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, 108 insertions, 0 deletions

diff --git a/Documentation/gpu/vc4.rst b/Documentation/gpu/vc4.rst
new file mode 100644
index 000000000..5e5e92e40
--- /dev/null
+++ b/Documentation/gpu/vc4.rst
@@ -0,0 +1,108 @@
+=====================================
+ drm/vc4 Broadcom VC4 Graphics Driver
+=====================================
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_drv.c
+   :doc: Broadcom VC4 Graphics Driver
+
+Display Hardware Handling
+=========================
+
+This section covers everything related to the display hardware including
+the mode setting infrastructure, plane, sprite and cursor handling and
+display, output probing and related topics.
+
+Pixel Valve (DRM CRTC)
+----------------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_crtc.c
+   :doc: VC4 CRTC module
+
+HVS
+---
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_hvs.c
+   :doc: VC4 HVS module.
+
+HVS planes
+----------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_plane.c
+   :doc: VC4 plane module
+
+HDMI encoder
+------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_hdmi.c
+   :doc: VC4 Falcon HDMI module
+
+DSI encoder
+-----------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_dsi.c
+   :doc: VC4 DSI0/DSI1 module
+
+DPI encoder
+-----------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_dpi.c
+   :doc: VC4 DPI module
+
+VEC (Composite TV out) encoder
+------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_vec.c
+   :doc: VC4 SDTV module
+
+KUnit Tests
+===========
+
+The VC4 Driver uses KUnit to perform driver-specific unit and
+integration tests.
+
+These tests are using a mock driver and can be ran using the
+command below, on either arm or arm64 architectures,
+
+.. code-block:: bash
+
+	$ ./tools/testing/kunit/kunit.py run \
+		--kunitconfig=drivers/gpu/drm/vc4/tests/.kunitconfig \
+		--cross_compile aarch64-linux-gnu- --arch arm64
+
+Parts of the driver that are currently covered by tests are:
+ * The HVS to PixelValve dynamic FIFO assignment, for the BCM2835-7
+   and BCM2711.
+
+Memory Management and 3D Command Submission
+===========================================
+
+This section covers the GEM implementation in the vc4 driver.
+
+GPU buffer object (BO) management
+---------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_bo.c
+   :doc: VC4 GEM BO management support
+
+V3D binner command list (BCL) validation
+----------------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_validate.c
+   :doc: Command list validator for VC4.
+
+V3D render command list (RCL) generation
+----------------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_render_cl.c
+   :doc: Render command list generation
+
+Shader validator for VC4
+---------------------------
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_validate_shaders.c
+   :doc: Shader validator for VC4.
+
+V3D Interrupts
+--------------
+
+.. kernel-doc:: drivers/gpu/drm/vc4/vc4_irq.c
+   :doc: Interrupt management for the V3D engine