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, 202 insertions, 0 deletions

diff --git a/Documentation/process/kernel-driver-statement.rst b/Documentation/process/kernel-driver-statement.rst
new file mode 100644
index 000000000..a849790a6
--- /dev/null
+++ b/Documentation/process/kernel-driver-statement.rst
@@ -0,0 +1,202 @@
+.. _process_statement_driver:
+
+Kernel Driver Statement
+-----------------------
+
+Position Statement on Linux Kernel Modules
+==========================================
+
+
+We, the undersigned Linux kernel developers, consider any closed-source
+Linux kernel module or driver to be harmful and undesirable. We have
+repeatedly found them to be detrimental to Linux users, businesses, and
+the greater Linux ecosystem. Such modules negate the openness,
+stability, flexibility, and maintainability of the Linux development
+model and shut their users off from the expertise of the Linux
+community. Vendors that provide closed-source kernel modules force their
+customers to give up key Linux advantages or choose new vendors.
+Therefore, in order to take full advantage of the cost savings and
+shared support benefits open source has to offer, we urge vendors to
+adopt a policy of supporting their customers on Linux with open-source
+kernel code.
+
+We speak only for ourselves, and not for any company we might work for
+today, have in the past, or will in the future.
+
+ - Dave Airlie
+ - Nick Andrew
+ - Jens Axboe
+ - Ralf Baechle
+ - Felipe Balbi
+ - Ohad Ben-Cohen
+ - Muli Ben-Yehuda
+ - Jiri Benc
+ - Arnd Bergmann
+ - Thomas Bogendoerfer
+ - Vitaly Bordug
+ - James Bottomley
+ - Josh Boyer
+ - Neil Brown
+ - Mark Brown
+ - David Brownell
+ - Michael Buesch
+ - Franck Bui-Huu
+ - Adrian Bunk
+ - François Cami
+ - Ralph Campbell
+ - Luiz Fernando N. Capitulino
+ - Mauro Carvalho Chehab
+ - Denis Cheng
+ - Jonathan Corbet
+ - Glauber Costa
+ - Alan Cox
+ - Magnus Damm
+ - Ahmed S. Darwish
+ - Robert P. J. Day
+ - Hans de Goede
+ - Arnaldo Carvalho de Melo
+ - Helge Deller
+ - Jean Delvare
+ - Mathieu Desnoyers
+ - Sven-Thorsten Dietrich
+ - Alexey Dobriyan
+ - Daniel Drake
+ - Alex Dubov
+ - Randy Dunlap
+ - Michael Ellerman
+ - Pekka Enberg
+ - Jan Engelhardt
+ - Mark Fasheh
+ - J. Bruce Fields
+ - Larry Finger
+ - Jeremy Fitzhardinge
+ - Mike Frysinger
+ - Kumar Gala
+ - Robin Getz
+ - Liam Girdwood
+ - Jan-Benedict Glaw
+ - Thomas Gleixner
+ - Brice Goglin
+ - Cyrill Gorcunov
+ - Andy Gospodarek
+ - Thomas Graf
+ - Krzysztof Halasa
+ - Harvey Harrison
+ - Stephen Hemminger
+ - Michael Hennerich
+ - Tejun Heo
+ - Benjamin Herrenschmidt
+ - Kristian Høgsberg
+ - Henrique de Moraes Holschuh
+ - Marcel Holtmann
+ - Mike Isely
+ - Takashi Iwai
+ - Olof Johansson
+ - Dave Jones
+ - Jesper Juhl
+ - Matthias Kaehlcke
+ - Kenji Kaneshige
+ - Jan Kara
+ - Jeremy Kerr
+ - Russell King
+ - Olaf Kirch
+ - Roel Kluin
+ - Hans-Jürgen Koch
+ - Auke Kok
+ - Peter Korsgaard
+ - Jiri Kosina
+ - Aaro Koskinen
+ - Mariusz Kozlowski
+ - Greg Kroah-Hartman
+ - Michael Krufky
+ - Aneesh Kumar
+ - Clemens Ladisch
+ - Christoph Lameter
+ - Gunnar Larisch
+ - Anders Larsen
+ - Grant Likely
+ - John W. Linville
+ - Yinghai Lu
+ - Tony Luck
+ - Pavel Machek
+ - Matt Mackall
+ - Paul Mackerras
+ - Roland McGrath
+ - Patrick McHardy
+ - Kyle McMartin
+ - Paul Menage
+ - Thierry Merle
+ - Eric Miao
+ - Akinobu Mita
+ - Ingo Molnar
+ - James Morris
+ - Andrew Morton
+ - Paul Mundt
+ - Oleg Nesterov
+ - Luca Olivetti
+ - S.Çağlar Onur
+ - Pierre Ossman
+ - Keith Owens
+ - Venkatesh Pallipadi
+ - Nick Piggin
+ - Nicolas Pitre
+ - Evgeniy Polyakov
+ - Richard Purdie
+ - Mike Rapoport
+ - Sam Ravnborg
+ - Gerrit Renker
+ - Stefan Richter
+ - David Rientjes
+ - Luis R. Rodriguez
+ - Stefan Roese
+ - Francois Romieu
+ - Rami Rosen
+ - Stephen Rothwell
+ - Maciej W. Rozycki
+ - Mark Salyzyn
+ - Yoshinori Sato
+ - Deepak Saxena
+ - Holger Schurig
+ - Amit Shah
+ - Yoshihiro Shimoda
+ - Sergei Shtylyov
+ - Kay Sievers
+ - Sebastian Siewior
+ - Rik Snel
+ - Jes Sorensen
+ - Alexey Starikovskiy
+ - Alan Stern
+ - Timur Tabi
+ - Hirokazu Takata
+ - Eliezer Tamir
+ - Eugene Teo
+ - Doug Thompson
+ - FUJITA Tomonori
+ - Dmitry Torokhov
+ - Marcelo Tosatti
+ - Steven Toth
+ - Theodore Tso
+ - Matthias Urlichs
+ - Geert Uytterhoeven
+ - Arjan van de Ven
+ - Ivo van Doorn
+ - Rik van Riel
+ - Wim Van Sebroeck
+ - Hans Verkuil
+ - Horst H. von Brand
+ - Dmitri Vorobiev
+ - Anton Vorontsov
+ - Daniel Walker
+ - Johannes Weiner
+ - Harald Welte
+ - Matthew Wilcox
+ - Dan J. Williams
+ - Darrick J. Wong
+ - David Woodhouse
+ - Chris Wright
+ - Bryan Wu
+ - Rafael J. Wysocki
+ - Herbert Xu
+ - Vlad Yasevich
+ - Peter Zijlstra
+ - Bartlomiej Zolnierkiewicz