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, 113 insertions, 0 deletions

diff --git a/Documentation/gpu/drm-ras.rst b/Documentation/gpu/drm-ras.rst
new file mode 100644
index 000000000..83c21853b
--- /dev/null
+++ b/Documentation/gpu/drm-ras.rst
@@ -0,0 +1,113 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+============================
+DRM RAS over Generic Netlink
+============================
+
+The DRM RAS (Reliability, Availability, Serviceability) interface provides a
+standardized way for GPU/accelerator drivers to expose error counters and
+other reliability nodes to user space via Generic Netlink. This allows
+diagnostic tools, monitoring daemons, or test infrastructure to query hardware
+health in a uniform way across different DRM drivers.
+
+Key Goals:
+
+* Provide a standardized RAS solution for GPU and accelerator drivers, enabling
+  data center monitoring and reliability operations.
+* Implement a single drm-ras Generic Netlink family to meet modern Netlink YAML
+  specifications and centralize all RAS-related communication in one namespace.
+* Support a basic error counter interface, addressing the immediate, essential
+  monitoring needs.
+* Offer a flexible, future-proof interface that can be extended to support
+  additional types of RAS data in the future.
+* Allow multiple nodes per driver, enabling drivers to register separate
+  nodes for different IP blocks, sub-blocks, or other logical subdivisions
+  as applicable.
+
+.. contents::
+
+Nodes
+=====
+
+Nodes are logical abstractions representing an error type or error source within
+the device. Currently, only error counter nodes is supported.
+
+Drivers are responsible for registering and unregistering nodes via the
+`drm_ras_node_register()` and `drm_ras_node_unregister()` APIs.
+
+Node Management
+-------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_ras.c
+   :doc: DRM RAS Node Management
+.. kernel-doc:: drivers/gpu/drm/drm_ras.c
+   :internal:
+
+Generic Netlink Usage
+=====================
+
+The interface is implemented as a Generic Netlink family named ``drm-ras``.
+User space tools can:
+
+* List registered nodes with the ``list-nodes`` command.
+* List all error counters in an node with the ``get-error-counter`` command with ``node-id``
+  as a parameter.
+* Query specific error counter values with the ``get-error-counter`` command, using both
+  ``node-id`` and ``error-id`` as parameters.
+* Clear specific error counters with the ``clear-error-counter`` command, using both
+  ``node-id`` and ``error-id`` as parameters.
+
+YAML-based Interface
+--------------------
+
+The interface is described in a YAML specification ``Documentation/netlink/specs/drm_ras.yaml``
+
+This YAML is used to auto-generate user space bindings via
+``tools/net/ynl/pyynl/ynl_gen_c.py``, and drives the structure of netlink
+attributes and operations.
+
+Usage Notes
+-----------
+
+* User space must first enumerate nodes to obtain their IDs.
+* Node IDs or Node names can be used for all further queries, such as error counters.
+* Error counters can be queried by either the Error ID or Error name.
+* Query Parameters should be defined as part of the uAPI to ensure user interface stability.
+* The interface supports future extension by adding new node types and
+  additional attributes.
+
+Example: List nodes using ynl
+
+.. code-block:: bash
+
+    sudo ynl --family drm_ras --dump list-nodes
+    [{'device-name': '0000:03:00.0',
+    'node-id': 0,
+    'node-name': 'correctable-errors',
+    'node-type': 'error-counter'},
+    {'device-name': '0000:03:00.0',
+     'node-id': 1,
+     'node-name': 'uncorrectable-errors',
+     'node-type': 'error-counter'}]
+
+Example: List all error counters using ynl
+
+.. code-block:: bash
+
+    sudo ynl --family drm_ras --dump get-error-counter --json '{"node-id":0}'
+    [{'error-id': 1, 'error-name': 'error_name1', 'error-value': 0},
+    {'error-id': 2, 'error-name': 'error_name2', 'error-value': 0}]
+
+Example: Query an error counter for a given node
+
+.. code-block:: bash
+
+    sudo ynl --family drm_ras --do get-error-counter --json '{"node-id":0, "error-id":1}'
+    {'error-id': 1, 'error-name': 'error_name1', 'error-value': 0}
+
+Example: Clear an error counter for a given node
+
+.. code-block:: bash
+
+    sudo ynl --family drm_ras --do clear-error-counter --json '{"node-id":0, "error-id":1}'
+    None