--- /dev/null
+ NO_HZ: Reducing Scheduling-Clock Ticks
+
+
+This document describes Kconfig options and boot parameters that can
+reduce the number of scheduling-clock interrupts, thereby improving energy
+efficiency and reducing OS jitter. Reducing OS jitter is important for
+some types of computationally intensive high-performance computing (HPC)
+applications and for real-time applications.
+
+There are two main contexts in which the number of scheduling-clock
+interrupts can be reduced compared to the old-school approach of sending
+a scheduling-clock interrupt to all CPUs every jiffy whether they need
+it or not (CONFIG_HZ_PERIODIC=y or CONFIG_NO_HZ=n for older kernels):
+
+1. Idle CPUs (CONFIG_NO_HZ_IDLE=y or CONFIG_NO_HZ=y for older kernels).
+
+2. CPUs having only one runnable task (CONFIG_NO_HZ_FULL=y).
+
+These two cases are described in the following two sections, followed
+by a third section on RCU-specific considerations and a fourth and final
+section listing known issues.
+
+
+IDLE CPUs
+
+If a CPU is idle, there is little point in sending it a scheduling-clock
+interrupt. After all, the primary purpose of a scheduling-clock interrupt
+is to force a busy CPU to shift its attention among multiple duties,
+and an idle CPU has no duties to shift its attention among.
+
+The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
+scheduling-clock interrupts to idle CPUs, which is critically important
+both to battery-powered devices and to highly virtualized mainframes.
+A battery-powered device running a CONFIG_HZ_PERIODIC=y kernel would
+drain its battery very quickly, easily 2-3 times as fast as would the
+same device running a CONFIG_NO_HZ_IDLE=y kernel. A mainframe running
+1,500 OS instances might find that half of its CPU time was consumed by
+unnecessary scheduling-clock interrupts. In these situations, there
+is strong motivation to avoid sending scheduling-clock interrupts to
+idle CPUs. That said, dyntick-idle mode is not free:
+
+1. It increases the number of instructions executed on the path
+ to and from the idle loop.
+
+2. On many architectures, dyntick-idle mode also increases the
+ number of expensive clock-reprogramming operations.
+
+Therefore, systems with aggressive real-time response constraints often
+run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
+in order to avoid degrading from-idle transition latencies.
+
+An idle CPU that is not receiving scheduling-clock interrupts is said to
+be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
+tickless". The remainder of this document will use "dyntick-idle mode".
+
+There is also a boot parameter "nohz=" that can be used to disable
+dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
+By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
+dyntick-idle mode.
+
+
+CPUs WITH ONLY ONE RUNNABLE TASK
+
+If a CPU has only one runnable task, there is little point in sending it
+a scheduling-clock interrupt because there is no other task to switch to.
+
+The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
+sending scheduling-clock interrupts to CPUs with a single runnable task,
+and such CPUs are said to be "adaptive-ticks CPUs". This is important
+for applications with aggressive real-time response constraints because
+it allows them to improve their worst-case response times by the maximum
+duration of a scheduling-clock interrupt. It is also important for
+computationally intensive short-iteration workloads: If any CPU is
+delayed during a given iteration, all the other CPUs will be forced to
+wait idle while the delayed CPU finishes. Thus, the delay is multiplied
+by one less than the number of CPUs. In these situations, there is
+again strong motivation to avoid sending scheduling-clock interrupts.
+
+By default, no CPU will be an adaptive-ticks CPU. The "nohz_full="
+boot parameter specifies the adaptive-ticks CPUs. For example,
+"nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
+CPUs. Note that you are prohibited from marking all of the CPUs as
+adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain
+online to handle timekeeping tasks in order to ensure that system calls
+like gettimeofday() returns accurate values on adaptive-tick CPUs.
+(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no
+running user processes to observe slight drifts in clock rate.)
+Therefore, the boot CPU is prohibited from entering adaptive-ticks
+mode. Specifying a "nohz_full=" mask that includes the boot CPU will
+result in a boot-time error message, and the boot CPU will be removed
+from the mask.
