This is an update of task isolation work that was originally done by Chris Metcalf <cmetcalf@xxxxxxxxxxxx> and maintained by him until November 2017. It is adapted to the current kernel and cleaned up to make this functionality both more complete (as in, prevent isolation breaking in situations that were not covered before) and cleaner (as in, avoid any dubious or fragile use of kernel interfaces, and provide clean and reliable isolation breaking procedure). I guess, I have to explain why such a thing exists. This is the result of development and maintenance of task isolation functionality that originally started based on task isolation patch v15 and was later updated to include v16. It provided RTOS-like predictable environment for userspace tasks running on arm64 processors alongside with full-featured Linux environment. It is intended to provide reliable interruption-free environment from the point when a userspace task enters isolation and until the moment it leaves isolation or receives a signal intentionally sent to it, and was successfully used for this purpose. While CPU isolation with nohz provides an environment that is close to this requirement, the remaining IPIs and other disturbances keep it from being usable for tasks that require complete predictability of CPU timing. It is clear that such isolation is neither possible nor necessary while a CPU is running kernel, userspace initialization or cleanup, so there is a need for a separate isolated state that a userspace task can enter and exit. This was the reason for using the original task isolation, and such reason still exists now. The alternative, running RTOS instead of Linux, is becoming more and more labor-consuming because modern CPUs and SoCs have very complex device/resource configuration and management procedures, and at this point for some hardware it is clearly in the realm of impractical to maintain an RTOS with hardware support on par with Linux kernel, reliable and secure at the same time. On the other hand, development of modern embedded-oriented SoCs had shown that numerous CPU cores may or may not share any hardware resources based on SoC designers' intention. Therefore OS ability to switch a CPU core into RTOS-ish mode and truly, really, at all levels, leave it alone until OS is needed there again, is an important feature for modern embedded systems development. Probably more important than even real-time interrupts latency and preemption, now that people, when they don't like how their interrupts are handled, can just add CPU cores. This is why we had to maintain task isolation, and I believe, after all improvements in CPU isolation, timer and interrupt management that was done in Linux since 2017, it is needed even more, as opposed to less. This set of patches only covers the implementation of task isolation, however the need for additional functionality, such as selective TLB flushes, is one of the reasons behind task_isolation_on_cpu() avoiding any non-isolation-specific data structures and the existence of fast_task_isolation_cpu_cleanup() function, that is always called on the CPU where isolated task is running. Reporting task isolation breaking in kernel log is now more informative and, if necessary, can be adapted to provide meaningful cause information to userspace software. I am not sure if such mechanism is needed -- development and reporting failures in production usually relies on kernel logs, and in production it is assumed that isolation breaking should not happen on its own. On the other hand, if application can collect a meaningful log where its events are matched to isolation failures, this may be better for developers than matching timing of records from multiple sources. For now, only log shows detailed descriptions. The userspace support and test program is now at https://github.com/abelits/libtmc . It was originally developed for earlier implementation, so it has some checks that may be redundant now but kept for compatibility. My thanks to Chris Metcalf for design and maintenance of the original task isolation patch, Francis Giraldeau <francis.giraldeau@xxxxxxxxx> and Yuri Norov <ynorov@xxxxxxxxxxx> for various contributions to this work, and Frederic Weisbecker <frederic@xxxxxxxxxx> for his work on CPU isolation and housekeeping that made possible to remove some less elegant solutions that I had to devise for earlier, <4.17 kernels. The previous patch (v16 by Chris Metcalf) is at: https://lore.kernel.org/lkml/1509728692-10460-1-git-send-email-cmetcalf@xxxxxxxxxxxx -- Alex
