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, 358 insertions, 0 deletions

diff --git a/tools/lib/subcmd/run-command.c b/tools/lib/subcmd/run-command.c
new file mode 100644
index 000000000..bd21b8bfd
--- /dev/null
+++ b/tools/lib/subcmd/run-command.c
@@ -0,0 +1,358 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <ctype.h>
+#include <fcntl.h>
+#include <string.h>
+#include <linux/compiler.h>
+#include <linux/string.h>
+#include <errno.h>
+#include <sys/wait.h>
+#include "subcmd-util.h"
+#include "run-command.h"
+#include "exec-cmd.h"
+
+#define STRERR_BUFSIZE 128
+
+static inline void close_pair(int fd[2])
+{
+	close(fd[0]);
+	close(fd[1]);
+}
+
+static inline void dup_devnull(int to)
+{
+	int fd = open("/dev/null", O_RDWR);
+	dup2(fd, to);
+	close(fd);
+}
+
+int start_command(struct child_process *cmd)
+{
+	int need_in, need_out, need_err;
+	int fdin[2], fdout[2], fderr[2];
+	char sbuf[STRERR_BUFSIZE];
+
+	/*
+	 * In case of errors we must keep the promise to close FDs
+	 * that have been passed in via ->in and ->out.
+	 */
+
+	need_in = !cmd->no_stdin && cmd->in < 0;
+	if (need_in) {
+		if (pipe(fdin) < 0) {
+			if (cmd->out > 0)
+				close(cmd->out);
+			return -ERR_RUN_COMMAND_PIPE;
+		}
+		cmd->in = fdin[1];
+	}
+
+	need_out = !cmd->no_stdout
+		&& !cmd->stdout_to_stderr
+		&& cmd->out < 0;
+	if (need_out) {
+		if (pipe(fdout) < 0) {
+			if (need_in)
+				close_pair(fdin);
+			else if (cmd->in)
+				close(cmd->in);
+			return -ERR_RUN_COMMAND_PIPE;
+		}
+		cmd->out = fdout[0];
+	}
+
+	need_err = !cmd->no_stderr && cmd->err < 0;
+	if (need_err) {
+		if (pipe(fderr) < 0) {
+			if (need_in)
+				close_pair(fdin);
+			else if (cmd->in)
+				close(cmd->in);
+			if (need_out)
+				close_pair(fdout);
+			else if (cmd->out)
+				close(cmd->out);
+			return -ERR_RUN_COMMAND_PIPE;
+		}
+		cmd->err = fderr[0];
+	}
+
+	fflush(NULL);
+	cmd->pid = fork();
+	if (!cmd->pid) {
+		if (cmd->no_stdin)
+			dup_devnull(0);
+		else if (need_in) {
+			dup2(fdin[0], 0);
+			close_pair(fdin);
+		} else if (cmd->in) {
+			dup2(cmd->in, 0);
+			close(cmd->in);
+		}
+
+		if (cmd->no_stderr)
+			dup_devnull(2);
+		else if (need_err) {
+			dup2(fderr[1], 2);
+			close_pair(fderr);
+		}
+
+		if (cmd->no_stdout)
+			dup_devnull(1);
+		else if (cmd->stdout_to_stderr)
+			dup2(2, 1);
+		else if (need_out) {
+			dup2(fdout[1], 1);
+			close_pair(fdout);
+		} else if (cmd->out > 1) {
+			dup2(cmd->out, 1);
+			close(cmd->out);
+		}
+
+		if (cmd->dir && chdir(cmd->dir))
+			die("exec %s: cd to %s failed (%s)", cmd->argv[0],
+			    cmd->dir, str_error_r(errno, sbuf, sizeof(sbuf)));
+		if (cmd->env) {
+			for (; *cmd->env; cmd->env++) {
+				if (strchr(*cmd->env, '='))
+					putenv((char*)*cmd->env);
+				else
+					unsetenv(*cmd->env);
+			}
+		}
+		if (cmd->preexec_cb)
+			cmd->preexec_cb();
+		if (cmd->no_exec_cmd)
+			exit(cmd->no_exec_cmd(cmd));
+		if (cmd->exec_cmd) {
+			execv_cmd(cmd->argv);
+		} else {
+			execvp(cmd->argv[0], (char *const*) cmd->argv);
+		}
+		exit(127);
+	}
+
+	if (cmd->pid < 0) {
+		int err = errno;
+		if (need_in)
+			close_pair(fdin);
+		else if (cmd->in)
+			close(cmd->in);
+		if (need_out)
+			close_pair(fdout);
+		else if (cmd->out)
+			close(cmd->out);
+		if (need_err)
+			close_pair(fderr);
+		return err == ENOENT ?
