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, 142 insertions, 0 deletions

diff --git a/drivers/input/misc/m68kspkr.c b/drivers/input/misc/m68kspkr.c
new file mode 100644
index 000000000..0542334df
--- /dev/null
+++ b/drivers/input/misc/m68kspkr.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  m68k beeper driver for Linux
+ *
+ *  Copyright (c) 2002 Richard Zidlicky
+ *  Copyright (c) 2002 Vojtech Pavlik
+ *  Copyright (c) 1992 Orest Zborowski
+ */
+
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/input.h>
+#include <linux/platform_device.h>
+#include <asm/machdep.h>
+#include <asm/io.h>
+
+MODULE_AUTHOR("Richard Zidlicky <rz@linux-m68k.org>");
+MODULE_DESCRIPTION("m68k beeper driver");
+MODULE_LICENSE("GPL");
+
+static struct platform_device *m68kspkr_platform_device;
+
+static int m68kspkr_event(struct input_dev *dev, unsigned int type, unsigned int code, int value)
+{
+	unsigned int count = 0;
+
+	if (type != EV_SND)
+		return -1;
+
+	switch (code) {
+		case SND_BELL: if (value) value = 1000;
+		case SND_TONE: break;
+		default: return -1;
+	}
+
+	if (value > 20 && value < 32767)
+		count = 1193182 / value;
+
+	mach_beep(count, -1);
+
+	return 0;
+}
+
+static int m68kspkr_probe(struct platform_device *dev)
+{
+	struct input_dev *input_dev;
+	int err;
+
+	input_dev = input_allocate_device();
+	if (!input_dev)
+		return -ENOMEM;
+
+	input_dev->name = "m68k beeper";
+	input_dev->phys = "m68k/generic";
+	input_dev->id.bustype = BUS_HOST;
+	input_dev->id.vendor  = 0x001f;
+	input_dev->id.product = 0x0001;
+	input_dev->id.version = 0x0100;
+	input_dev->dev.parent = &dev->dev;
+
+	input_dev->evbit[0] = BIT_MASK(EV_SND);
+	input_dev->sndbit[0] = BIT_MASK(SND_BELL) | BIT_MASK(SND_TONE);
+	input_dev->event = m68kspkr_event;
+
+	err = input_register_device(input_dev);
+	if (err) {
+		input_free_device(input_dev);
+		return err;
+	}
+
+	platform_set_drvdata(dev, input_dev);
+
+	return 0;
+}
+
+static void m68kspkr_remove(struct platform_device *dev)
+{
+	struct input_dev *input_dev = platform_get_drvdata(dev);
+
+	input_unregister_device(input_dev);
+	/* turn off the speaker */
+	m68kspkr_event(NULL, EV_SND, SND_BELL, 0);
+}
+
+static void m68kspkr_shutdown(struct platform_device *dev)
+{
+	/* turn off the speaker */
+	m68kspkr_event(NULL, EV_SND, SND_BELL, 0);
+}
+
+static struct platform_driver m68kspkr_platform_driver = {
+	.driver		= {
+		.name	= "m68kspkr",
+	},
+	.probe		= m68kspkr_probe,
+	.remove		= m68kspkr_remove,
+	.shutdown	= m68kspkr_shutdown,
+};
+
+static int __init m68kspkr_init(void)
+{
+	int err;
+
+	if (!mach_beep) {
+		printk(KERN_INFO "m68kspkr: no lowlevel beep support\n");
+		return -ENODEV;
+        }
+
+	err = platform_driver_register(&m68kspkr_platform_driver);
+	if (err)
+		return err;
+
+	m68kspkr_platform_device = platform_device_alloc("m68kspkr", -1);
+	if (!m68kspkr_platform_device) {
+		err = -ENOMEM;
+		goto err_unregister_driver;
+	}
+
+	err = platform_device_add(m68kspkr_platform_device);
+	if (err)
+		goto err_free_device;
+
+	return 0;
+
+ err_free_device:
+	platform_device_put(m68kspkr_platform_device);
+ err_unregister_driver:
+	platform_driver_unregister(&m68kspkr_platform_driver);
+
+	return err;
+}
+
+static void __exit m68kspkr_exit(void)
+{
+	platform_device_unregister(m68kspkr_platform_device);
+	platform_driver_unregister(&m68kspkr_platform_driver);
+}
+
+module_init(m68kspkr_init);
+module_exit(m68kspkr_exit);