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>

3 files changed, 271 insertions, 0 deletions

diff --git a/samples/auxdisplay/.gitignore b/samples/auxdisplay/.gitignore
new file mode 100644
index 000000000..d02381684
--- /dev/null
+++ b/samples/auxdisplay/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/cfag12864b-example
diff --git a/samples/auxdisplay/Makefile b/samples/auxdisplay/Makefile
new file mode 100644
index 000000000..19d556893
--- /dev/null
+++ b/samples/auxdisplay/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0
+userprogs-always-y += cfag12864b-example
diff --git a/samples/auxdisplay/cfag12864b-example.c b/samples/auxdisplay/cfag12864b-example.c
new file mode 100644
index 000000000..2e3bb7375
--- /dev/null
+++ b/samples/auxdisplay/cfag12864b-example.c
@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *    Filename: cfag12864b-example.c
+ *     Version: 0.1.0
+ * Description: cfag12864b LCD userspace example program
+ *
+ *      Author: Copyright (C) Miguel Ojeda <ojeda@kernel.org>
+ *        Date: 2006-10-31
+ */
+
+/*
+ * ------------------------
+ * start of cfag12864b code
+ * ------------------------
+ */
+
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+
+#define CFAG12864B_WIDTH		(128)
+#define CFAG12864B_HEIGHT		(64)
+#define CFAG12864B_SIZE			(128 * 64 / 8)
+#define CFAG12864B_BPB			(8)
+#define CFAG12864B_ADDRESS(x, y)	((y) * CFAG12864B_WIDTH / \
+					CFAG12864B_BPB + (x) / CFAG12864B_BPB)
+#define CFAG12864B_BIT(n)		(((unsigned char) 1) << (n))
+
+#undef CFAG12864B_DOCHECK
+#ifdef CFAG12864B_DOCHECK
+	#define CFAG12864B_CHECK(x, y)		((x) < CFAG12864B_WIDTH && \
+						(y) < CFAG12864B_HEIGHT)
+#else
+	#define CFAG12864B_CHECK(x, y)		(1)
+#endif
+
+int cfag12864b_fd;
+unsigned char * cfag12864b_mem;
+unsigned char cfag12864b_buffer[CFAG12864B_SIZE];
+
+/*
+ * init a cfag12864b framebuffer device
+ *
+ * No error:       return = 0
+ * Unable to open: return = -1
+ * Unable to mmap: return = -2
+ */
+static int cfag12864b_init(char *path)
+{
+	cfag12864b_fd = open(path, O_RDWR);
+	if (cfag12864b_fd == -1)
+		return -1;
+
+	cfag12864b_mem = mmap(0, CFAG12864B_SIZE, PROT_READ | PROT_WRITE,
+		MAP_SHARED, cfag12864b_fd, 0);
+	if (cfag12864b_mem == MAP_FAILED) {
+		close(cfag12864b_fd);
+		return -2;
+	}
+
+	return 0;
+}
+
+/*
+ * exit a cfag12864b framebuffer device
+ */
+static void cfag12864b_exit(void)
+{
+	munmap(cfag12864b_mem, CFAG12864B_SIZE);
+	close(cfag12864b_fd);
+}
+
+/*
+ * set (x, y) pixel
+ */
+static void cfag12864b_set(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] |=
+			CFAG12864B_BIT(x % CFAG12864B_BPB);
+}
+
+/*
+ * unset (x, y) pixel
+ */
+static void cfag12864b_unset(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] &=
+			~CFAG12864B_BIT(x % CFAG12864B_BPB);
+}
+
+/*
+ * is set (x, y) pixel?
