sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 109 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kunit/index.rst b/Documentation/dev-tools/kunit/index.rst
new file mode 100644
index 000000000..b3593ae29
--- /dev/null
+++ b/Documentation/dev-tools/kunit/index.rst
@@ -0,0 +1,109 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+KUnit - Linux Kernel Unit Testing
+=================================
+
+.. toctree::
+	:maxdepth: 2
+	:caption: Contents:
+
+	start
+	architecture
+	run_wrapper
+	run_manual
+	usage
+	api/index
+	style
+	faq
+	running_tips
+
+This section details the kernel unit testing framework.
+
+Introduction
+============
+
+KUnit (Kernel unit testing framework) provides a common framework for
+unit tests within the Linux kernel. Using KUnit, you can define groups
+of test cases called test suites. The tests either run on kernel boot
+if built-in, or load as a module. KUnit automatically flags and reports
+failed test cases in the kernel log. The test results appear in
+:doc:`KTAP (Kernel - Test Anything Protocol) format</dev-tools/ktap>`.
+It is inspired by JUnit, Python’s unittest.mock, and GoogleTest/GoogleMock
+(C++ unit testing framework).
+
+KUnit tests are part of the kernel, written in the C (programming)
+language, and test parts of the Kernel implementation (example: a C
+language function). Excluding build time, from invocation to
+completion, KUnit can run around 100 tests in less than 10 seconds.
+KUnit can test any kernel component, for example: file system, system
+calls, memory management, device drivers and so on.
+
+KUnit follows the white-box testing approach. The test has access to
+internal system functionality. KUnit runs in kernel space and is not
+restricted to things exposed to user-space.
+
+In addition, KUnit has kunit_tool, a script (``tools/testing/kunit/kunit.py``)
+that configures the Linux kernel, runs KUnit tests under QEMU or UML
+(:doc:`User Mode Linux </virt/uml/user_mode_linux_howto_v2>`),
+parses the test results and
+displays them in a user friendly manner.
+
+Features
+--------
+
+- Provides a framework for writing unit tests.
+- Runs tests on any kernel architecture.
+- Runs a test in milliseconds.
+
+Prerequisites
+-------------
+
+- Any Linux kernel compatible hardware.
+- For Kernel under test, Linux kernel version 5.5 or greater.
+
+Unit Testing
+============
+
+A unit test tests a single unit of code in isolation. A unit test is the finest
+granularity of testing and allows all possible code paths to be tested in the
+code under test. This is possible if the code under test is small and does not
+have any external dependencies outside of the test's control like hardware.
+
+
+Write Unit Tests
+----------------
+
+To write good unit tests, there is a simple but powerful pattern:
+Arrange-Act-Assert. This is a great way to structure test cases and
+defines an order of operations.
+
+- Arrange inputs and targets: At the start of the test, arrange the data
+  that allows a function to work. Example: initialize a statement or
+  object.
+- Act on the target behavior: Call your function/code under test.
+- Assert expected outcome: Verify that the result (or resulting state) is as
+  expected.
+
+Unit Testing Advantages
+-----------------------
+
+- Increases testing speed and development in the long run.
+- Detects bugs at initial stage and therefore decreases bug fix cost
+  compared to acceptance testing.
+- Improves code quality.
+- Encourages writing testable code.
+
+Read also :ref:`kinds-of-tests`.
+
+How do I use it?
+================
+
+You can find a step-by-step guide to writing and running KUnit tests in
+Documentation/dev-tools/kunit/start.rst
+
+Alternatively, feel free to look through the rest of the KUnit documentation,
+or to experiment with tools/testing/kunit/kunit.py and the example test under
+lib/kunit/kunit-example-test.c
+
+Happy testing!