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/process/generated-content.rst b/Documentation/process/generated-content.rst
new file mode 100644
index 000000000..08621e50a
--- /dev/null
+++ b/Documentation/process/generated-content.rst
@@ -0,0 +1,109 @@
+============================================
+Kernel Guidelines for Tool-Generated Content
+============================================
+
+Purpose
+=======
+
+Kernel contributors have been using tooling to generate contributions
+for a long time. These tools can increase the volume of contributions.
+At the same time, reviewer and maintainer bandwidth is a scarce
+resource. Understanding which portions of a contribution come from
+humans versus tools is helpful to maintain those resources and keep
+kernel development healthy.
+
+The goal here is to clarify community expectations around tools. This
+lets everyone become more productive while also maintaining high
+degrees of trust between submitters and reviewers.
+
+Out of Scope
+============
+
+These guidelines do not apply to tools that make trivial tweaks to
+preexisting content. Nor do they pertain to tooling that helps with
+menial tasks. Some examples:
+
+ - Spelling and grammar fix ups, like rephrasing to imperative voice
+ - Typing aids like identifier completion, common boilerplate or
+   trivial pattern completion
+ - Purely mechanical transformations like variable renaming
+ - Reformatting, like running Lindent, ``clang-format`` or
+   ``rust-fmt``
+
+Even whenever your tool use is out of scope, you should still always
+consider if it would help reviewing your contribution if the reviewer
+knows about the tool that you used.
+
+In Scope
+========
+
+These guidelines apply when a meaningful amount of content in a kernel
+contribution was not written by a person in the Signed-off-by chain,
+but was instead created by a tool.
+
+Detection of a problem and testing the fix for it is also part of the
+development process; if a tool was used to find a problem addressed by
+a change, that should be noted in the changelog. This not only gives
+credit where it is due, it also helps fellow developers find out about
+these tools.
+
+Some examples:
+ - Any tool-suggested fix such as ``checkpatch.pl --fix``
+ - Coccinelle scripts
+ - A chatbot generated a new function in your patch to sort list entries.
+ - A .c file in the patch was originally generated by a coding
+   assistant but cleaned up by hand.
+ - The changelog was generated by handing the patch to a generative AI
+   tool and asking it to write the changelog.
+ - The changelog was translated from another language.
+
+If in doubt, choose transparency and assume these guidelines apply to
+your contribution.
+
+Guidelines
+==========
+
+First, read the Developer's Certificate of Origin:
+Documentation/process/submitting-patches.rst. Its rules are simple
+and have been in place for a long time. They have covered many
+tool-generated contributions. Ensure that you understand your entire
+submission and are prepared to respond to review comments.
+
+Second, when making a contribution, be transparent about the origin of
+content in cover letters and changelogs. You can be more transparent
+by adding information like this:
+
+ - What tools were used?
+ - The input to the tools you used, like the Coccinelle source script.
+ - If code was largely generated from a single or short set of
+   prompts, include those prompts. For longer sessions, include a
+   summary of the prompts and the nature of resulting assistance.
+ - Which portions of the content were affected by that tool?
+ - How is the submission tested and what tools were used to test the
+   fix?
+
+As with all contributions, individual maintainers have discretion to
+choose how they handle the contribution. For example, they might:
+
+ - Treat it just like any other contribution.
+ - Reject it outright.
+ - Treat the contribution specially, for example, asking for extra
+   testing, reviewing with extra scrutiny, or reviewing at a lower
+   priority than human-generated content.
+ - Ask for some other special steps, like asking the contributor to
+   elaborate on how the tool or model was trained.
+ - Ask the submitter to explain in more detail about the contribution
+   so that the maintainer can be assured that the submitter fully
+   understands how the code works.
+ - Suggest a better prompt instead of suggesting specific code changes.
+
+If tools permit you to generate a contribution automatically, expect
+additional scrutiny in proportion to how much of it was generated.
+
+As with the output of any tooling, the result may be incorrect or
+inappropriate. You are expected to understand and to be able to defend
+everything you submit. If you are unable to do so, then do not submit
+the resulting changes.
+
+If you do so anyway, maintainers are entitled to reject your series
+without detailed review.