+ *
+ * Pixel off: return = 0
+ * Pixel on:  return = 1
+ */
+static unsigned char cfag12864b_isset(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		if (cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] &
+			CFAG12864B_BIT(x % CFAG12864B_BPB))
+			return 1;
+
+	return 0;
+}
+
+/*
+ * not (x, y) pixel
+ */
+static void cfag12864b_not(unsigned char x, unsigned char y)
+{
+	if (cfag12864b_isset(x, y))
+		cfag12864b_unset(x, y);
+	else
+		cfag12864b_set(x, y);
+}
+
+/*
+ * fill (set all pixels)
+ */
+static void cfag12864b_fill(void)
+{
+	unsigned short i;
+
+	for (i = 0; i < CFAG12864B_SIZE; i++)
+		cfag12864b_buffer[i] = 0xFF;
+}
+
+/*
+ * clear (unset all pixels)
+ */
+static void cfag12864b_clear(void)
+{
+	unsigned short i;
+
+	for (i = 0; i < CFAG12864B_SIZE; i++)
+		cfag12864b_buffer[i] = 0;
+}
+
+/*
+ * format a [128*64] matrix
+ *
+ * Pixel off: src[i] = 0
+ * Pixel on:  src[i] > 0
+ */
+static void cfag12864b_format(unsigned char * matrix)
+{
+	unsigned char i, j, n;
+
+	for (i = 0; i < CFAG12864B_HEIGHT; i++)
+	for (j = 0; j < CFAG12864B_WIDTH / CFAG12864B_BPB; j++) {
+		cfag12864b_buffer[i * CFAG12864B_WIDTH / CFAG12864B_BPB +
+			j] = 0;
+		for (n = 0; n < CFAG12864B_BPB; n++)
+			if (matrix[i * CFAG12864B_WIDTH +
+				j * CFAG12864B_BPB + n])
+				cfag12864b_buffer[i * CFAG12864B_WIDTH /
+					CFAG12864B_BPB + j] |=
+					CFAG12864B_BIT(n);
+	}
+}
+
+/*
+ * blit buffer to lcd
+ */
+static void cfag12864b_blit(void)
+{
+	memcpy(cfag12864b_mem, cfag12864b_buffer, CFAG12864B_SIZE);
+}
+
+/*
+ * ----------------------
+ * end of cfag12864b code
+ * ----------------------
+ */
+
+#include <stdio.h>
+
+#define EXAMPLES	6
+
+static void example(unsigned char n)
+{
+	unsigned short i, j;
+	unsigned char matrix[CFAG12864B_WIDTH * CFAG12864B_HEIGHT];
+
+	if (n > EXAMPLES)
+		return;
+
+	printf("Example %i/%i - ", n, EXAMPLES);
+
+	switch (n) {
+	case 1:
+		printf("Draw points setting bits");
+		cfag12864b_clear();
+		for (i = 0; i < CFAG12864B_WIDTH; i += 2)
+			for (j = 0; j < CFAG12864B_HEIGHT; j += 2)
+				cfag12864b_set(i, j);
+		break;
+
+	case 2:
+		printf("Clear the LCD");
+		cfag12864b_clear();
+		break;
+
+	case 3:
+		printf("Draw rows formatting a [128*64] matrix");
+		memset(matrix, 0, CFAG12864B_WIDTH * CFAG12864B_HEIGHT);
+		for (i = 0; i < CFAG12864B_WIDTH; i++)
+			for (j = 0; j < CFAG12864B_HEIGHT; j += 2)
+				matrix[j * CFAG12864B_WIDTH + i] = 1;
+		cfag12864b_format(matrix);
+		break;
+
+	case 4:
+		printf("Fill the lcd");
+		cfag12864b_fill();
+		break;
+
+	case 5:
+		printf("Draw columns unsetting bits");
+		for (i = 0; i < CFAG12864B_WIDTH; i += 2)
+			for (j = 0; j < CFAG12864B_HEIGHT; j++)
+				cfag12864b_unset(i, j);
+		break;
+
+	case 6:
+		printf("Do negative not-ing all bits");
+		for (i = 0; i < CFAG12864B_WIDTH; i++)
+			for (j = 0; j < CFAG12864B_HEIGHT; j ++)
+				cfag12864b_not(i, j);
+		break;
+	}
+
+	puts(" - [Press Enter]");
+}
+
+int main(int argc, char *argv[])
+{
+	unsigned char n;
+
+	if (argc != 2) {
+		printf(
+			"Syntax:  %s fbdev\n"
+			"Usually: /dev/fb0, /dev/fb1...\n", argv[0]);
+		return -1;
+	}
+
+	if (cfag12864b_init(argv[1])) {
+		printf("Can't init %s fbdev\n", argv[1]);
+		return -2;
+	}
+
+	for (n = 1; n <= EXAMPLES; n++) {
+		example(n);
+		cfag12864b_blit();
+		while (getchar() != '\n');
+	}
+
+	cfag12864b_exit();
+
+	return 0;
+}