sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

alb_break_llc() decides whether to break LLC preference during active
load balance. It does so by testing that every runnable fair task on the
source rq prefers its LLC:

	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable

But the two counters cover different sets. nr_pref_llc_running is updated
in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
clear_delayed() and drops delay-dequeued tasks.

So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
alb_break_llc() returns false, and active balance is free to pull a task
off its preferred LLC. Active balance only moves runnable tasks, and this
is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
skips the per-task test in can_migrate_task(). The runnable set is the one
we want.

Fix it on the counter side. A task should be counted in
nr_pref_llc_running exactly while it is both queued on its preferred LLC
(pref_llc_queued) and runnable (!sched_delayed). Define that membership
once in task_pref_llc_runnable(), and adjust the counter only through
pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
change either input: account_llc_enqueue(), account_llc_dequeue(),
set_delayed() and clear_delayed(). Gating every update on the same
predicate keeps the delay, wake and dequeue paths from double-counting
or underflowing; see the comments at those sites for the ordering.

nr_llc_running and sd->llc_counts are not touched and stay on queued
semantics.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 69 insertions, 0 deletions

diff --git a/Documentation/gpu/nova/guidelines.rst b/Documentation/gpu/nova/guidelines.rst
new file mode 100644
index 000000000..13ab13984
--- /dev/null
+++ b/Documentation/gpu/nova/guidelines.rst
@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+==========
+Guidelines
+==========
+
+This document describes the general project guidelines that apply to nova-core
+and nova-drm.
+
+Language
+========
+
+The Nova project uses the Rust programming language. In this context, all rules
+of the Rust for Linux project as documented in
+:doc:`../../rust/general-information` apply. Additionally, the following rules
+apply.
+
+- Unless technically necessary otherwise (e.g. uAPI), any driver code is written
+  in Rust.
+
+- Unless technically necessary, unsafe Rust code must be avoided. In case of
+  technical necessity, unsafe code should be isolated in a separate component
+  providing a safe API for other driver code to use.
+
+Style
+-----
+
+All rules of the Rust for Linux project as documented in
+:doc:`../../rust/coding-guidelines` apply.
+
+For a submit checklist, please also see the `Rust for Linux Submit checklist
+addendum <https://rust-for-linux.com/contributing#submit-checklist-addendum>`_.
+
+Documentation
+=============
+
+The availability of proper documentation is essential in terms of scalability,
+accessibility for new contributors and maintainability of a project in general,
+but especially for a driver running as complex hardware as Nova is targeting.
+
+Hence, adding documentation of any kind is very much encouraged by the project.
+
+Besides that, there are some minimum requirements.
+
+- Every non-private structure needs at least a brief doc comment explaining the
+  semantical sense of the structure, as well as potential locking and lifetime
+  requirements. It is encouraged to have the same minimum documentation for
+  non-trivial private structures.
+
+- uAPIs must be fully documented with kernel-doc comments; additionally, the
+  semantical behavior must be explained including potential special or corner
+  cases.
+
+- The APIs connecting the 1st level driver (nova-core) with 2nd level drivers
+  must be fully documented. This includes doc comments, potential locking and
+  lifetime requirements, as well as example code if applicable.
+
+- Abbreviations must be explained when introduced; terminology must be uniquely
+  defined.
+
+- Register addresses, layouts, shift values and masks must be defined properly;
+  unless obvious, the semantical sense must be documented. This only applies if
+  the author is able to obtain the corresponding information.
+
+Acceptance Criteria
+===================
+
+- Patches must only be applied if reviewed by at least one other person on the
+  mailing list; this also applies for maintainers.