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, 99 insertions, 0 deletions

diff --git a/lib/notifier-error-inject.c b/lib/notifier-error-inject.c
new file mode 100644
index 000000000..954c3412d
--- /dev/null
+++ b/lib/notifier-error-inject.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+
+#include "notifier-error-inject.h"
+
+static int debugfs_errno_set(void *data, u64 val)
+{
+	*(int *)data = clamp_t(int, val, -MAX_ERRNO, 0);
+	return 0;
+}
+
+static int debugfs_errno_get(void *data, u64 *val)
+{
+	*val = *(int *)data;
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE_SIGNED(fops_errno, debugfs_errno_get, debugfs_errno_set,
+			"%lld\n");
+
+static struct dentry *debugfs_create_errno(const char *name, umode_t mode,
+				struct dentry *parent, int *value)
+{
+	return debugfs_create_file(name, mode, parent, value, &fops_errno);
+}
+
+static int notifier_err_inject_callback(struct notifier_block *nb,
+				unsigned long val, void *p)
+{
+	int err = 0;
+	struct notifier_err_inject *err_inject =
+		container_of(nb, struct notifier_err_inject, nb);
+	struct notifier_err_inject_action *action;
+
+	for (action = err_inject->actions; action->name; action++) {
+		if (action->val == val) {
+			err = action->error;
+			break;
+		}
+	}
+	if (err)
+		pr_info("Injecting error (%d) to %s\n", err, action->name);
+
+	return notifier_from_errno(err);
+}
+
+struct dentry *notifier_err_inject_dir;
+EXPORT_SYMBOL_GPL(notifier_err_inject_dir);
+
+struct dentry *notifier_err_inject_init(const char *name, struct dentry *parent,
+			struct notifier_err_inject *err_inject, int priority)
+{
+	struct notifier_err_inject_action *action;
+	umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
+	struct dentry *dir;
+	struct dentry *actions_dir;
+
+	err_inject->nb.notifier_call = notifier_err_inject_callback;
+	err_inject->nb.priority = priority;
+
+	dir = debugfs_create_dir(name, parent);
+
+	actions_dir = debugfs_create_dir("actions", dir);
+
+	for (action = err_inject->actions; action->name; action++) {
+		struct dentry *action_dir;
+
+		action_dir = debugfs_create_dir(action->name, actions_dir);
+
+		/*
+		 * Create debugfs r/w file containing action->error. If
+		 * notifier call chain is called with action->val, it will
+		 * fail with the error code
+		 */
+		debugfs_create_errno("error", mode, action_dir, &action->error);
+	}
+	return dir;
+}
+EXPORT_SYMBOL_GPL(notifier_err_inject_init);
+
+static int __init err_inject_init(void)
+{
+	notifier_err_inject_dir =
+		debugfs_create_dir("notifier-error-inject", NULL);
+
+	return 0;
+}
+
+static void __exit err_inject_exit(void)
+{
+	debugfs_remove_recursive(notifier_err_inject_dir);
+}
+
+module_init(err_inject_init);
+module_exit(err_inject_exit);
+
+MODULE_DESCRIPTION("Notifier error injection module");
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Akinobu Mita <akinobu.mita@gmail.com>");