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It is really time. LoongArch has all the required architecture related changes, that have been identified over time, in order to enable PREEMPT_RT. With the recent printk changes, the last known road block has been addressed. Allow to enable PREEMPT_RT on LoongArch. Below are the latency data from cyclictest on a 4-core Loongson-3A5000 machine, with a "make -j8" kernel building workload in the background. 1. PREEMPT kernel with default configuration: ./cyclictest -a -t -m -i200 -d0 -p99 policy: fifo: loadavg: 8.78 8.96 8.64 10/296 64800 T: 0 ( 4592) P:99 I:200 C:14838617 Min: 3 Act: 6 Avg: 8 Max: 844 T: 1 ( 4593) P:99 I:200 C:14838765 Min: 3 Act: 9 Avg: 8 Max: 909 T: 2 ( 4594) P:99 I:200 C:14838510 Min: 3 Act: 7 Avg: 8 Max: 832 T: 3 ( 4595) P:99 I:200 C:14838631 Min: 3 Act: 8 Avg: 8 Max: 931 2. PREEMPT_RT kernel with default configuration: ./cyclictest -a -t -m -i200 -d0 -p99 policy: fifo: loadavg: 10.38 10.47 10.35 9/336 77788 T: 0 ( 3941) P:99 I:200 C:19439626 Min: 3 Act: 12 Avg: 8 Max: 227 T: 1 ( 3942) P:99 I:200 C:19439624 Min: 2 Act: 11 Avg: 8 Max: 184 T: 2 ( 3943) P:99 I:200 C:19439623 Min: 3 Act: 4 Avg: 7 Max: 223 T: 3 ( 3944) P:99 I:200 C:19439623 Min: 2 Act: 10 Avg: 7 Max: 226 3. PREEMPT_RT kernel with tuned configuration: ./cyclictest -a -t -m -i200 -d0 -p99 policy: fifo: loadavg: 10.52 10.66 10.62 12/334 109397 T: 0 ( 4765) P:99 I:200 C:29335186 Min: 3 Act: 6 Avg: 8 Max: 62 T: 1 ( 4766) P:99 I:200 C:29335185 Min: 3 Act: 10 Avg: 8 Max: 52 T: 2 ( 4767) P:99 I:200 C:29335184 Min: 3 Act: 8 Avg: 8 Max: 64 T: 3 ( 4768) P:99 I:200 C:29335183 Min: 3 Act: 12 Avg: 8 Max: 53 Main instruments of tuned configuration include: Disable the boot rom space in BIOS, in order to avoid kernel's speculative access to low- speed memory (i.e. boot rom space); Disable CPUFreq scaling; Disable RTC synchronization in the ntpd/chronyd service (also avoid other RTC accesses when running low-latency workloads). Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.