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, 109 insertions, 0 deletions

diff --git a/Documentation/process/contribution-maturity-model.rst b/Documentation/process/contribution-maturity-model.rst
new file mode 100644
index 000000000..b87ab34de
--- /dev/null
+++ b/Documentation/process/contribution-maturity-model.rst
@@ -0,0 +1,109 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+Linux Kernel Contribution Maturity Model
+========================================
+
+
+Background
+==========
+
+As a part of the 2021 Linux Kernel Maintainers’ Summit, there was a
+`discussion <https://lwn.net/Articles/870581/>`_ about the challenges in
+recruiting kernel maintainers as well as maintainer succession.  Some of
+the conclusions from that discussion included that companies which are a
+part of the Linux Kernel community need to allow engineers to be
+maintainers as part of their job, so they can grow into becoming
+respected leaders and eventually, kernel maintainers.  To support a
+strong talent pipeline, developers should be allowed and encouraged to
+take on upstream contributions such as reviewing other people’s patches,
+refactoring kernel infrastructure, and writing documentation.
+
+To that end, the Linux Foundation Technical Advisory Board (TAB)
+proposes this Linux Kernel Contribution Maturity Model. These common
+expectations for upstream community engagement aim to increase the
+influence of individual developers, increase the collaboration of
+organizations, and improve the overall health of the Linux Kernel
+ecosystem.
+
+The TAB urges organizations to continuously evaluate their Open Source
+maturity model and commit to improvements to align with this model.  To
+be effective, this evaluation should incorporate feedback from across
+the organization, including management and developers at all seniority
+levels.  In the spirit of Open Source, we encourage organizations to
+publish their evaluations and plans to improve their engagement with the
+upstream community.
+
+Level 0
+=======
+
+* Software Engineers are not allowed to contribute patches to the Linux
+  kernel.
+
+
+Level 1
+=======
+
+* Software Engineers are allowed to contribute patches to the Linux
+  kernel, either as part of their job responsibilities or on their own
+  time.
+
+Level 2
+=======
+
+* Software Engineers are expected to contribute to the Linux Kernel as
+  part of their job responsibilities.
+* Software Engineers will be supported to attend Linux-related
+  conferences as a part of their job.
+* A Software Engineer’s upstream code contributions will be considered
+  in promotion and performance reviews.
+
+Level 3
+=======
+
+* Software Engineers are expected to review patches (including patches
+  authored by engineers from other companies) as part of their job
+  responsibilities
+* Contributing presentations or papers to Linux-related or academic
+  conferences (such those organized by the Linux Foundation, Usenix,
+  ACM, etc.), are considered part of an engineer’s work.
+* A Software Engineer’s community contributions will be considered in
+  promotion and performance reviews.
+* Organizations will regularly report metrics of their open source
+  contributions and track these metrics over time.  These metrics may be
+  published only internally within the organization, or at the
+  organization’s discretion, some or all may be published externally.
+  Metrics that are strongly suggested include:
+
+  * The number of upstream kernel contributions by team or organization
+    (e.g., all people reporting up to a manager, director, or VP).
+  * The percentage of kernel developers who have made upstream
+    contributions relative to the total kernel developers in the
+    organization.
+  * The time interval between kernels used in the organization’s servers
+    and/or products, and the publication date of the upstream kernel
+    upon which the internal kernel is based.
+  * The number of out-of-tree commits present in internal kernels.
+
+Level 4
+=======
+
+* Software Engineers are encouraged to spend a portion of their work
+  time focused on Upstream Work, which is defined as reviewing patches,
+  serving on program committees, improving core project infrastructure
+  such as writing or maintaining tests, upstream tech debt reduction,
+  writing documentation, etc.
+* Software Engineers are supported in helping to organize Linux-related
+  conferences.
+* Organizations will consider community member feedback in official
+  performance reviews.
+
+Level 5
+=======
+
+* Upstream kernel development is considered a formal job position, with
+  at least a third of the engineer’s time spent doing Upstream Work.
+* Organizations will actively seek out community member feedback as a
+  factor in official performance reviews.
+* Organizations will regularly report internally on the ratio of
+  Upstream Work to work focused on directly pursuing business goals.