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, 75 insertions, 0 deletions

diff --git a/Documentation/driver-api/tty/index.rst b/Documentation/driver-api/tty/index.rst
new file mode 100644
index 000000000..c1ffe3d1e
--- /dev/null
+++ b/Documentation/driver-api/tty/index.rst
@@ -0,0 +1,75 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===
+TTY
+===
+
+Teletypewriter (TTY) layer takes care of all those serial devices. Including
+the virtual ones like pseudoterminal (PTY).
+
+TTY structures
+==============
+
+There are several major TTY structures. Every TTY device in a system has a
+corresponding struct tty_port. These devices are maintained by a TTY driver
+which is struct tty_driver. This structure describes the driver but also
+contains a reference to operations which could be performed on the TTYs. It is
+struct tty_operations. Then, upon open, a struct tty_struct is allocated and
+lives until the final close. During this time, several callbacks from struct
+tty_operations are invoked by the TTY layer.
+
+Every character received by the kernel (both from devices and users) is passed
+through a preselected :doc:`tty_ldisc` (in
+short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters
+as defined by a particular ldisc or by user too. The default one is n_tty,
+implementing echoes, signal handling, jobs control, special characters
+processing, and more. The transformed characters are passed further to
+user/device, depending on the source.
+
+In-detail description of the named TTY structures is in separate documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   tty_driver
+   tty_port
+   tty_struct
+   tty_ldisc
+   tty_buffer
+   tty_ioctl
+   tty_internals
+   console
+
+Writing TTY Driver
+==================
+
+Before one starts writing a TTY driver, they must consider
+:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>`
+layers first. Drivers for serial devices can often use one of these specific
+layers to implement a serial driver. Only special devices should be handled
+directly by the TTY Layer. If you are about to write such a driver, read on.
+
+A *typical* sequence a TTY driver performs is as follows:
+
+#. Allocate and register a TTY driver (module init)
+#. Create and register TTY devices as they are probed (probe function)
+#. Handle TTY operations and events like interrupts (TTY core invokes the
+   former, the device the latter)
+#. Remove devices as they are going away (remove function)
+#. Unregister and free the TTY driver (module exit)
+
+Steps regarding driver, i.e. 1., 3., and 5. are described in detail in
+:doc:`tty_driver`. For the other two (devices handling), look into
+:doc:`tty_port`.
+
+Other Documentation
+===================
+
+Miscellaneous documentation can be further found in these documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   moxa-smartio
+   n_gsm
+   n_tty