+			-ERR_RUN_COMMAND_EXEC :
+			-ERR_RUN_COMMAND_FORK;
+	}
+
+	if (need_in)
+		close(fdin[0]);
+	else if (cmd->in)
+		close(cmd->in);
+
+	if (need_out)
+		close(fdout[1]);
+	else if (cmd->out)
+		close(cmd->out);
+
+	if (need_err)
+		close(fderr[1]);
+
+	return 0;
+}
+
+static int wait_or_whine(struct child_process *cmd, bool block)
+{
+	bool finished;
+	int result;
+
+	if (cmd->pid <= 0) {
+		cmd->finished = 1;
+		if (cmd->pid < 0 && cmd->finish_result == 0)
+			cmd->finish_result = -ERR_RUN_COMMAND_FORK;
+		return cmd->finish_result;
+	}
+
+	finished = cmd->finished;
+	result = cmd->finish_result;
+
+	while (!finished) {
+		int status, code;
+		pid_t waiting = waitpid(cmd->pid, &status, block ? 0 : WNOHANG);
+
+		if (!block && waiting == 0)
+			break;
+
+		if (waiting < 0 && errno == EINTR)
+			continue;
+
+		finished = true;
+		if (waiting < 0) {
+			char sbuf[STRERR_BUFSIZE];
+
+			fprintf(stderr, " Error: waitpid failed (%s)",
+				str_error_r(errno, sbuf, sizeof(sbuf)));
+			result = -ERR_RUN_COMMAND_WAITPID;
+		} else if (waiting != cmd->pid) {
+			result = -ERR_RUN_COMMAND_WAITPID_WRONG_PID;
+		} else if (WIFSIGNALED(status)) {
+			result = -ERR_RUN_COMMAND_WAITPID_SIGNAL;
+		} else if (!WIFEXITED(status)) {
+			result = -ERR_RUN_COMMAND_WAITPID_NOEXIT;
+		} else {
+			code = WEXITSTATUS(status);
+			switch (code) {
+			case 127:
+				result = -ERR_RUN_COMMAND_EXEC;
+				break;
+			case 0:
+				result = 0;
+				break;
+			default:
+				result = -code;
+				break;
+			}
+		}
+	}
+	if (finished) {
+		cmd->finished = 1;
+		cmd->finish_result = result;
+	}
+	return result;
+}
+
+/*
+ * Conservative estimate of number of characaters needed to hold an a decoded
+ * integer, assume each 3 bits needs a character byte and plus a possible sign
+ * character.
+ */
+#ifndef is_signed_type
+#define is_signed_type(type) (((type)(-1)) < (type)1)
+#endif
+#define MAX_STRLEN_TYPE(type) (sizeof(type) * 8 / 3 + (is_signed_type(type) ? 1 : 0))
+
+int check_if_command_finished(struct child_process *cmd)
+{
+#ifdef __linux__
+	char filename[6 + MAX_STRLEN_TYPE(typeof(cmd->pid)) + 7 + 1];
+	char status_line[256];
+	FILE *status_file;
+#endif
+
+	if (cmd->finished)
+		return 1;
+	if (cmd->pid <= 0) {
+		cmd->finished = 1;
+		if (cmd->pid < 0 && cmd->finish_result == 0)
+			cmd->finish_result = -ERR_RUN_COMMAND_FORK;
+		return 1;
+	}
+
+#ifdef __linux__
+	/*
+	 * Check by reading /proc/<pid>/status as calling waitpid causes
+	 * stdout/stderr to be closed and data lost.
+	 */
+	sprintf(filename, "/proc/%u/status", cmd->pid);
+	status_file = fopen(filename, "r");
+	if (status_file == NULL) {
+		int status;
+		pid_t waiting;
+
+		/*
+		 * fopen() can fail with ENOENT if the process has been reaped.
+		 * It can also fail with EMFILE/ENFILE if RLIMIT_NOFILE is reached.
+		 * In those cases, use waitpid(..., WNOHANG) to robustly check
+		 * and reap the process if it has exited.
+		 */
+		if (errno == ENOENT)
+			return 1;
+
+		waiting = waitpid(cmd->pid, &status, WNOHANG);
+		if (waiting == cmd->pid) {
+			int result;
+			int code;
+
+			cmd->finished = 1;
+			if (WIFSIGNALED(status)) {
+				result = -ERR_RUN_COMMAND_WAITPID_SIGNAL;
+			} else if (!WIFEXITED(status)) {
+				result = -ERR_RUN_COMMAND_WAITPID_NOEXIT;
+			} else {
+				code = WEXITSTATUS(status);
+				switch (code) {
+				case 127:
+					result = -ERR_RUN_COMMAND_EXEC;
+					break;
+				case 0:
+					result = 0;
+					break;
+				default:
+					result = -code;
+					break;
+				}
+			}
+			cmd->finish_result = result;
+			return 1;
+		}
+		if (waiting < 0 && (errno == ECHILD || errno == ESRCH))
+			return 1;
+		return 0;
+	}
+	while (fgets(status_line, sizeof(status_line), status_file) != NULL) {
+		char *p;
+
+		if (strncmp(status_line, "State:", 6))
+			continue;
+
+		fclose(status_file);
+		p = status_line + 6;
+		while (isspace(*p))
+			p++;
+		return *p == 'Z' ? 1 : 0;
+	}
+	/* Read failed assume finish_command was called. */
+	fclose(status_file);
+	return 1;
+#else
+	wait_or_whine(cmd, /*block=*/false);
+	return cmd->finished;
+#endif
+}
+
+int finish_command(struct child_process *cmd)
+{
+	return wait_or_whine(cmd, /*block=*/true);
+}
+
+int run_command(struct child_process *cmd)
+{
+	int code = start_command(cmd);
+	if (code)
+		return code;
+	return finish_command(cmd);
+}
+
+static void prepare_run_command_v_opt(struct child_process *cmd,
+				      const char **argv,
+				      int opt)
+{
+	memset(cmd, 0, sizeof(*cmd));
+	cmd->argv = argv;
+	cmd->no_stdin = opt & RUN_COMMAND_NO_STDIN ? 1 : 0;
+	cmd->exec_cmd = opt & RUN_EXEC_CMD ? 1 : 0;
+	cmd->stdout_to_stderr = opt & RUN_COMMAND_STDOUT_TO_STDERR ? 1 : 0;
+}
+
+int run_command_v_opt(const char **argv, int opt)
+{
+	struct child_process cmd;
+	prepare_run_command_v_opt(&cmd, argv, opt);
+	return run_command(&cmd);
+}