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, 66 insertions, 0 deletions

diff --git a/arch/alpha/Makefile b/arch/alpha/Makefile
new file mode 100644
index 000000000..7074602dc
--- /dev/null
+++ b/arch/alpha/Makefile
@@ -0,0 +1,66 @@
+#
+# alpha/Makefile
+#
+# This file is subject to the terms and conditions of the GNU General Public
+# License.  See the file "COPYING" in the main directory of this archive
+# for more details.
+#
+# Copyright (C) 1994 by Linus Torvalds
+#
+
+NM := $(NM) -B
+
+LDFLAGS_vmlinux	:= -static -N #-relax
+CHECKFLAGS	+= -D__alpha__
+cflags-y	:= -pipe -mno-fp-regs -ffixed-8
+cflags-y	+= $(call cc-option, -fno-jump-tables)
+
+cpuflags-$(CONFIG_ALPHA_EV56)		:= -mcpu=ev56
+cpuflags-$(CONFIG_ALPHA_POLARIS)	:= -mcpu=pca56
+cpuflags-$(CONFIG_ALPHA_SX164)		:= -mcpu=pca56
+cpuflags-$(CONFIG_ALPHA_EV6)		:= -mcpu=ev6
+cpuflags-$(CONFIG_ALPHA_EV67)		:= -mcpu=ev67
+# If GENERIC, make sure to turn off any instruction set extensions that
+# the host compiler might have on by default.
+cpuflags-$(CONFIG_ALPHA_GENERIC)	:= -mcpu=ev56 -mtune=ev6
+
+cflags-y				+= $(cpuflags-y)
+
+
+KBUILD_CFLAGS += $(cflags-y)
+
+# For TSUNAMI, we must have the assembler not emulate our instructions.
+# The same is true for IRONGATE, POLARIS, PYXIS.
+# BWX is most important, but we don't really want any emulation ever.
+# Only gas emulates instructions the target does not implement, and only gas
+# understands -mev6.  LLVM's integrated assembler never emulates.
+ifdef CONFIG_AS_IS_GNU
+KBUILD_CFLAGS += -Wa,-mev6
+endif
+
+libs-y				+= arch/alpha/lib/
+
+# export what is needed by arch/alpha/boot/Makefile
+LIBS_Y := $(patsubst %/, %/lib.a, $(libs-y))
+export LIBS_Y
+
+boot := arch/alpha/boot
+
+#Default target when executing make with no arguments
+all boot: $(boot)/vmlinux.gz
+
+$(boot)/vmlinux.gz: vmlinux
+	$(Q)$(MAKE) $(build)=$(boot) $@
+
+bootimage bootpfile bootpzfile: vmlinux
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+archheaders:
+	$(Q)$(MAKE) $(build)=arch/alpha/kernel/syscalls all
+
+define archhelp
+  echo '* boot		- Compressed kernel image (arch/alpha/boot/vmlinux.gz)'
+  echo '  bootimage	- SRM bootable image (arch/alpha/boot/bootimage)'
+  echo '  bootpfile	- BOOTP bootable image (arch/alpha/boot/bootpfile)'
+  echo '  bootpzfile	- compressed kernel BOOTP image (arch/alpha/boot/bootpzfile)'
+endef