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, 222 insertions, 0 deletions

diff --git a/drivers/input/serio/parkbd.c b/drivers/input/serio/parkbd.c
new file mode 100644
index 000000000..09a24413d
--- /dev/null
+++ b/drivers/input/serio/parkbd.c
@@ -0,0 +1,222 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  Parallel port to Keyboard port adapter driver for Linux
+ *
+ *  Copyright (c) 1999-2004 Vojtech Pavlik
+ */
+
+
+/*
+ * To connect an AT or XT keyboard to the parallel port, a fairly simple adapter
+ * can be made:
+ * 
+ *  Parallel port            Keyboard port
+ *
+ *     +5V --------------------- +5V (4)
+ *  
+ *                 ______
+ *     +5V -------|______|--.
+ *                          |
+ *     ACK (10) ------------|
+ *                          |--- KBD CLOCK (5)
+ *     STROBE (1) ---|<|----'
+ *     
+ *                 ______
+ *     +5V -------|______|--.
+ *                          |
+ *     BUSY (11) -----------|
+ *                          |--- KBD DATA (1)
+ *     AUTOFD (14) --|<|----'
+ *
+ *     GND (18-25) ------------- GND (3)
+ *     
+ * The diodes can be fairly any type, and the resistors should be somewhere
+ * around 5 kOhm, but the adapter will likely work without the resistors,
+ * too.
+ *
+ * The +5V source can be taken either from USB, from mouse or keyboard ports,
+ * or from a joystick port. Unfortunately, the parallel port of a PC doesn't
+ * have a +5V pin, and feeding the keyboard from signal pins is out of question
+ * with 300 mA power reqirement of a typical AT keyboard.
+ */
+
+#include <linux/module.h>
+#include <linux/parport.h>
+#include <linux/slab.h>
+#include <linux/init.h>
+#include <linux/serio.h>
+
+MODULE_AUTHOR("Vojtech Pavlik <vojtech@ucw.cz>");
+MODULE_DESCRIPTION("Parallel port to Keyboard port adapter driver");
+MODULE_LICENSE("GPL");
+
+static unsigned int parkbd_pp_no;
+module_param_named(port, parkbd_pp_no, int, 0);
+MODULE_PARM_DESC(port, "Parallel port the adapter is connected to (default is 0)");
+
+static unsigned int parkbd_mode = SERIO_8042;
+module_param_named(mode, parkbd_mode, uint, 0);
+MODULE_PARM_DESC(mode, "Mode of operation: XT = 0/AT = 1 (default)");
+
+#define PARKBD_CLOCK	0x01	/* Strobe & Ack */
+#define PARKBD_DATA	0x02	/* AutoFd & Busy */
+
+static int parkbd_buffer;
+static int parkbd_counter;
+static unsigned long parkbd_last;
+static int parkbd_writing;
+static unsigned long parkbd_start;
+
+static struct pardevice *parkbd_dev;
+static struct serio *parkbd_port;
+
+static int parkbd_readlines(void)
+{
+	return (parport_read_status(parkbd_dev->port) >> 6) ^ 2;
+}
+
+static void parkbd_writelines(int data)
+{
+	parport_write_control(parkbd_dev->port, (~data & 3) | 0x10);
+}
+
+static int parkbd_write(struct serio *port, unsigned char c)
+{
+	unsigned char p;
+
+	if (!parkbd_mode) return -1;
+
+        p = c ^ (c >> 4);
+	p = p ^ (p >> 2);
+	p = p ^ (p >> 1);
+
+	parkbd_counter = 0;
+	parkbd_writing = 1;
+	parkbd_buffer = c | (((int) (~p & 1)) << 8) | 0x600;
+
+	parkbd_writelines(2);
+
+	return 0;
+}
+
+static void parkbd_interrupt(void *dev_id)
+{
+
+	if (parkbd_writing) {
+
+		if (parkbd_counter && ((parkbd_counter == 11) || time_after(jiffies, parkbd_last + HZ/100))) {
+			parkbd_counter = 0;
+			parkbd_buffer = 0;
+			parkbd_writing = 0;
+			parkbd_writelines(3);
+			return;
+		}
+
+		parkbd_writelines(((parkbd_buffer >> parkbd_counter++) & 1) | 2);
+
+		if (parkbd_counter == 11) {
+			parkbd_counter = 0;
+			parkbd_buffer = 0;
+			parkbd_writing = 0;
+			parkbd_writelines(3);
+		}
+
+	} else {
+
+		if ((parkbd_counter == parkbd_mode + 10) || time_after(jiffies, parkbd_last + HZ/100)) {
+			parkbd_counter = 0;
+			parkbd_buffer = 0;
+		}
+
+		parkbd_buffer |= (parkbd_readlines() >> 1) << parkbd_counter++;
+
+		if (parkbd_counter == parkbd_mode + 10)
+			serio_interrupt(parkbd_port, (parkbd_buffer >> (2 - parkbd_mode)) & 0xff, 0);
+	}
+
+	parkbd_last = jiffies;
+}
+
+static int parkbd_getport(struct parport *pp)
+{
+	struct pardev_cb parkbd_parport_cb;
+
+	memset(&parkbd_parport_cb, 0, sizeof(parkbd_parport_cb));
+	parkbd_parport_cb.irq_func = parkbd_interrupt;
+	parkbd_parport_cb.flags = PARPORT_FLAG_EXCL;
+
+	parkbd_dev = parport_register_dev_model(pp, "parkbd",
+						&parkbd_parport_cb, 0);
+
+	if (!parkbd_dev)
+		return -ENODEV;
+
+	if (parport_claim(parkbd_dev)) {
+		parport_unregister_device(parkbd_dev);
+		return -EBUSY;
+	}
+
+	parkbd_start = jiffies;
+
+	return 0;
+}
+
+static struct serio *parkbd_allocate_serio(void)
+{
+	struct serio *serio;
+
+	serio = kzalloc_obj(*serio);
+	if (serio) {
+		serio->id.type = parkbd_mode;
+		serio->write = parkbd_write;
+		strscpy(serio->name, "PARKBD AT/XT keyboard adapter", sizeof(serio->name));
+		snprintf(serio->phys, sizeof(serio->phys), "%s/serio0", parkbd_dev->port->name);
+	}
+
+	return serio;
+}
+
+static void parkbd_attach(struct parport *pp)
+{
+	if (pp->number != parkbd_pp_no) {
+		pr_debug("Not using parport%d.\n", pp->number);
+		return;
+	}
+
+	if (parkbd_getport(pp))
+		return;
+
+	parkbd_port = parkbd_allocate_serio();
+	if (!parkbd_port) {
+		parport_release(parkbd_dev);
+		parport_unregister_device(parkbd_dev);
+		return;
+	}
+
+	parkbd_writelines(3);
+
+	serio_register_port(parkbd_port);
+
+	printk(KERN_INFO "serio: PARKBD %s adapter on %s\n",
+                        parkbd_mode ? "AT" : "XT", parkbd_dev->port->name);
+
+	return;
+}
+
+static void parkbd_detach(struct parport *port)
+{
+	if (!parkbd_port || port->number != parkbd_pp_no)
+		return;
+
+	parport_release(parkbd_dev);
+	serio_unregister_port(parkbd_port);
+	parport_unregister_device(parkbd_dev);
+	parkbd_port = NULL;
+}
+
+static struct parport_driver parkbd_parport_driver = {
+	.name = "parkbd",
+	.match_port = parkbd_attach,
+	.detach = parkbd_detach,
+};
+module_parport_driver(parkbd_parport_driver);