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, 172 insertions, 0 deletions

diff --git a/Documentation/gpu/automated_testing.rst b/Documentation/gpu/automated_testing.rst
new file mode 100644
index 000000000..8a7328aef
--- /dev/null
+++ b/Documentation/gpu/automated_testing.rst
@@ -0,0 +1,172 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=========================================
+Automated testing of the DRM subsystem
+=========================================
+
+Introduction
+============
+
+Making sure that changes to the core or drivers don't introduce regressions can
+be very time-consuming when lots of different hardware configurations need to
+be tested. Moreover, it isn't practical for each person interested in this
+testing to have to acquire and maintain what can be a considerable amount of
+hardware.
+
+Also, it is desirable for developers to check for regressions in their code by
+themselves, instead of relying on the maintainers to find them and then
+reporting back.
+
+There are facilities in gitlab.freedesktop.org to automatically test Mesa that
+can be used as well for testing the DRM subsystem. This document explains how
+people interested in testing it can use this shared infrastructure to save
+quite some time and effort.
+
+
+Relevant files
+==============
+
+drivers/gpu/drm/ci/gitlab-ci.yml
+--------------------------------
+
+This is the root configuration file for GitLab CI. Among other less interesting
+bits, it specifies the specific version of the scripts to be used. There are
+some variables that can be modified to change the behavior of the pipeline:
+
+DRM_CI_PROJECT_PATH
+    Repository that contains the Mesa software infrastructure for CI
+
+DRM_CI_COMMIT_SHA
+    A particular revision to use from that repository
+
+UPSTREAM_REPO
+    URL to git repository containing the target branch
+
+TARGET_BRANCH
+    Branch to which this branch is to be merged into
+
+IGT_VERSION
+    Revision of igt-gpu-tools being used, from
+    https://gitlab.freedesktop.org/drm/igt-gpu-tools
+
+drivers/gpu/drm/ci/testlist.txt
+-------------------------------
+
+IGT tests to be run on all drivers (unless mentioned in a driver's \*-skips.txt
+file, see below).
+
+drivers/gpu/drm/ci/${DRIVER_NAME}-${HW_REVISION}-fails.txt
+----------------------------------------------------------
+
+Lists the known failures for a given driver on a specific hardware revision.
+
+drivers/gpu/drm/ci/${DRIVER_NAME}-${HW_REVISION}-flakes.txt
+-----------------------------------------------------------
+
+Lists the tests that for a given driver on a specific hardware revision are
+known to behave unreliably. These tests won't cause a job to fail regardless of
+the result. They will still be run.
+
+Each new flake entry must be associated with a link to the email reporting the
+bug to the author of the affected driver or the relevant GitLab issue. The entry
+must also include the board name or Device Tree name, the first kernel version
+affected, the IGT version used for tests, and an approximation of the failure rate.
+
+They should be provided under the following format::
+
+  # Bug Report: $LORE_URL_OR_GITLAB_ISSUE
+  # Board Name: broken-board.dtb
+  # Linux Version: 6.6-rc1
+  # IGT Version: 1.28-gd2af13d9f
+  # Failure Rate: 100
+  flaky-test
+
+Use the appropriate link below to create a GitLab issue:
+amdgpu driver: https://gitlab.freedesktop.org/drm/amd/-/issues
+i915 driver: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues
+msm driver: https://gitlab.freedesktop.org/drm/msm/-/issues
+xe driver: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues
+
+drivers/gpu/drm/ci/${DRIVER_NAME}-${HW_REVISION}-skips.txt
+-----------------------------------------------------------
+
+Lists the tests that won't be run for a given driver on a specific hardware
+revision. These are usually tests that interfere with the running of the test
+list due to hanging the machine, causing OOM, taking too long, etc.
+
+
+How to enable automated testing on your tree
+============================================
+
+1. Create a Linux tree in https://gitlab.freedesktop.org/ if you don't have one
+yet, by forking https://gitlab.freedesktop.org/drm/kernel (this allows GitLab
+to internally track that these are the same git objects).
+
+2. In your kernel repo's configuration (eg.
+https://gitlab.freedesktop.org/janedoe/linux/-/settings/ci_cd), change the
+CI/CD configuration file from .gitlab-ci.yml to
+drivers/gpu/drm/ci/gitlab-ci.yml.
+
+3. Request to be added to the drm/ci-ok group so that your user has the
+necessary privileges to run the CI on https://gitlab.freedesktop.org/drm/ci-ok
+
+4. Next time you push to this repository, you will see a CI pipeline being
+created (eg. https://gitlab.freedesktop.org/janedoe/linux/-/pipelines)
+
+5. The various jobs will be run and when the pipeline is finished, all jobs
+should be green unless a regression has been found.
+
+6. Warnings in the pipeline indicate that lockdep
+(see Documentation/locking/lockdep-design.rst) issues have been detected
+during the tests.
+
+
+How to update test expectations
+===============================
+
+If your changes to the code fix any tests, you will have to remove one or more
+lines from one or more of the files in
+drivers/gpu/drm/ci/${DRIVER_NAME}_*_fails.txt, for each of the test platforms
+affected by the change.
+
+
+How to expand coverage
+======================
+
+If your code changes make it possible to run more tests (by solving reliability
+issues, for example), you can remove tests from the flakes and/or skips lists,
+and then the expected results if there are any known failures.
+
+If there is a need for updating the version of IGT being used (maybe you have
+added more tests to it), update the IGT_VERSION variable at the top of the
+gitlab-ci.yml file.
+
+
+How to test your changes to the scripts
+=======================================
+
+For testing changes to the scripts in the drm-ci repo, change the
+DRM_CI_PROJECT_PATH and DRM_CI_COMMIT_SHA variables in
+drivers/gpu/drm/ci/gitlab-ci.yml to match your fork of the project (eg.
+janedoe/drm-ci). This fork needs to be in https://gitlab.freedesktop.org/.
+
+
+How to incorporate external fixes in your testing
+=================================================
+
+Often, regressions in other trees will prevent testing changes local to the
+tree under test. These fixes will be automatically merged in during the build
+jobs from a branch in the target tree that is named as
+${TARGET_BRANCH}-external-fixes.
+
+If the pipeline is not in a merge request and a branch with the same name
+exists in the local tree, commits from that branch will be merged in as well.
+
+
+How to deal with automated testing labs that may be down
+========================================================
+
+If a hardware farm is down and thus causing pipelines to fail that would
+otherwise pass, one can disable all jobs that would be submitted to that farm
+by editing the file at
+https://gitlab.freedesktop.org/gfx-ci/lab-status/-/blob/main/lab-status.yml.