--- /dev/null
+REDUCING OS JITTER DUE TO PER-CPU KTHREADS
+
+This document lists per-CPU kthreads in the Linux kernel and presents
+options to control their OS jitter. Note that non-per-CPU kthreads are
+not listed here. To reduce OS jitter from non-per-CPU kthreads, bind
+them to a "housekeeping" CPU dedicated to such work.
+
+
+REFERENCES
+
+o Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
+
+o Documentation/cgroups: Using cgroups to bind tasks to sets of CPUs.
+
+o man taskset: Using the taskset command to bind tasks to sets
+ of CPUs.
+
+o man sched_setaffinity: Using the sched_setaffinity() system
+ call to bind tasks to sets of CPUs.
+
+o /sys/devices/system/cpu/cpuN/online: Control CPU N's hotplug state,
+ writing "0" to offline and "1" to online.
+
+o In order to locate kernel-generated OS jitter on CPU N:
+
+ cd /sys/kernel/debug/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 handled
+separately 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.)
+BLOCK_IOPOLL_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. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
+ CONFIG_NO_HZ_FULL=y, and, in addition, 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. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y,
+ CONFIG_NO_HZ_FULL=y, and, in addition 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 and CONFIG_RCU_FAST_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: 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 CONFIG_RCU_NOCB_CPU_ALL=y,
+ which offloads all RCU callbacks to kthreads that can be moved
+ off of 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: rcuob/%d, rcuop/%d, and rcuos/%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_CPUS=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.
+
+Name: watchdog/%u
+Purpose: Detect software lockups on each CPU.
+To reduce its OS jitter, do at least one of the following:
+1. Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these
+ kthreads from being created in the first place.
+2. Echo a zero to /proc/sys/kernel/watchdog to disable the
+ watchdog timer.
+3. Echo a large number of /proc/sys/kernel/watchdog_thresh in
+ order to reduce the frequency of OS jitter due to the watchdog
+ timer down to a level that is acceptable for your workload.
projects / mirror_ubuntu-artful-kernel.git / commitdiff