Chapter 13. Real-Time Kernel

The Red Hat Enterprise Linux for Real Time Kernel is designed to enable fine-tuning for systems with extremely high determinism requirements. The major increase in the consistency of results can, and should, be achieved by tuning the standard kernel. The real-time kernel enables gaining a small increase on top of increase achieved by tuning the standard kernel.

The real-time kernel is available in the rhel-7-server-rt-rpms repository. The Installation Guide contains the installation instructions and the rest of the documentation is available at Product Documentation for Red Hat Enterprise Linux for Real Time.

The kernel-rt sources have been upgraded to be based on the latest Red Hat Enterprise Linux kernel source tree, which provides a number of bug fixes and enhancements over the previous version. (BZ#1553351)

The SCHED_DEADLINE scheduler class for the real-time kernel, which was introduced in Red Hat Enterprise Linux 7.4 as a Technology Preview, is now fully supported. The scheduler enables predictable task scheduling based on application deadlines. SCHED_DEADLINE benefits periodic workloads by guaranteeing timing isolation, which is based not only on a fixed priority but also on the applications' timing requirements. (BZ#1297061)

The chrony daemon, chronyd, enables or disables network timestamping, which activates a static key within the kernel. When a static key is enabled or disabled, three inter-processor interrupt (IPIs) are generated to notify other processors of the activation.

Previously, rapid activation and deactivation of the chronyd static keys led to a delay of a realtime task. Consequently, a latency spike occurred. With this update, systemd starts the rt-entsk program, which keeps timestamping enabled and prevents the IPIs from being generated. As a result, IPI generation no longer occurs in a rapid succession, and realtime tasks are no longer delayed due to this bug. (BZ#1616038)