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, 109 insertions, 0 deletions

diff --git a/fs/jfs/jfs_debug.h b/fs/jfs/jfs_debug.h
new file mode 100644
index 000000000..48e2150c0
--- /dev/null
+++ b/fs/jfs/jfs_debug.h
@@ -0,0 +1,109 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ *   Copyright (C) International Business Machines Corp., 2000-2002
+ *   Portions Copyright (C) Christoph Hellwig, 2001-2002
+ */
+#ifndef _H_JFS_DEBUG
+#define _H_JFS_DEBUG
+
+/*
+ *	jfs_debug.h
+ *
+ * global debug message, data structure/macro definitions
+ * under control of CONFIG_JFS_DEBUG, CONFIG_JFS_STATISTICS;
+ */
+
+/*
+ * Create /proc/fs/jfs if procfs is enabled andeither
+ * CONFIG_JFS_DEBUG or CONFIG_JFS_STATISTICS is defined
+ */
+#if defined(CONFIG_PROC_FS) && (defined(CONFIG_JFS_DEBUG) || defined(CONFIG_JFS_STATISTICS))
+#define PROC_FS_JFS
+extern void jfs_proc_init(void);
+extern void jfs_proc_clean(void);
+#endif
+
+/*
+ *	assert with traditional printf/panic
+ */
+#define assert(p) do {	\
+	if (!(p)) {	\
+		printk(KERN_CRIT "BUG at %s:%d assert(%s)\n",	\
+		       __FILE__, __LINE__, #p);			\
+		BUG();	\
+	}		\
+} while (0)
+
+/*
+ *	debug ON
+ *	--------
+ */
+#ifdef CONFIG_JFS_DEBUG
+#define ASSERT(p) assert(p)
+
+/* printk verbosity */
+#define JFS_LOGLEVEL_ERR 1
+#define JFS_LOGLEVEL_WARN 2
+#define JFS_LOGLEVEL_DEBUG 3
+#define JFS_LOGLEVEL_INFO 4
+
+extern int jfsloglevel;
+
+int jfs_txanchor_proc_show(struct seq_file *m, void *v);
+
+/* information message: e.g., configuration, major event */
+#define jfs_info(fmt, arg...) do {			\
+	if (jfsloglevel >= JFS_LOGLEVEL_INFO)		\
+		printk(KERN_INFO fmt "\n", ## arg);	\
+} while (0)
+
+/* debug message: ad hoc */
+#define jfs_debug(fmt, arg...) do {			\
+	if (jfsloglevel >= JFS_LOGLEVEL_DEBUG)		\
+		printk(KERN_DEBUG fmt "\n", ## arg);	\
+} while (0)
+
+/* warn message: */
+#define jfs_warn(fmt, arg...) do {			\
+	if (jfsloglevel >= JFS_LOGLEVEL_WARN)		\
+		printk(KERN_WARNING fmt "\n", ## arg);	\
+} while (0)
+
+/* error event message: e.g., i/o error */
+#define jfs_err(fmt, arg...) do {			\
+	if (jfsloglevel >= JFS_LOGLEVEL_ERR)		\
+		printk(KERN_ERR fmt "\n", ## arg);	\
+} while (0)
+
+/*
+ *	debug OFF
+ *	---------
+ */
+#else				/* CONFIG_JFS_DEBUG */
+#define ASSERT(p) do {} while (0)
+#define jfs_info(fmt, arg...) do {} while (0)
+#define jfs_debug(fmt, arg...) do {} while (0)
+#define jfs_warn(fmt, arg...) do {} while (0)
+#define jfs_err(fmt, arg...) do {} while (0)
+#endif				/* CONFIG_JFS_DEBUG */
+
+/*
+ *	statistics
+ *	----------
+ */
+#ifdef	CONFIG_JFS_STATISTICS
+int jfs_lmstats_proc_show(struct seq_file *m, void *v);
+int jfs_txstats_proc_show(struct seq_file *m, void *v);
+int jfs_mpstat_proc_show(struct seq_file *m, void *v);
+int jfs_xtstat_proc_show(struct seq_file *m, void *v);
+
+#define	INCREMENT(x)		((x)++)
+#define	DECREMENT(x)		((x)--)
+#define	HIGHWATERMARK(x,y)	((x) = max((x), (y)))
+#else
+#define	INCREMENT(x)
+#define	DECREMENT(x)
+#define	HIGHWATERMARK(x,y)
+#endif				/* CONFIG_JFS_STATISTICS */
+
+#endif				/* _H_JFS_DEBUG */