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diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt new file mode 100644 index 000000000000..5b5322024067 --- /dev/null +++ b/Documentation/timers/NO_HZ.txt @@ -0,0 +1,273 @@ + 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. |