+
+Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies
+that all CPUs other than the boot CPU are adaptive-ticks CPUs. This
+Kconfig parameter will be overridden by the "nohz_full=" boot parameter,
+so that if both the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter and
+the "nohz_full=1" boot parameter is specified, the boot parameter will
+prevail so that only CPU 1 will be an adaptive-ticks CPU.
+
+Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
+This is covered in the "RCU IMPLICATIONS" section below.
+
+Normally, a CPU remains in adaptive-ticks mode as long as possible.
+In particular, transitioning to kernel mode does not automatically change
+the mode. Instead, the CPU will exit adaptive-ticks mode only if needed,
+for example, if that CPU enqueues an RCU callback.
+
+Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
+not come for free:
+
+1. CONFIG_NO_HZ_FULL selects CONFIG_NO_HZ_COMMON, so you cannot run
+ adaptive ticks without also running dyntick idle. This dependency
+ extends down into the implementation, so that all of the costs
+ of CONFIG_NO_HZ_IDLE are also incurred by CONFIG_NO_HZ_FULL.
+
+2. The user/kernel transitions are slightly more expensive due
+ to the need to inform kernel subsystems (such as RCU) about
+ the change in mode.
+
+3. POSIX CPU timers on adaptive-tick CPUs may miss their deadlines
+ (perhaps indefinitely) because they currently rely on
+ scheduling-tick interrupts. This will likely be fixed in
+ one of two ways: (1) Prevent CPUs with POSIX CPU timers from
+ entering adaptive-tick mode, or (2) Use hrtimers or other
+ adaptive-ticks-immune mechanism to cause the POSIX CPU timer to
+ fire properly.
+
+4. If there are more perf events pending than the hardware can
+ accommodate, they are normally round-robined so as to collect
+ all of them over time. Adaptive-tick mode may prevent this
+ round-robining from happening. This will likely be fixed by
+ preventing CPUs with large numbers of perf events pending from
+ entering adaptive-tick mode.
+
+5. Scheduler statistics for adaptive-tick CPUs may be computed
+ slightly differently than those for non-adaptive-tick CPUs.
+ This might in turn perturb load-balancing of real-time tasks.
+
+6. The LB_BIAS scheduler feature is disabled by adaptive ticks.
+
+Although improvements are expected over time, adaptive ticks is quite
+useful for many types of real-time and compute-intensive applications.
+However, the drawbacks listed above mean that adaptive ticks should not
+(yet) be enabled by default.
+
+
+RCU IMPLICATIONS
+
+There are situations in which idle CPUs cannot be permitted to
+enter either dyntick-idle mode or adaptive-tick mode, the most
+common being when that CPU has RCU callbacks pending.
+
+The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
+to enter dyntick-idle mode or adaptive-tick mode anyway. In this case,
+a timer will awaken these CPUs every four jiffies in order to ensure
+that the RCU callbacks are processed in a timely fashion.
+
+Another approach is to offload RCU callback processing to "rcuo" kthreads
+using the CONFIG_RCU_NOCB_CPU=y Kconfig option. The specific CPUs to
+offload may be selected via several methods:
+
+1. One of three mutually exclusive Kconfig options specify a
+ build-time default for the CPUs to offload:
+
+ a. The CONFIG_RCU_NOCB_CPU_NONE=y Kconfig option results in
+ no CPUs being offloaded.
+
+ b. The CONFIG_RCU_NOCB_CPU_ZERO=y Kconfig option causes
+ CPU 0 to be offloaded.
+
+ c. The CONFIG_RCU_NOCB_CPU_ALL=y Kconfig option causes all
+ CPUs to be offloaded. Note that the callbacks will be
+ offloaded to "rcuo" kthreads, and that those kthreads
+ will in fact run on some CPU. However, this approach
+ gives fine-grained control on exactly which CPUs the
+ callbacks run on, along with their scheduling priority
+ (including the default of SCHED_OTHER), and it further
+ allows this control to be varied dynamically at runtime.
+
+2. The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
+ list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
+ 3, 4, and 5. The specified CPUs will be offloaded in addition to
+ any CPUs specified as offloaded by CONFIG_RCU_NOCB_CPU_ZERO=y or
+ CONFIG_RCU_NOCB_CPU_ALL=y. This means that the "rcu_nocbs=" boot
+ parameter has no effect for kernels built with RCU_NOCB_CPU_ALL=y.
