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, 152 insertions, 0 deletions

diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
new file mode 100644
index 000000000..6601a2586
--- /dev/null
+++ b/Documentation/driver-api/index.rst
@@ -0,0 +1,152 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Driver implementer's API guide
+==============================
+
+The kernel offers a wide variety of interfaces to support the development
+of device drivers.  This document is an only somewhat organized collection
+of some of those interfaces — it will hopefully get better over time!  The
+available subsections can be seen below.
+
+
+General information for driver authors
+======================================
+
+This section contains documentation that should, at some point or other, be
+of interest to most developers working on device drivers.
+
+.. toctree::
+   :maxdepth: 1
+
+   basics
+   driver-model/index
+   device_link
+   infrastructure
+   ioctl
+   pm/index
+
+Useful support libraries
+========================
+
+This section contains documentation that should, at some point or other, be
+of interest to most developers working on device drivers.
+
+.. toctree::
+   :maxdepth: 1
+
+   early-userspace/index
+   connector
+   device-io
+   devfreq
+   dma-buf
+   component
+   io-mapping
+   io_ordering
+   uio-howto
+   vfio-mediated-device
+   vfio
+   vfio-pci-device-specific-driver-acceptance
+
+Bus-level documentation
+=======================
+
+.. toctree::
+   :maxdepth: 1
+
+   auxiliary_bus
+   cxl/index
+   eisa
+   firewire
+   i3c/index
+   isa
+   men-chameleon-bus
+   pci/index
+   rapidio/index
+   slimbus
+   usb/index
+   virtio/index
+   vme
+   w1
+   xillybus
+
+
+Subsystem-specific APIs
+=======================
+
+.. toctree::
+   :maxdepth: 1
+
+   80211/index
+   acpi/index
+   backlight/lp855x-driver.rst
+   clk
+   coco/index
+   console
+   crypto/index
+   dmaengine/index
+   dpll
+   edac
+   extcon
+   firmware/index
+   fpga/index
+   frame-buffer
+   aperture
+   generic-counter
+   generic_pt
+   gpio/index
+   hsi
+   hte/index
+   hw-recoverable-errors
+   i2c
+   iio/index
+   infiniband
+   input
+   interconnect
+   ipmb
+   ipmi
+   issei/index
+   libata
+   mailbox
+   md/index
+   media/index
+   mei/index
+   memory-devices/index
+   message-based
+   misc_devices
+   miscellaneous
+   mmc/index
+   mtd/index
+   mtdnand
+   nfc/index
+   ntb
+   nvdimm/index
+   nvmem
+   parport-lowlevel
+   phy/index
+   pin-control
+   pldmfw/index
+   pps
+   ptp
+   pwm
+   pwrseq
+   regulator
+   reset
+   rfkill
+   s390-drivers
+   scsi
+   serial/index
+   sm501
+   soundwire/index
+   spi
+   surface_aggregator/index
+   switchtec
+   sync_file
+   target
+   tee
+   thermal/index
+   tty/index
+   wbrf
+   wmi
+   xilinx/index
+   zorro