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, 144 insertions, 0 deletions

diff --git a/drivers/acpi/ec_sys.c b/drivers/acpi/ec_sys.c
new file mode 100644
index 000000000..c074a0fae
--- /dev/null
+++ b/drivers/acpi/ec_sys.c
@@ -0,0 +1,144 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * ec_sys.c
+ *
+ * Copyright (C) 2010 SUSE Products GmbH/Novell
+ * Author:
+ *      Thomas Renninger <trenn@suse.de>
+ */
+
+#include <linux/kernel.h>
+#include <linux/acpi.h>
+#include <linux/debugfs.h>
+#include <linux/module.h>
+#include <linux/uaccess.h>
+#include "internal.h"
+
+MODULE_AUTHOR("Thomas Renninger <trenn@suse.de>");
+MODULE_DESCRIPTION("ACPI EC sysfs access driver");
+MODULE_LICENSE("GPL");
+
+static bool write_support;
+module_param_hw(write_support, bool, other, 0644);
+MODULE_PARM_DESC(write_support, "Dangerous, reboot and removal of battery may "
+		 "be needed.");
+
+#define EC_SPACE_SIZE 256
+
+static struct dentry *acpi_ec_debugfs_dir;
+
+static ssize_t acpi_ec_read_io(struct file *f, char __user *buf,
+			       size_t count, loff_t *off)
+{
+	/* Use this if support reading/writing multiple ECs exists in ec.c:
+	 * struct acpi_ec *ec = ((struct seq_file *)f->private_data)->private;
+	 */
+	unsigned int size = EC_SPACE_SIZE;
+	loff_t init_off = *off;
+	int err = 0;
+
+	if (*off >= size)
+		return 0;
+	if (*off + count >= size) {
+		size -= *off;
+		count = size;
+	} else
+		size = count;
+
+	while (size) {
+		u8 byte_read;
+		err = ec_read(*off, &byte_read);
+		if (err)
+			return err;
+		if (put_user(byte_read, buf + *off - init_off)) {
+			if (*off - init_off)
+				return *off - init_off; /* partial read */
+			return -EFAULT;
+		}
+		*off += 1;
+		size--;
+	}
+	return count;
+}
+
+static ssize_t acpi_ec_write_io(struct file *f, const char __user *buf,
+				size_t count, loff_t *off)
+{
+	/* Use this if support reading/writing multiple ECs exists in ec.c:
+	 * struct acpi_ec *ec = ((struct seq_file *)f->private_data)->private;
+	 */
+
+	unsigned int size = count;
+	loff_t init_off = *off;
+	int err = 0;
+
+	if (!write_support)
+		return -EINVAL;
+
+	if (*off >= EC_SPACE_SIZE)
+		return 0;
+	if (*off + count >= EC_SPACE_SIZE) {
+		size = EC_SPACE_SIZE - *off;
+		count = size;
+	}
+
+	while (size) {
+		u8 byte_write;
+		if (get_user(byte_write, buf + *off - init_off)) {
+			if (*off - init_off)
+				return *off - init_off; /* partial write */
+			return -EFAULT;
+		}
+		err = ec_write(*off, byte_write);
+		if (err)
+			return err;
+
+		*off += 1;
+		size--;
+	}
+	return count;
+}
+
+static const struct file_operations acpi_ec_io_ops = {
+	.owner = THIS_MODULE,
+	.open  = simple_open,
+	.read  = acpi_ec_read_io,
+	.write = acpi_ec_write_io,
+	.llseek = default_llseek,
+};
+
+static void acpi_ec_add_debugfs(struct acpi_ec *ec, unsigned int ec_device_count)
+{
+	struct dentry *dev_dir;
+	char name[64];
+	umode_t mode = 0400;
+
+	if (ec_device_count == 0)
+		acpi_ec_debugfs_dir = debugfs_create_dir("ec", NULL);
+
+	sprintf(name, "ec%u", ec_device_count);
+	dev_dir = debugfs_create_dir(name, acpi_ec_debugfs_dir);
+
+	debugfs_create_x32("gpe", 0444, dev_dir, &first_ec->gpe);
+	debugfs_create_bool("use_global_lock", 0444, dev_dir,
+			    &first_ec->global_lock);
+
+	if (write_support)
+		mode = 0600;
+	debugfs_create_file("io", mode, dev_dir, ec, &acpi_ec_io_ops);
+}
+
+static int __init acpi_ec_sys_init(void)
+{
+	if (first_ec)
+		acpi_ec_add_debugfs(first_ec, 0);
+	return 0;
+}
+
+static void __exit acpi_ec_sys_exit(void)
+{
+	debugfs_remove_recursive(acpi_ec_debugfs_dir);
+}
+
+module_init(acpi_ec_sys_init);
+module_exit(acpi_ec_sys_exit);