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, 122 insertions, 0 deletions

diff --git a/drivers/usb/serial/empeg.c b/drivers/usb/serial/empeg.c
new file mode 100644
index 000000000..4937575ca
--- /dev/null
+++ b/drivers/usb/serial/empeg.c
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * USB Empeg empeg-car player driver
+ *
+ *	Copyright (C) 2000, 2001
+ *	    Gary Brubaker (xavyer@ix.netcom.com)
+ *
+ *	Copyright (C) 1999 - 2001
+ *	    Greg Kroah-Hartman (greg@kroah.com)
+ *
+ * See Documentation/usb/usb-serial.rst for more information on using this
+ * driver
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/slab.h>
+#include <linux/tty.h>
+#include <linux/module.h>
+#include <linux/spinlock.h>
+#include <linux/usb.h>
+#include <linux/usb/serial.h>
+
+#define DRIVER_AUTHOR "Greg Kroah-Hartman <greg@kroah.com>, Gary Brubaker <xavyer@ix.netcom.com>"
+#define DRIVER_DESC "USB Empeg Mark I/II Driver"
+
+#define EMPEG_VENDOR_ID			0x084f
+#define EMPEG_PRODUCT_ID		0x0001
+
+/* function prototypes for an empeg-car player */
+static int  empeg_startup(struct usb_serial *serial);
+static void empeg_init_termios(struct tty_struct *tty);
+
+static const struct usb_device_id id_table[] = {
+	{ USB_DEVICE(EMPEG_VENDOR_ID, EMPEG_PRODUCT_ID) },
+	{ }					/* Terminating entry */
+};
+
+MODULE_DEVICE_TABLE(usb, id_table);
+
+static struct usb_serial_driver empeg_device = {
+	.driver = {
+		.name =		"empeg",
+	},
+	.id_table =		id_table,
+	.num_ports =		1,
+	.bulk_out_size =	256,
+	.throttle =		usb_serial_generic_throttle,
+	.unthrottle =		usb_serial_generic_unthrottle,
+	.attach =		empeg_startup,
+	.init_termios =		empeg_init_termios,
+};
+
+static struct usb_serial_driver * const serial_drivers[] = {
+	&empeg_device, NULL
+};
+
+static int empeg_startup(struct usb_serial *serial)
+{
+	int r;
+
+	if (serial->dev->actconfig->desc.bConfigurationValue != 1) {
+		dev_err(&serial->dev->dev, "active config #%d != 1 ??\n",
+			serial->dev->actconfig->desc.bConfigurationValue);
+		return -ENODEV;
+	}
+
+	r = usb_reset_configuration(serial->dev);
+
+	/* continue on with initialization */
+	return r;
+}
+
+static void empeg_init_termios(struct tty_struct *tty)
+{
+	struct ktermios *termios = &tty->termios;
+
+	/*
+	 * The empeg-car player wants these particular tty settings.
+	 * You could, for example, change the baud rate, however the
+	 * player only supports 115200 (currently), so there is really
+	 * no point in support for changes to the tty settings.
+	 * (at least for now)
+	 *
+	 * The default requirements for this device are:
+	 */
+	termios->c_iflag
+		&= ~(IGNBRK	/* disable ignore break */
+		| BRKINT	/* disable break causes interrupt */
+		| PARMRK	/* disable mark parity errors */
+		| ISTRIP	/* disable clear high bit of input characters */
+		| INLCR		/* disable translate NL to CR */
+		| IGNCR		/* disable ignore CR */
+		| ICRNL		/* disable translate CR to NL */
+		| IXON);	/* disable enable XON/XOFF flow control */
+
+	termios->c_oflag
+		&= ~OPOST;	/* disable postprocess output characters */
+
+	termios->c_lflag
+		&= ~(ECHO	/* disable echo input characters */
+		| ECHONL	/* disable echo new line */
+		| ICANON	/* disable erase, kill, werase, and rprnt special characters */
+		| ISIG		/* disable interrupt, quit, and suspend special characters */
+		| IEXTEN);	/* disable non-POSIX special characters */
+
+	termios->c_cflag
+		&= ~(CSIZE	/* no size */
+		| PARENB	/* disable parity bit */
+		| CBAUD);	/* clear current baud rate */
+
+	termios->c_cflag
+		|= CS8;		/* character size 8 bits */
+
+	tty_encode_baud_rate(tty, 115200, 115200);
+}
+
+module_usb_serial_driver(serial_drivers, id_table);
+
+MODULE_AUTHOR(DRIVER_AUTHOR);
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_LICENSE("GPL v2");