqueue".  That said, the workqueues
0171         maintainer would like to caution people against indiscriminately
0172         sprinkling WQ_SYSFS across all the workqueues.  The reason for
0173         caution is that it is easy to add WQ_SYSFS, but because sysfs is
0174         part of the formal user/kernel API, it can be nearly impossible
0175         to remove it, even if its addition was a mistake.
0176 3.      Do any of the following needed to avoid jitter that your
0177         application cannot tolerate:
0178         a.      Build your kernel with CONFIG_SLUB=y rather than
0179                 CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
0180                 use of each CPU's workqueues to run its cache_reap()
0181                 function.
0182         b.      Avoid using oprofile, thus avoiding OS jitter from
0183                 wq_sync_buffer().
0184         c.      Limit your CPU frequency so that a CPU-frequency
0185                 governor is not required, possibly enlisting the aid of
0186                 special heatsinks or other cooling technologies.  If done
0187                 correctly, and if you CPU architecture permits, you should
0188                 be able to build your kernel with CONFIG_CPU_FREQ=n to
0189                 avoid the CPU-frequency governor periodically running
0190                 on each CPU, including cs_dbs_timer() and od_dbs_timer().
0191                 WARNING:  Please check your CPU specifications to
0192                 make sure that this is safe on your particular system.
0193         d.      As of v3.18, Christoph Lameter's on-demand vmstat workers
0194                 commit prevents OS jitter due to vmstat_update() on
0195                 CONFIG_SMP=y systems.  Before v3.18, is not possible
0196                 to entirely get rid of the OS jitter, but you can
0197                 decrease its frequency by writing a large value to
0198                 /proc/sys/vm/stat_interval.  The default value is HZ,
0199                 for an interval of one second.  Of course, larger values
0200                 will make your virtual-memory statistics update more
0201                 slowly.  Of course, you can also run your workload at
0202                 a real-time priority, thus preempting vmstat_update(),
0203                 but if your workload is CPU-bound, this is a bad idea.
0204                 However, there is an RFC patch from Christoph Lameter
0205                 (based on an earlier one from Gilad Ben-Yossef) that
0206                 reduces or even eliminates vmstat overhead for some
0207                 workloads at https://lkml.org/lkml/2013/9/4/379.
0208         e.      Boot with "elevator=noop" to avoid workqueue use by
0209                 the block layer.
0210         f.      If running on high-end powerpc servers, build with
0211                 CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS
0212                 daemon from running on each CPU every second or so.
0213                 (This will require editing Kconfig files and will defeat
0214                 this platform's RAS functionality.)  This avoids jitter
0215                 due to the rtas_event_scan() function.
0216                 WARNING:  Please check your CPU specifications to
0217                 make sure that this is safe on your particular system.
0218         g.      If running on Cell Processor, build your kernel with
0219                 CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
0220                 spu_gov_work().
0221                 WARNING:  Please check your CPU specifications to
0222                 make sure that this is safe on your particular system.
0223         h.      If running on PowerMAC, build your kernel with
0224                 CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
0225                 avoiding OS jitter from rackmeter_do_timer().
0226 
0227 Name: rcuc/%u
0228 Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
0229 To reduce its OS jitter, do at least one of the following:
0230 1.      Build the kernel with CONFIG_PREEMPT=n.  This prevents these
0231         kthreads from being created in the first place, and also obviates
0232         the need for RCU priority boosting.  This approach is feasible
0233         for workloads that do not require high degrees of responsiveness.
0234 2.      Build the kernel with CONFIG_RCU_BOOST=n.  This prevents these
0235         kthreads from being created in the first place.  This approach
0236         is feasible only if your workload never requires RCU priority
0237         boosting, for example, if you ensure frequent idle time on all
0238         CPUs that might execute within the kernel.
0239 3.      Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y,
0240         which offloads all RCU callbacks to kthreads that can be moved
0241         off of CPUs susceptible to OS jitter.  This approach prevents the
0242         rcuc/%u kthreads from having any work to do, so that they are
0243         never awakened.
0244 4.      Ensure that the CPU never enters the kernel, and, in particular,
0245         avoid initiating any CPU hotplug operations on this CPU.  This is
0246         another way of preventing any callbacks from being queued on the
0247         CPU, again preventing the rcuc/%u kthreads from having any work
0248         to do.
0249 
0250 Name: rcuob/%d, rcuop/%d, and rcuos/%d
0251 Purpose: Offload RCU callbacks from the corresponding CPU.
0252 To reduce its OS jitter, do at least one of the following:
0253 1.      Use affinity, cgroups, or other mechanism to force these kthreads
0254         to execute on some other CPU.
0255 2.      Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
0256         kthreads from being created in the first place.  However, please
0257         note that this will not eliminate OS jitter, but will instead
0258         shift it to RCU_SOFTIRQ.
0259 
0260 Name: watchdog/%u
0261 Purpose: Detect software lockups on each CPU.
0262 To reduce its OS jitter, do at least one of the following:
0263 1.      Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
0264         kthreads from being created in the first place.
0265 2.      Boot with "nosoftlockup=0", which will also prevent these kthreads
0266         from being created.  Other related watchdog and softlockup boot
0267         parameters may be found in Documentation/admin-guide/kernel-parameters.rst
0268         and Documentation/watchdog/watchdog-parameters.txt.
0269 3.      Echo a zero to /proc/sys/kernel/watchdog to disable the
0270         watchdog timer.
0271 4.      Echo a large number of /proc/sys/kernel/watchdog_thresh in
0272         order to reduce the frequency of OS jitter due to the watchdog
0273         timer down to a level that is acceptable for your workload.