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

3 files changed, 152 insertions, 0 deletions

diff --git a/lib/tests/module/.gitignore b/lib/tests/module/.gitignore
new file mode 100644
index 000000000..8be7891b2
--- /dev/null
+++ b/lib/tests/module/.gitignore
@@ -0,0 +1,4 @@
+test_kallsyms_a.c
+test_kallsyms_b.c
+test_kallsyms_c.c
+test_kallsyms_d.c
diff --git a/lib/tests/module/Makefile b/lib/tests/module/Makefile
new file mode 100644
index 000000000..2f3e1a772
--- /dev/null
+++ b/lib/tests/module/Makefile
@@ -0,0 +1,14 @@
+obj-$(CONFIG_TEST_KALLSYMS_A) += test_kallsyms_a.o
+obj-$(CONFIG_TEST_KALLSYMS_B) += test_kallsyms_b.o
+obj-$(CONFIG_TEST_KALLSYMS_C) += test_kallsyms_c.o
+obj-$(CONFIG_TEST_KALLSYMS_D) += test_kallsyms_d.o
+
+quiet_cmd_gen_test_kallsyms = GEN     $@
+	cmd_gen_test_kallsyms = $< $@ \
+	$(CONFIG_TEST_KALLSYMS_NUMSYMS) \
+	$(CONFIG_TEST_KALLSYMS_SCALE_FACTOR)
+
+$(obj)/%.c: $(src)/gen_test_kallsyms.sh FORCE
+	$(call if_changed,gen_test_kallsyms)
+
+targets += $(foreach x, a b c d, test_kallsyms_$(x).c)
diff --git a/lib/tests/module/gen_test_kallsyms.sh b/lib/tests/module/gen_test_kallsyms.sh
new file mode 100755
index 000000000..31fe4ed63
--- /dev/null
+++ b/lib/tests/module/gen_test_kallsyms.sh
@@ -0,0 +1,134 @@
+#!/usr/bin/env bash
+
+TARGET=$(basename $1)
+DIR=lib/tests/module
+TARGET="$DIR/$TARGET"
+NUM_SYMS=$2
+SCALE_FACTOR=$3
+TEST_TYPE=$(echo $TARGET | sed -e 's|lib/tests/module/test_kallsyms_||g')
+TEST_TYPE=$(echo $TEST_TYPE | sed -e 's|.c||g')
+FIRST_B_LOOKUP=1
+
+if [[ $NUM_SYMS -gt 2 ]]; then
+	FIRST_B_LOOKUP=$((NUM_SYMS/2))
+fi
+
+gen_template_module_header()
+{
+	cat <<____END_MODULE
+// SPDX-License-Identifier: GPL-2.0-or-later OR copyleft-next-0.3.1
+/*
+ * Copyright (C) 2023 Luis Chamberlain <mcgrof@kernel.org>
+ *
+ * Automatically generated code for testing, do not edit manually.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+
+____END_MODULE
+}
+
+gen_num_syms()
+{
+	PREFIX=$1
+	NUM=$2
+	for i in $(seq 1 $NUM); do
+		printf "int auto_test_%s_%010d = 0;\n" $PREFIX $i
+		printf "EXPORT_SYMBOL_GPL(auto_test_%s_%010d);\n" $PREFIX $i
+	done
+	echo
+}
+
+gen_template_module_data_a()
+{
+	gen_num_syms a $1
+	cat <<____END_MODULE
+static int auto_runtime_test(void)
+{
+	return 0;
+}
+
+____END_MODULE
+}
+
+gen_template_module_data_b()
+{
+	printf "\nextern int auto_test_a_%010d;\n\n" $FIRST_B_LOOKUP
+	echo "static int auto_runtime_test(void)"
+	echo "{"
+	printf "\nreturn auto_test_a_%010d;\n" $FIRST_B_LOOKUP
+	echo "}"
+}
+
+gen_template_module_data_c()
+{
+	gen_num_syms c $1
+	cat <<____END_MODULE
+static int auto_runtime_test(void)
+{
+	return 0;
+}
+
+____END_MODULE
+}
+
+gen_template_module_data_d()
+{
+	gen_num_syms d $1
+	cat <<____END_MODULE
+static int auto_runtime_test(void)
+{
+	return 0;
+}
+
+____END_MODULE
+}
+
+gen_template_module_exit()
+{
+	cat <<____END_MODULE
+static int __init auto_test_module_init(void)
+{
+	return auto_runtime_test();
+}
+module_init(auto_test_module_init);
+
+static void __exit auto_test_module_exit(void)
+{
+}
+module_exit(auto_test_module_exit);
+
+MODULE_AUTHOR("Luis Chamberlain <mcgrof@kernel.org>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Test module for kallsyms");
+____END_MODULE
+}
+
+case $TEST_TYPE in
+	a)
+		gen_template_module_header > $TARGET
+		gen_template_module_data_a $NUM_SYMS >> $TARGET
+		gen_template_module_exit >> $TARGET
+		;;
+	b)
+		gen_template_module_header > $TARGET
+		gen_template_module_data_b >> $TARGET
+		gen_template_module_exit >> $TARGET
+		;;
+	c)
+		gen_template_module_header > $TARGET
+		gen_template_module_data_c $((NUM_SYMS * SCALE_FACTOR)) >> $TARGET
+		gen_template_module_exit >> $TARGET
+		;;
+	d)
+		gen_template_module_header > $TARGET
+		gen_template_module_data_d $((NUM_SYMS * SCALE_FACTOR * 2)) >> $TARGET
+		gen_template_module_exit >> $TARGET
+		;;
+	*)
+		;;
+esac