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, 53 insertions, 0 deletions

diff --git a/Documentation/driver-api/spi.rst b/Documentation/driver-api/spi.rst
new file mode 100644
index 000000000..74eca6735
--- /dev/null
+++ b/Documentation/driver-api/spi.rst
@@ -0,0 +1,53 @@
+Serial Peripheral Interface (SPI)
+=================================
+
+SPI is the "Serial Peripheral Interface", widely used with embedded
+systems because it is a simple and efficient interface: basically a
+multiplexed shift register. Its three signal wires hold a clock (SCK,
+often in the range of 1-20 MHz), a "Master Out, Slave In" (MOSI) data
+line, and a "Master In, Slave Out" (MISO) data line. SPI is a full
+duplex protocol; for each bit shifted out the MOSI line (one per clock)
+another is shifted in on the MISO line. Those bits are assembled into
+words of various sizes on the way to and from system memory. An
+additional chipselect line is usually active-low (nCS); four signals are
+normally used for each peripheral, plus sometimes an interrupt.
+
+The SPI bus facilities listed here provide a generalized interface to
+declare SPI buses and devices, manage them according to the standard
+Linux driver model, and perform input/output operations. At this time,
+only "master" side interfaces are supported, where Linux talks to SPI
+peripherals and does not implement such a peripheral itself. (Interfaces
+to support implementing SPI slaves would necessarily look different.)
+
+The programming interface is structured around two kinds of driver, and
+two kinds of device. A "Controller Driver" abstracts the controller
+hardware, which may be as simple as a set of GPIO pins or as complex as
+a pair of FIFOs connected to dual DMA engines on the other side of the
+SPI shift register (maximizing throughput). Such drivers bridge between
+whatever bus they sit on (often the platform bus) and SPI, and expose
+the SPI side of their device as a :c:type:`struct spi_controller
+<spi_controller>`. SPI devices are children of that master,
+represented as a :c:type:`struct spi_device <spi_device>` and
+manufactured from :c:type:`struct spi_board_info
+<spi_board_info>` descriptors which are usually provided by
+board-specific initialization code. A :c:type:`struct spi_driver
+<spi_driver>` is called a "Protocol Driver", and is bound to a
+spi_device using normal driver model calls.
+
+The I/O model is a set of queued messages. Protocol drivers submit one
+or more :c:type:`struct spi_message <spi_message>` objects,
+which are processed and completed asynchronously. (There are synchronous
+wrappers, however.) Messages are built from one or more
+:c:type:`struct spi_transfer <spi_transfer>` objects, each of
+which wraps a full duplex SPI transfer. A variety of protocol tweaking
+options are needed, because different chips adopt very different
+policies for how they use the bits transferred with SPI.
+
+.. kernel-doc:: include/linux/spi/spi.h
+   :internal:
+
+.. kernel-doc:: drivers/spi/spi.c
+   :functions: spi_register_board_info
+
+.. kernel-doc:: drivers/spi/spi.c
+   :export: