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, 319 insertions, 0 deletions

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
new file mode 100644
index 000000000..726516fb0
--- /dev/null
+++ b/drivers/char/misc.c
@@ -0,0 +1,319 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/drivers/char/misc.c
+ *
+ * Generic misc open routine by Johan Myreen
+ *
+ * Based on code from Linus
+ *
+ * Teemu Rantanen's Microsoft Busmouse support and Derrick Cole's
+ *   changes incorporated into 0.97pl4
+ *   by Peter Cervasio (pete%q106fm.uucp@wupost.wustl.edu) (08SEP92)
+ *   See busmouse.c for particulars.
+ *
+ * Made things a lot mode modular - easy to compile in just one or two
+ * of the misc drivers, as they are now completely independent. Linus.
+ *
+ * Support for loadable modules. 8-Sep-95 Philip Blundell <pjb27@cam.ac.uk>
+ *
+ * Fixed a failing symbol register to free the device registration
+ *		Alan Cox <alan@lxorguk.ukuu.org.uk> 21-Jan-96
+ *
+ * Dynamic minors and /proc/mice by Alessandro Rubini. 26-Mar-96
+ *
+ * Renamed to misc and miscdevice to be more accurate. Alan Cox 26-Mar-96
+ *
+ * Handling of mouse minor numbers for kerneld:
+ *  Idea by Jacques Gelinas <jack@solucorp.qc.ca>,
+ *  adapted by Bjorn Ekwall <bj0rn@blox.se>
+ *  corrected by Alan Cox <alan@lxorguk.ukuu.org.uk>
+ *
+ * Changes for kmod (from kerneld):
+ *	Cyrus Durgin <cider@speakeasy.org>
+ *
+ * Added devfs support. Richard Gooch <rgooch@atnf.csiro.au>  10-Jan-1998
+ */
+
+#include <linux/module.h>
+
+#include <linux/fs.h>
+#include <linux/errno.h>
+#include <linux/miscdevice.h>
+#include <linux/kernel.h>
+#include <linux/major.h>
+#include <linux/mutex.h>
+#include <linux/proc_fs.h>
+#include <linux/seq_file.h>
+#include <linux/stat.h>
+#include <linux/init.h>
+#include <linux/device.h>
+#include <linux/tty.h>
+#include <linux/kmod.h>
+#include <linux/gfp.h>
+
+/*
+ * Head entry for the doubly linked miscdevice list
+ */
+static LIST_HEAD(misc_list);
+static DEFINE_MUTEX(misc_mtx);
+
+/*
+ * Assigned numbers.
+ */
+static DEFINE_IDA(misc_minors_ida);
+
+static int misc_minor_alloc(int minor)
+{
+	int ret = 0;
+
+	if (minor == MISC_DYNAMIC_MINOR) {
+		/* allocate free id */
+		ret = ida_alloc_range(&misc_minors_ida, MISC_DYNAMIC_MINOR + 1,
+				      MINORMASK, GFP_KERNEL);
+	} else {
+		ret = ida_alloc_range(&misc_minors_ida, minor, minor, GFP_KERNEL);
+	}
+	return ret;
+}
+
+static void misc_minor_free(int minor)
+{
+	ida_free(&misc_minors_ida, minor);
+}
+
+#ifdef CONFIG_PROC_FS
+static void *misc_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	mutex_lock(&misc_mtx);
+	return seq_list_start(&misc_list, *pos);
+}
+
+static void *misc_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	return seq_list_next(v, &misc_list, pos);
+}
+
+static void misc_seq_stop(struct seq_file *seq, void *v)
+{
+	mutex_unlock(&misc_mtx);
+}
+
+static int misc_seq_show(struct seq_file *seq, void *v)
+{
+	const struct miscdevice *p = list_entry(v, struct miscdevice, list);
+
+	seq_printf(seq, "%3i %s\n", p->minor, p->name ? p->name : "");
+	return 0;
+}
+
+
+static const struct seq_operations misc_seq_ops = {
+	.start = misc_seq_start,
+	.next  = misc_seq_next,
+	.stop  = misc_seq_stop,
+	.show  = misc_seq_show,
+};
+#endif
+
+static int misc_open(struct inode *inode, struct file *file)
+{
+	int minor = iminor(inode);
+	struct miscdevice *c = NULL, *iter;
+	int err = -ENODEV;
+	const struct file_operations *new_fops = NULL;
+
+	mutex_lock(&misc_mtx);
+
+	list_for_each_entry(iter, &misc_list, list) {
+		if (iter->minor != minor)
+			continue;
+		c = iter;
+		new_fops = fops_get(iter->fops);
+		break;
+	}
+
+	/* Only request module for fixed minor code */
+	if (!new_fops && minor < MISC_DYNAMIC_MINOR) {
+		mutex_unlock(&misc_mtx);
+		request_module("char-major-%d-%d", MISC_MAJOR, minor);
+		mutex_lock(&misc_mtx);
+
+		list_for_each_entry(iter, &misc_list, list) {
+			if (iter->minor != minor)
+				continue;
+			c = iter;
+			new_fops = fops_get(iter->fops);
+			break;
+		}
+	}
+
+	if (!new_fops)
+		goto fail;
+
+	/*
+	 * Place the miscdevice in the file's
+	 * private_data so it can be used by the
+	 * file operations, including f_op->open below
+	 */
+	file->private_data = c;
+
+	err = 0;
+	replace_fops(file, new_fops);
+	if (file->f_op->open)
+		err = file->f_op->open(inode, file);
+fail:
+	mutex_unlock(&misc_mtx);
+	return err;
+}
+
+static char *misc_devnode(const struct device *dev, umode_t *mode)
+{
+	const struct miscdevice *c = dev_get_drvdata(dev);
+
+	if (mode && c->mode)
+		*mode = c->mode;
+	if (c->nodename)
+		return kstrdup(c->nodename, GFP_KERNEL);
+	return NULL;
+}
+
+static const struct class misc_class = {
+	.name		= "misc",
+	.devnode	= misc_devnode,
+};
+
+static const struct file_operations misc_fops = {
+	.owner		= THIS_MODULE,
+	.open		= misc_open,
+	.llseek		= noop_llseek,
+};
+
+/**
+ *	misc_register	-	register a miscellaneous device
+ *	@misc: device structure
+ *
+ *	Register a miscellaneous device with the kernel. If the minor
+ *	number is set to %MISC_DYNAMIC_MINOR a minor number is assigned
+ *	and placed in the minor field of the structure. For other cases
+ *	the minor number requested is used.
