Real-Time Linux Kernel Scheduler

The -rt patchset, or just -rt, adds real-time scheduling capabilities to the Linux kernel.

Many market sectors, such as financial trading, defense, industry automation and gaming, long have had a need for low latencies and deterministic response time. Traditionally, custom-built hardware and software were used to meet these real-time requirements. However, for some soft real-time requirements, where predictability of response times is advantageous and not mandatory, this is an expensive solution. With the advent of the PREEMPT_RT patchset, referred to as -rt henceforth, led by Ingo Molnar, Linux has made great progress in the world of real-time operating systems for “enterprise real-time” applications. A number of modifications were made to the general-purpose Linux kernel to make Linux a viable choice for real time, such as the scheduler, interrupt handling, locking mechanism and so on.

A real-time system is one that provides guaranteed system response times for events and transactions—that is, every operation is expected to be completed within a certain rigid time period. A system is classified as hard real-time if missed deadlines cause system failure and soft real-time if the system can tolerate some missed time constraints.

Design Goal

Real-time systems require that tasks be executed in a strict priority order. This necessitates that only the N highest-priority tasks be running at any given point in time, where N is the number of CPUs. A variation to this requirement could be strict priority-ordered scheduling in a given subset of CPUs or scheduling domains (explained later in this article). In both cases, when a task is runnable, the scheduler must ensure that it be put on a runqueue on which it can be run immediately—that is, the real-time scheduler has to ensure system-wide strict real-time priority scheduling (SWSRPS). Unlike non-real-time systems where the scheduler needs to look only at its runqueue of tasks to make scheduling decisions, a real-time scheduler makes global scheduling decisions, taking into account all the tasks in the system at any given point. Real-time task balancing also has to be performed frequently. Task balancing can introduce cache thrashing and contention for global data (such as runqueue locks) and can degrade throughput and scalability. A real-time task scheduler would trade off throughput in favor of correctness, but at the same time, it must ensure minimal task ping-ponging.

The standard Linux kernel provides two real-time scheduling policies, SCHED_FIFO and SCHED_RR. The main real-time policy is SCHED_FIFO. It implements a first-in, first-out scheduling algorithm. When a SCHED_FIFO task starts running, it continues to run until it voluntarily yields the processor, blocks or is preempted by a higher-priority real-time task. It has no timeslices. All other tasks of lower priority will not be scheduled until it relinquishes the CPU. Two equal-priority SCHED_FIFO tasks do not preempt each other. SCHED_RR is similar to SCHED_FIFO, except that such tasks are allotted timeslices based on their priority and run until they exhaust their timeslice. Non-real-time tasks use the SCHED_NORMAL scheduling policy (older kernels had a policy named SCHED_OTHER).

In the standard Linux kernel, real-time priorities range from zero to (MAX_RT_PRIO-1), inclusive. By default, MAX_RT_PRIO is 100. Non-real-time tasks have priorities in the range of MAX_RT_PRIO to (MAX_RT_PRIO + 40). This constitutes the nice values of SCHED_NORMAL tasks. By default, the –20 to 19 nice range maps directly onto the priority range of 100 to 139.

This article assumes that readers are aware of the basics of a task scheduler. See Resources for more information about the Linux Completely Fair Scheduler (CFS).

Overview of the -rt Patchset Scheduling Algorithm

The real-time scheduler of the -rt patchset adopts an active push-pull strategy developed by Steven Rostedt and Gregory Haskins for balancing tasks across CPUs. The scheduler has to address several scenarios:

  1. Where to place a task optimally on wakeup (that is, pre-balance).

  2. What to do with a lower-priority task when it wakes up but is on a runqueue running a task of higher priority.

  3. What to do with a low-priority task when a higher-priority task on the same runqueue wakes up and preempts it.

  4. What to do when a task lowers its priority and thereby causes a previously lower-priority task to have the higher priority.

A push operation is initiated in cases 2 and 3 above. The push algorithm considers all the runqueues within its root domain (discussed later) to find the one that is of a lower priority than the task being pushed.

A pull operation is performed for case 4 above. Whenever a runqueue is about to schedule a task that is lower in priority than the previous one, it checks to see whether it can pull tasks of higher priority from other runqueues.

Real-time tasks are affected only by the push and pull operations. The CFS load-balancing algorithm does not handle real-time tasks at all, as it has been observed that the load balancing pulls real-time tasks away from runqueues to which they were correctly assigned, inducing unnecessary latencies.