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, 79 insertions, 0 deletions

diff --git a/tools/perf/util/path.c b/tools/perf/util/path.c
new file mode 100644
index 000000000..2e62f272f
--- /dev/null
+++ b/tools/perf/util/path.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "path.h"
+#include "cache.h"
+#include <linux/kernel.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <dirent.h>
+#include <unistd.h>
+
+static char *cleanup_path(char *path)
+{
+	/* Clean it up */
+	if (!memcmp(path, "./", 2)) {
+		path += 2;
+		while (*path == '/')
+			path++;
+	}
+	return path;
+}
+
+char *mkpath(char *path_buf, size_t sz, const char *fmt, ...)
+{
+	va_list args;
+	unsigned len;
+
+	va_start(args, fmt);
+	len = vsnprintf(path_buf, sz, fmt, args);
+	va_end(args);
+	if (len >= sz)
+		strncpy(path_buf, "/bad-path/", sz);
+	return cleanup_path(path_buf);
+}
+
+int path__join(char *bf, size_t size, const char *path1, const char *path2)
+{
+	return scnprintf(bf, size, "%s%s%s", path1, path1[0] ? "/" : "", path2);
+}
+
+int path__join3(char *bf, size_t size, const char *path1, const char *path2, const char *path3)
+{
+	return scnprintf(bf, size, "%s%s%s%s%s", path1, path1[0] ? "/" : "",
+			 path2, path2[0] ? "/" : "", path3);
+}
+
+bool is_regular_file(const char *file)
+{
+	struct stat st;
+
+	if (stat(file, &st))
+		return false;
+
+	return S_ISREG(st.st_mode);
+}
+
+/* Helper function for filesystems that return a dent->d_type DT_UNKNOWN */
+bool is_directory(const char *base_path, const struct dirent *dent)
+{
+	char path[PATH_MAX];
+	struct stat st;
+
+	snprintf(path, sizeof(path), "%s/%s", base_path, dent->d_name);
+	if (stat(path, &st))
+		return false;
+
+	return S_ISDIR(st.st_mode);
+}
+
+bool is_directory_at(int dir_fd, const char *path)
+{
+	struct stat st;
+
+	if (fstatat(dir_fd, path, &st, /*flags=*/0))
+		return false;
+
+	return S_ISDIR(st.st_mode);
+}