sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 304 insertions, 0 deletions

diff --git a/drivers/usb/serial/console.c b/drivers/usb/serial/console.c
new file mode 100644
index 000000000..29f091908
--- /dev/null
+++ b/drivers/usb/serial/console.c
@@ -0,0 +1,304 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * USB Serial Console driver
+ *
+ * Copyright (C) 2001 - 2002 Greg Kroah-Hartman (greg@kroah.com)
+ *
+ * Thanks to Randy Dunlap for the original version of this code.
+ *
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/tty.h>
+#include <linux/console.h>
+#include <linux/serial.h>
+#include <linux/usb.h>
+#include <linux/usb/serial.h>
+
+struct usbcons_info {
+	int			magic;
+	int			break_flag;
+	struct usb_serial_port	*port;
+};
+
+static struct usbcons_info usbcons_info;
+static struct console usbcons;
+
+/*
+ * ------------------------------------------------------------
+ * USB Serial console driver
+ *
+ * Much of the code here is copied from drivers/char/serial.c
+ * and implements a phony serial console in the same way that
+ * serial.c does so that in case some software queries it,
+ * it will get the same results.
+ *
+ * Things that are different from the way the serial port code
+ * does things, is that we call the lower level usb-serial
+ * driver code to initialize the device, and we set the initial
+ * console speeds based on the command line arguments.
+ * ------------------------------------------------------------
+ */
+
+static const struct tty_operations usb_console_fake_tty_ops = {
+};
+
+/*
+ * The parsing of the command line works exactly like the
+ * serial.c code, except that the specifier is "ttyUSB" instead
+ * of "ttyS".
+ */
+static int usb_console_setup(struct console *co, char *options)
+{
+	struct usbcons_info *info = &usbcons_info;
+	int baud = 9600;
+	int bits = 8;
+	int parity = 'n';
+	int doflow = 0;
+	int cflag = CREAD | HUPCL | CLOCAL;
+	char *s;
+	struct usb_serial *serial;
+	struct usb_serial_port *port;
+	int retval;
+	struct tty_struct *tty = NULL;
+	struct ktermios dummy;
+
+	if (options) {
+		baud = simple_strtoul(options, NULL, 10);
+		s = options;
+		while (*s >= '0' && *s <= '9')
+			s++;
+		if (*s)
+			parity = *s++;
+		if (*s)
+			bits   = *s++ - '0';
+		if (*s)
+			doflow = (*s++ == 'r');
+	}
+
+	/* Sane default */
+	if (baud == 0)
+		baud = 9600;
+
+	switch (bits) {
+	case 7:
+		cflag |= CS7;
+		break;
+	default:
+	case 8:
+		cflag |= CS8;
+		break;
+	}
+	switch (parity) {
+	case 'o': case 'O':
+		cflag |= PARODD;
+		break;
+	case 'e': case 'E':
+		cflag |= PARENB;
+		break;
+	}
+
+	if (doflow)
+		cflag |= CRTSCTS;
+
+	/*
+	 * no need to check the index here: if the index is wrong, console
+	 * code won't call us
+	 */
+	port = usb_serial_port_get_by_minor(co->index);
+	if (port == NULL) {
+		/* no device is connected yet, sorry :( */
+		pr_err("No USB device connected to ttyUSB%i\n", co->index);
+		return -ENODEV;
+	}
+	serial = port->serial;
+
+	retval = usb_autopm_get_interface(serial->interface);
+	if (retval)
+		goto error_get_interface;
+
+	tty_port_tty_set(&port->port, NULL);
+
+	info->port = port;
+
+	++port->port.count;
+	if (!tty_port_initialized(&port->port)) {
+		if (serial->type->set_termios) {
+			/*
+			 * allocate a fake tty so the driver can initialize
+			 * the termios structure, then later call set_termios to
+			 * configure according to command line arguments
+			 */
+			tty = kzalloc_obj(*tty);
+			if (!tty) {
+				retval = -ENOMEM;
+				goto reset_open_count;
+			}
+			kref_init(&tty->kref);
+			tty->driver = usb_serial_tty_driver;
+			tty->index = co->index;
+			init_ldsem(&tty->ldisc_sem);
+			spin_lock_init(&tty->files_lock);
+			INIT_LIST_HEAD(&tty->tty_files);
+			kref_get(&tty->driver->kref);
+			__module_get(tty->driver->owner);
+			tty->ops = &usb_console_fake_tty_ops;
+			tty_init_termios(tty);
+			tty_port_tty_set(&port->port, tty);
