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, 166 insertions, 0 deletions

diff --git a/drivers/input/serio/q40kbd.c b/drivers/input/serio/q40kbd.c
new file mode 100644
index 000000000..2f553efbe
--- /dev/null
+++ b/drivers/input/serio/q40kbd.c
@@ -0,0 +1,166 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (c) 2000-2001 Vojtech Pavlik
+ *
+ *  Based on the work of:
+ *	Richard Zidlicky <Richard.Zidlicky@stud.informatik.uni-erlangen.de>
+ */
+
+/*
+ * Q40 PS/2 keyboard controller driver for Linux/m68k
+ */
+
+#include <linux/module.h>
+#include <linux/serio.h>
+#include <linux/interrupt.h>
+#include <linux/err.h>
+#include <linux/bitops.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+#include <asm/io.h>
+#include <linux/uaccess.h>
+#include <asm/q40_master.h>
+#include <asm/irq.h>
+#include <asm/q40ints.h>
+
+#define DRV_NAME	"q40kbd"
+
+MODULE_AUTHOR("Vojtech Pavlik <vojtech@ucw.cz>");
+MODULE_DESCRIPTION("Q40 PS/2 keyboard controller driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:" DRV_NAME);
+
+struct q40kbd {
+	struct serio *port;
+	spinlock_t lock;
+};
+
+static irqreturn_t q40kbd_interrupt(int irq, void *dev_id)
+{
+	struct q40kbd *q40kbd = dev_id;
+
+	guard(spinlock_irqsave)(&q40kbd->lock);
+
+	if (Q40_IRQ_KEYB_MASK & master_inb(INTERRUPT_REG))
+		serio_interrupt(q40kbd->port, master_inb(KEYCODE_REG), 0);
+
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * q40kbd_flush() flushes all data that may be in the keyboard buffers
+ */
+
+static void q40kbd_flush(struct q40kbd *q40kbd)
+{
+	int maxread = 100;
+
+	guard(spinlock_irqsave)(&q40kbd->lock);
+
+	while (maxread-- && (Q40_IRQ_KEYB_MASK & master_inb(INTERRUPT_REG)))
+		master_inb(KEYCODE_REG);
+}
+
+static void q40kbd_stop(void)
+{
+	master_outb(0, KEY_IRQ_ENABLE_REG);
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+}
+
+/*
+ * q40kbd_open() is called when a port is open by the higher layer.
+ * It allocates the interrupt and enables in in the chip.
+ */
+
+static int q40kbd_open(struct serio *port)
+{
+	struct q40kbd *q40kbd = port->port_data;
+
+	q40kbd_flush(q40kbd);
+
+	/* off we go */
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+	master_outb(1, KEY_IRQ_ENABLE_REG);
+
+	return 0;
+}
+
+static void q40kbd_close(struct serio *port)
+{
+	struct q40kbd *q40kbd = port->port_data;
+
+	q40kbd_stop();
+	q40kbd_flush(q40kbd);
+}
+
+static int q40kbd_probe(struct platform_device *pdev)
+{
+	struct q40kbd *q40kbd;
+	struct serio *port;
+	int error;
+
+	q40kbd = kzalloc_obj(*q40kbd);
+	port = kzalloc_obj(*port);
+	if (!q40kbd || !port) {
+		error = -ENOMEM;
+		goto err_free_mem;
+	}
+
+	q40kbd->port = port;
+	spin_lock_init(&q40kbd->lock);
+
+	port->id.type = SERIO_8042;
+	port->open = q40kbd_open;
+	port->close = q40kbd_close;
+	port->port_data = q40kbd;
+	port->dev.parent = &pdev->dev;
+	strscpy(port->name, "Q40 Kbd Port", sizeof(port->name));
+	strscpy(port->phys, "Q40", sizeof(port->phys));
+
+	q40kbd_stop();
+
+	error = request_irq(Q40_IRQ_KEYBOARD, q40kbd_interrupt, 0,
+			    DRV_NAME, q40kbd);
+	if (error) {
+		dev_err(&pdev->dev, "Can't get irq %d.\n", Q40_IRQ_KEYBOARD);
+		goto err_free_mem;
+	}
+
+	serio_register_port(q40kbd->port);
+
+	platform_set_drvdata(pdev, q40kbd);
+	printk(KERN_INFO "serio: Q40 kbd registered\n");
+
+	return 0;
+
+err_free_mem:
+	kfree(port);
+	kfree(q40kbd);
+	return error;
+}
+
+static void q40kbd_remove(struct platform_device *pdev)
+{
+	struct q40kbd *q40kbd = platform_get_drvdata(pdev);
+
+	/*
+	 * q40kbd_close() will be called as part of unregistering
+	 * and will ensure that IRQ is turned off, so it is safe
+	 * to unregister port first and free IRQ later.
+	 */
+	serio_unregister_port(q40kbd->port);
+	free_irq(Q40_IRQ_KEYBOARD, q40kbd);
+	kfree(q40kbd);
+}
+
+static struct platform_driver q40kbd_driver = {
+	.driver		= {
+		.name	= "q40kbd",
+	},
+	.remove		= q40kbd_remove,
+};
+
+module_platform_driver_probe(q40kbd_driver, q40kbd_probe);