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, 56 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-css b/Documentation/ABI/testing/sysfs-bus-css
new file mode 100644
index 000000000..d4d5cfb63
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-css
@@ -0,0 +1,56 @@
+What:		/sys/bus/css/devices/.../type
+Date:		March 2008
+Contact:	linux-s390@vger.kernel.org
+Description:	Contains the subchannel type, as reported by the hardware.
+		This attribute is present for all subchannel types.
+
+What:		/sys/bus/css/devices/.../modalias
+Date:		March 2008
+Contact:	linux-s390@vger.kernel.org
+Description:	Contains the module alias as reported with uevents.
+		It is of the format css:t<type> and present for all
+		subchannel types.
+
+What:		/sys/bus/css/drivers/io_subchannel/.../chpids
+Date:		December 2002
+Contact:	linux-s390@vger.kernel.org
+Description:	Contains the ids of the channel paths used by this
+		subchannel, as reported by the channel subsystem
+		during subchannel recognition.
+
+		Note: This is an I/O-subchannel specific attribute.
+Users:		s390-tools, HAL
+
+What:		/sys/bus/css/drivers/io_subchannel/.../pimpampom
+Date:		December 2002
+Contact:	linux-s390@vger.kernel.org
+Description:	Contains the PIM/PAM/POM values, as reported by the
+		channel subsystem when last queried by the common I/O
+		layer (this implies that this attribute is not necessarily
+		in sync with the values current in the channel subsystem).
+
+		Note: This is an I/O-subchannel specific attribute.
+Users:		s390-tools, HAL
+
+What:		/sys/bus/css/devices/.../driver_override
+Date:		June 2019
+Contact:	linux-s390@vger.kernel.org
+Description:	This file allows the driver for a device to be specified. When
+		specified, only a driver with a name matching the value written
+		to driver_override will have an opportunity to bind to the
+		device. The override is specified by writing a string to the
+		driver_override file (echo vfio-ccw > driver_override) and
+		may be cleared with an empty string (echo > driver_override).
+		This returns the device to standard matching rules binding.
+		Writing to driver_override does not automatically unbind the
+		device from its current driver or make any attempt to
+		automatically load the specified driver.  If no driver with a
+		matching name is currently loaded in the kernel, the device
+		will not bind to any driver.  This also allows devices to
+		opt-out of driver binding using a driver_override name such as
+		"none".  Only a single driver may be specified in the override,
+		there is no support for parsing delimiters.
+
+		Note that unlike the mechanism of the same name for pci, this
+		file does not allow to override basic matching rules. I.e.,
+		the driver must still match the subchannel type of the device.