+ *
+ *	The structure passed is linked into the kernel and may not be
+ *	destroyed until it has been unregistered. By default, an open()
+ *	syscall to the device sets file->private_data to point to the
+ *	structure. Drivers don't need open in fops for this.
+ *
+ *	A zero is returned on success and a negative errno code for
+ *	failure.
+ */
+
+int misc_register(struct miscdevice *misc)
+{
+	dev_t dev;
+	int err = 0;
+	bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
+
+	if (misc->minor > MISC_DYNAMIC_MINOR) {
+		pr_err("Invalid fixed minor %d for miscdevice '%s'\n",
+		       misc->minor, misc->name);
+		return -EINVAL;
+	}
+
+	INIT_LIST_HEAD(&misc->list);
+
+	mutex_lock(&misc_mtx);
+
+	if (is_dynamic) {
+		int i = misc_minor_alloc(misc->minor);
+
+		if (i < 0) {
+			err = -EBUSY;
+			goto out;
+		}
+		misc->minor = i;
+	} else {
+		struct miscdevice *c;
+		int i;
+
+		list_for_each_entry(c, &misc_list, list) {
+			if (c->minor == misc->minor) {
+				err = -EBUSY;
+				goto out;
+			}
+		}
+
+		i = misc_minor_alloc(misc->minor);
+		if (i < 0) {
+			err = -EBUSY;
+			goto out;
+		}
+	}
+
+	dev = MKDEV(MISC_MAJOR, misc->minor);
+
+	misc->this_device =
+		device_create_with_groups(&misc_class, misc->parent, dev,
+					  misc, misc->groups, "%s", misc->name);
+	if (IS_ERR(misc->this_device)) {
+		misc_minor_free(misc->minor);
+		if (is_dynamic) {
+			misc->minor = MISC_DYNAMIC_MINOR;
+		}
+		err = PTR_ERR(misc->this_device);
+		goto out;
+	}
+
+	/*
+	 * Add it to the front, so that later devices can "override"
+	 * earlier defaults
+	 */
+	list_add(&misc->list, &misc_list);
+ out:
+	mutex_unlock(&misc_mtx);
+	return err;
+}
+EXPORT_SYMBOL(misc_register);
+
+/**
+ *	misc_deregister - unregister a miscellaneous device
+ *	@misc: device to unregister
+ *
+ *	Unregister a miscellaneous device that was previously
+ *	successfully registered with misc_register().
+ */
+
+void misc_deregister(struct miscdevice *misc)
+{
+	mutex_lock(&misc_mtx);
+	list_del_init(&misc->list);
+	device_destroy(&misc_class, MKDEV(MISC_MAJOR, misc->minor));
+	misc_minor_free(misc->minor);
+	if (misc->minor > MISC_DYNAMIC_MINOR)
+		misc->minor = MISC_DYNAMIC_MINOR;
+	mutex_unlock(&misc_mtx);
+}
+EXPORT_SYMBOL(misc_deregister);
+
+static int __init misc_init(void)
+{
+	int err;
+	struct proc_dir_entry *misc_proc_file;
+
+	misc_proc_file = proc_create_seq("misc", 0, NULL, &misc_seq_ops);
+	err = class_register(&misc_class);
+	if (err)
+		goto fail_remove;
+
+	err = __register_chrdev(MISC_MAJOR, 0, MINORMASK + 1, "misc", &misc_fops);
+	if (err < 0)
+		goto fail_printk;
+	return 0;
+
+fail_printk:
+	pr_err("unable to get major %d for misc devices\n", MISC_MAJOR);
+	class_unregister(&misc_class);
+fail_remove:
+	if (misc_proc_file)
+		remove_proc_entry("misc", NULL);
+	return err;
+}
+subsys_initcall(misc_init);