+
+The offloaded CPUs will never queue RCU callbacks, and therefore RCU
+never prevents offloaded CPUs from entering either dyntick-idle mode
+or adaptive-tick mode. That said, note that it is up to userspace to
+pin the "rcuo" kthreads to specific CPUs if desired. Otherwise, the
+scheduler will decide where to run them, which might or might not be
+where you want them to run.
+
+
+KNOWN ISSUES
+
+o Dyntick-idle slows transitions to and from idle slightly.
+ In practice, this has not been a problem except for the most
+ aggressive real-time workloads, which have the option of disabling
+ dyntick-idle mode, an option that most of them take. However,
+ some workloads will no doubt want to use adaptive ticks to
+ eliminate scheduling-clock interrupt latencies. Here are some
+ options for these workloads:
+
+ a. Use PMQOS from userspace to inform the kernel of your
+ latency requirements (preferred).
+
+ b. On x86 systems, use the "idle=mwait" boot parameter.
+
+ c. On x86 systems, use the "intel_idle.max_cstate=" to limit
+ ` the maximum C-state depth.
+
+ d. On x86 systems, use the "idle=poll" boot parameter.
+ However, please note that use of this parameter can cause
+ your CPU to overheat, which may cause thermal throttling
+ to degrade your latencies -- and that this degradation can
+ be even worse than that of dyntick-idle. Furthermore,
+ this parameter effectively disables Turbo Mode on Intel
+ CPUs, which can significantly reduce maximum performance.
+
+o Adaptive-ticks slows user/kernel transitions slightly.
+ This is not expected to be a problem for computationally intensive
+ workloads, which have few such transitions. Careful benchmarking
+ will be required to determine whether or not other workloads
+ are significantly affected by this effect.
+
+o Adaptive-ticks does not do anything unless there is only one
+ runnable task for a given CPU, even though there are a number
+ of other situations where the scheduling-clock tick is not
+ needed. To give but one example, consider a CPU that has one
+ runnable high-priority SCHED_FIFO task and an arbitrary number
+ of low-priority SCHED_OTHER tasks. In this case, the CPU is
+ required to run the SCHED_FIFO task until it either blocks or
+ some other higher-priority task awakens on (or is assigned to)
+ this CPU, so there is no point in sending a scheduling-clock
+ interrupt to this CPU. However, the current implementation
+ nevertheless sends scheduling-clock interrupts to CPUs having a
+ single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
+ tasks, even though these interrupts are unnecessary.
+
+ Better handling of these sorts of situations is future work.
+
+o A reboot is required to reconfigure both adaptive idle and RCU
+ callback offloading. Runtime reconfiguration could be provided
+ if needed, however, due to the complexity of reconfiguring RCU at
+ runtime, there would need to be an earthshakingly good reason.
+ Especially given that you have the straightforward option of
+ simply offloading RCU callbacks from all CPUs and pinning them
+ where you want them whenever you want them pinned.
+
+o Additional configuration is required to deal with other sources
+ of OS jitter, including interrupts and system-utility tasks
+ and processes. This configuration normally involves binding
+ interrupts and tasks to particular CPUs.
+
+o Some sources of OS jitter can currently be eliminated only by
+ constraining the workload. For example, the only way to eliminate
+ OS jitter due to global TLB shootdowns is to avoid the unmapping
+ operations (such as kernel module unload operations) that
+ result in these shootdowns. For another example, page faults
+ and TLB misses can be reduced (and in some cases eliminated) by
+ using huge pages and by constraining the amount of memory used
+ by the application. Pre-faulting the working set can also be
+ helpful, especially when combined with the mlock() and mlockall()
+ system calls.
+
+o Unless all CPUs are idle, at least one CPU must keep the
+ scheduling-clock interrupt going in order to support accurate
+ timekeeping.
+
+o If there are adaptive-ticks CPUs, there will be at least one
+ CPU keeping the scheduling-clock interrupt going, even if all
+ CPUs are otherwise idle.
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