[PATCH v5 0/9] "Task_isolation" mode

Alex Belits
2020-11-23T17:44:06Z

This is an update of task isolation work that was originally done by
Chris Metcalf <cmetcalf@mellanox.com> and maintained by him until
November 2017. It is adapted to the current kernel and cleaned up to
implement its functionality in a more complete and cleaner manner.

Previous version is at
https://lore.kernel.org/netdev/04be044c1bcd76b7438b7563edc35383417f12c8.camel@marvell.com/

The last version by Chris Metcalf (now obsolete but may be relevant
for comparison and understanding the origin of the changes) is at
https://lore.kernel.org/lkml/1509728692-10460-1-git-send-email-cmetcalf@mellanox.com

Supported architectures

This version includes only architecture-independent code and arm64
support. x86 and arm support, and everything related to virtualization
will be re-added later when new kernel entry/exit implementation will
be accommodated. Support for other architectures can be added in a
somewhat modular manner, however it heavily depends on the details of
a kernel entry/exit support on any particular architecture.
Development of common entry/exit and conversion to it should simplify
that task. For now, this is the version that is currently being
developed on arm64.

Major changes since v4

The goal was to make isolation-breaking detection as generic as
possible, and remove everything related to determining, _why_
isolation was broken. Originally reporting isolation breaking was done
with a large number of of hooks in specific code (hardware interrupts,
syscalls, IPIs, page faults, etc.), and it was necessary to cover all
possible such events to have a reliable notification of a task about
its isolation being broken. To avoid such a fragile mechanism, this
version relies on mere fact of kernel being entered in isolation
mode. As a result, reporting happens later in kernel code, however it
covers everything.

This means that now there is no specific reporting, in kernel log or
elsewhere, about the reasons for breaking isolation. Information about
that may be valuable at runtime, so a separate mechanism for generic
reporting "why did CPU enter kernel" (with isolation or under other
conditions) may be a good thing. That can be done later, however at
this point it's important that task isolation does not require it, and
such mechanism will not be developed with the limited purpose of
supporting isolation alone.

General description

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 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.

This set of patches only covers the implementation of task isolation,
however additional functionality, such as selective TLB flushes, may
be implemented to avoid other kinds of disturbances that affect
latency and performance of isolated tasks.

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@gmail.com>
and Yuri Norov <ynorov@marvell.com> for various contributions to this
work, Frederic Weisbecker <frederic@kernel.org> 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, and
Nitesh Narayan Lal <nitesh@redhat.com> for adapting earlier patches
related to interrupt and work distribution in presence of CPU
isolation.

--
Alex