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, 96 insertions, 0 deletions

diff --git a/Documentation/process/coding-assistants.rst b/Documentation/process/coding-assistants.rst
new file mode 100644
index 000000000..051f0d819
--- /dev/null
+++ b/Documentation/process/coding-assistants.rst
@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _coding_assistants:
+
+AI Coding Assistants
+++++++++++++++++++++
+
+This document provides guidance for AI tools and developers using AI
+assistance when contributing to the Linux kernel.
+
+AI tools helping with Linux kernel development should follow the standard
+kernel development process:
+
+* Documentation/process/development-process.rst
+* Documentation/process/coding-style.rst
+* Documentation/process/submitting-patches.rst
+
+For guidelines on content generated by AI coding assistants see:
+
+* Documentation/process/generated-content.rst
+
+Licensing and Legal Requirements
+================================
+
+All contributions must comply with the kernel's licensing requirements:
+
+* All code must be compatible with GPL-2.0-only
+* Use appropriate SPDX license identifiers
+* See Documentation/process/license-rules.rst for details
+
+Signed-off-by and Developer Certificate of Origin
+=================================================
+
+AI agents MUST NOT add Signed-off-by tags. Only humans can legally
+certify the Developer Certificate of Origin (DCO). The human submitter
+is responsible for:
+
+* Reviewing all AI-generated code
+* Ensuring compliance with licensing requirements
+* Adding their own Signed-off-by tag to certify the DCO
+* Taking full responsibility for the contribution
+
+Attribution
+===========
+
+When AI tools contribute to kernel development, proper attribution
+helps track the evolving role of AI in the development process.
+Contributions should include an Assisted-by tag in the following format::
+
+  Assisted-by: LLM [TOOL1] [TOOL2]
+
+* ``[TOOL1] [TOOL2]`` are optional specialized analysis tools used
+  (e.g., coccinelle, sparse, smatch, clang-tidy)
+
+Basic development tools (git, gcc, make, editors) should not be listed.
+
+Example::
+
+  Assisted-by: LLM coccinelle sparse
+
+Procedure for finding and fixing bugs
+=====================================
+
+When an AI assistant is used to find and fix bugs, it **MUST** follow at least
+these steps:
+
+1. Before starting, read the whole process documentation listed above, as well
+   as any other document mentioned in the request. Do not rely on isolated
+   parts found by keyword search.
+2. Note the commit ID and Locate a bug as instructed.
+3. For any bug found that is not trivial, verify that it looks real by
+   attempting to create a reproducer to demonstrate it. Lacking it may cause
+   the report to be ignored, as many unverified bug reports sent to maintainers
+   happen to be invalid. Stop here if it finally looks wrong.
+4. Write a fix for the bug. This part is not optional: except in a few very
+   rare cases, an AI assistant able to find a bug is able to fix it. Note that
+   fixes written in the same session as used to find the bug will generally
+   lead to better and more accurate fixes as the LLM's reasoning context
+   remains present.
+5. Build and verify that the fix works either using the reproducer or by
+   re-running a complete analysis; drop any fix that doesn't work and try
+   another one. The fix must not add build warnings and must pass the
+   checkpatch.pl checks (see submitting-patches.rst).
+6. Commit the working fix with a detailed message describing the problem, the
+   solution and a Fixes tag. Do not add a Signed-off-by tag, and add an
+   Assisted-by tag, as described above.
+7. Identify the maintainers and lists using scripts/get_maintainer.pl.
+   Documentation/process/security-bugs.rst shows how to do that.
+8. Indicate what could not be done. If the fix could not be built or tested, or
+   if no reproducer could be produced, say so explicitly: maintainers currently
+   waste too much time analyzing unverified reports and untested fixes.
+9. Read Documentation/process/threat-model.rst to determine whether the bug is
+   a vulnerability or a regular bug, and leave the result to the reporter for
+   review (the assistant must never send anything itself). Regular bugs are
+   submitted as described in Documentation/process/submitting-patches.rst,
+   vulnerabilities as described in Documentation/process/security-bugs.rst.