+		}
+
+		/* only call the device specific open if this
+		 * is the first time the port is opened */
+		retval = serial->type->open(NULL, port);
+		if (retval) {
+			dev_err(&port->dev, "could not open USB console port\n");
+			goto fail;
+		}
+
+		if (serial->type->set_termios) {
+			tty->termios.c_cflag = cflag;
+			tty_termios_encode_baud_rate(&tty->termios, baud, baud);
+			memset(&dummy, 0, sizeof(struct ktermios));
+			serial->type->set_termios(tty, port, &dummy);
+
+			tty_port_tty_set(&port->port, NULL);
+			tty_save_termios(tty);
+			tty_kref_put(tty);
+		}
+		tty_port_set_initialized(&port->port, true);
+	}
+	/* Now that any required fake tty operations are completed restore
+	 * the tty port count */
+	--port->port.count;
+	/* The console is special in terms of closing the device so
+	 * indicate this port is now acting as a system console. */
+	port->port.console = 1;
+
+	mutex_unlock(&serial->disc_mutex);
+	return retval;
+
+ fail:
+	tty_port_tty_set(&port->port, NULL);
+	tty_kref_put(tty);
+ reset_open_count:
+	port->port.count = 0;
+	info->port = NULL;
+	usb_autopm_put_interface(serial->interface);
+ error_get_interface:
+	mutex_unlock(&serial->disc_mutex);
+	usb_serial_put(serial);
+	return retval;
+}
+
+static void usb_console_write(struct console *co,
+					const char *buf, unsigned count)
+{
+	static struct usbcons_info *info = &usbcons_info;
+	struct usb_serial_port *port = info->port;
+	struct usb_serial *serial;
+	int retval = -ENODEV;
+
+	if (!port || port->serial->dev->state == USB_STATE_NOTATTACHED)
+		return;
+	serial = port->serial;
+
+	if (count == 0)
+		return;
+
+	dev_dbg(&port->dev, "%s - %d byte(s)\n", __func__, count);
+
+	if (!port->port.console) {
+		dev_dbg(&port->dev, "%s - port not opened\n", __func__);
+		return;
+	}
+
+	while (count) {
+		unsigned int i;
+		unsigned int lf;
+		/* search for LF so we can insert CR if necessary */
+		for (i = 0, lf = 0 ; i < count ; i++) {
+			if (*(buf + i) == 10) {
+				lf = 1;
+				i++;
+				break;
+			}
+		}
+		/* pass on to the driver specific version of this function if
+		   it is available */
+		retval = serial->type->write(NULL, port, buf, i);
+		dev_dbg(&port->dev, "%s - write: %d\n", __func__, retval);
+		if (lf) {
+			/* append CR after LF */
+			unsigned char cr = 13;
+			retval = serial->type->write(NULL, port, &cr, 1);
+			dev_dbg(&port->dev, "%s - write cr: %d\n",
+							__func__, retval);
+		}
+		buf += i;
+		count -= i;
+	}
+}
+
+static struct tty_driver *usb_console_device(struct console *co, int *index)
+{
+	struct tty_driver **p = (struct tty_driver **)co->data;
+
+	if (!*p)
+		return NULL;
+
+	*index = co->index;
+	return *p;
+}
+
+static struct console usbcons = {
+	.name =		"ttyUSB",
+	.write =	usb_console_write,
+	.device =	usb_console_device,
+	.setup =	usb_console_setup,
+	.flags =	CON_PRINTBUFFER,
+	.index =	-1,
+	.data = 	&usb_serial_tty_driver,
+};
+
+void usb_serial_console_disconnect(struct usb_serial *serial)
+{
+	if (serial->port[0] && serial->port[0] == usbcons_info.port) {
+		usb_serial_console_exit();
+		usb_serial_put(serial);
+	}
+}
+
+void usb_serial_console_init(int minor)
+{
+	if (minor == 0) {
+		/*
+		 * Call register_console() if this is the first device plugged
+		 * in.  If we call it earlier, then the callback to
+		 * console_setup() will fail, as there is not a device seen by
+		 * the USB subsystem yet.
+		 */
+		/*
+		 * Register console.
+		 * NOTES:
+		 * console_setup() is called (back) immediately (from
+		 * register_console). console_write() is called immediately
+		 * from register_console iff CON_PRINTBUFFER is set in flags.
+		 */
+		pr_debug("registering the USB serial console.\n");
+		register_console(&usbcons);
+	}
+}
+
+void usb_serial_console_exit(void)
+{
+	if (usbcons_info.port) {
+		unregister_console(&usbcons);
+		usbcons_info.port->port.console = 0;
+		usbcons_info.port = NULL;
+	}
+}
+