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, 43 insertions, 0 deletions

diff --git a/drivers/char/Makefile b/drivers/char/Makefile
new file mode 100644
index 000000000..a46d7bf7c
--- /dev/null
+++ b/drivers/char/Makefile
@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the kernel character device drivers.
+#
+
+obj-y				+= mem.o random.o
+obj-$(CONFIG_TTY_PRINTK)	+= ttyprintk.o
+obj-y				+= misc.o
+obj-$(CONFIG_TEST_MISC_MINOR)	+= misc_minor_kunit.o
+obj-$(CONFIG_ATARI_DSP56K)	+= dsp56k.o
+obj-$(CONFIG_VIRTIO_CONSOLE)	+= virtio_console.o
+obj-$(CONFIG_UV_MMTIMER)	+= uv_mmtimer.o
+obj-$(CONFIG_IBM_BSR)		+= bsr.o
+
+obj-$(CONFIG_PRINTER)		+= lp.o
+
+obj-$(CONFIG_APM_EMULATION)	+= apm-emulation.o
+
+obj-$(CONFIG_SONYPI)		+= sonypi.o
+obj-$(CONFIG_HPET)		+= hpet.o
+obj-$(CONFIG_XILINX_HWICAP)	+= xilinx_hwicap/
+obj-$(CONFIG_NVRAM)		+= nvram.o
+obj-$(CONFIG_TOSHIBA)		+= toshiba.o
+obj-$(CONFIG_DS1620)		+= ds1620.o
+obj-$(CONFIG_HW_RANDOM)		+= hw_random/
+obj-$(CONFIG_PPDEV)		+= ppdev.o
+obj-$(CONFIG_NWBUTTON)		+= nwbutton.o
+obj-$(CONFIG_NWFLASH)		+= nwflash.o
+obj-$(CONFIG_SCx200_GPIO)	+= scx200_gpio.o
+obj-$(CONFIG_PC8736x_GPIO)	+= pc8736x_gpio.o
+obj-$(CONFIG_NSC_GPIO)		+= nsc_gpio.o
+obj-$(CONFIG_TELCLOCK)		+= tlclk.o
+
+obj-y				+= agp/
+
+obj-$(CONFIG_HANGCHECK_TIMER)	+= hangcheck-timer.o
+obj-$(CONFIG_TCG_TPM)		+= tpm/
+
+obj-$(CONFIG_PS3_FLASH)		+= ps3flash.o
+
+obj-$(CONFIG_XILLYBUS_CLASS)	+= xillybus/
+obj-$(CONFIG_POWERNV_OP_PANEL)	+= powernv-op-panel.o
+obj-$(CONFIG_ADI)		+= adi.o