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, 51 insertions, 0 deletions

diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst
new file mode 100644
index 000000000..b44ef68f6
--- /dev/null
+++ b/Documentation/admin-guide/LSM/index.rst
@@ -0,0 +1,51 @@
+===========================
+Linux Security Module Usage
+===========================
+
+The Linux Security Module (LSM) framework provides a mechanism for
+various security checks to be hooked by new kernel extensions. The name
+"module" is a bit of a misnomer since these extensions are not actually
+loadable kernel modules. Instead, they are selectable at build-time via
+CONFIG_DEFAULT_SECURITY and can be overridden at boot-time via the
+``"security=..."`` kernel command line argument, in the case where multiple
+LSMs were built into a given kernel.
+
+The primary users of the LSM interface are Mandatory Access Control
+(MAC) extensions which provide a comprehensive security policy. Examples
+include SELinux, Smack, Tomoyo, and AppArmor. In addition to the larger
+MAC extensions, other extensions can be built using the LSM to provide
+specific changes to system operation when these tweaks are not available
+in the core functionality of Linux itself.
+
+The Linux capabilities modules will always be included. This may be
+followed by any number of "minor" modules and at most one "major" module.
+For more details on capabilities, see ``capabilities(7)`` in the Linux
+man-pages project.
+
+A list of the active security modules can be found by reading
+``/sys/kernel/security/lsm``. This is a comma separated list, and
+will always include the capability module. The list reflects the
+order in which checks are made. The capability module will always
+be first, followed by any "minor" modules (e.g. Yama) and then
+the one "major" module (e.g. SELinux) if there is one configured.
+
+Process attributes associated with "major" security modules should
+be accessed and maintained using the special files in ``/proc/.../attr``.
+A security module may maintain a module specific subdirectory there,
+named after the module. ``/proc/.../attr/smack`` is provided by the Smack
+security module and contains all its special files. The files directly
+in ``/proc/.../attr`` remain as legacy interfaces for modules that provide
+subdirectories.
+
+.. toctree::
+   :maxdepth: 1
+
+   apparmor
+   LoadPin
+   SELinux
+   Smack
+   tomoyo
+   Yama
+   SafeSetID
+   ipe
+   landlock