perf daemon: Add daemon command

Add a daemon skeleton with a minimal base (non) functionality, covering
various setup in start command.

Add an initial perf-daemon.txt with basic info.

This is in response to pople asking for the possibility to be able run
record long running sessions on the background.

The patchset that starts with this adds support to configure and run
record sessions on background via new 'perf daemon' command.

This is useful for being able to use perf as a flight recorder that one
can interact with asking for events to be enabled or disabled, added or
removed, etc.

Signed-off-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Alexei Budankov <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Michael Petlan <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Build

···
 perf-y += builtin-data.o
 perf-y += builtin-version.o
 perf-y += builtin-c2c.o
+perf-y += builtin-daemon.o
 
 perf-$(CONFIG_TRACE) += builtin-trace.o
 perf-$(CONFIG_LIBELF) += builtin-probe.o

tools/perf/Documentation/perf-daemon.txt

···
+perf-daemon(1)
+==============
+
+
+NAME
+----
+perf-daemon - Run record sessions on background
+
+
+SYNOPSIS
+--------
+[verse]
+'perf daemon'
+'perf daemon' [<options>]
+'perf daemon start'  [<options>]
+
+
+DESCRIPTION
+-----------
+This command allows to run simple daemon process that starts and
+monitors configured record sessions.
+
+
+OPTIONS
+-------
+-v::
+--verbose::
+	Be more verbose.
+
+All generic options are available also under commands.
+
+
+START COMMAND
+-------------
+The start command creates the daemon process.
+
+
+SEE ALSO
+--------
+linkperf:perf-record[1], linkperf:perf-config[1]

tools/perf/builtin-daemon.c

···
+// SPDX-License-Identifier: GPL-2.0
+#include <subcmd/parse-options.h>
+#include <linux/limits.h>
+#include <string.h>
+#include <signal.h>
+#include <stdio.h>
+#include <unistd.h>
+#include "builtin.h"
+#include "perf.h"
+#include "debug.h"
+#include "util.h"
+
+struct daemon {
+	char			*base;
+	FILE			*out;
+	char			 perf[PATH_MAX];
+};
+
+static struct daemon __daemon = { };
+
+static const char * const daemon_usage[] = {
+	"perf daemon start [<options>]",
+	"perf daemon [<options>]",
+	NULL
+};
+
+static bool done;
+
+static void sig_handler(int sig __maybe_unused)
+{
+	done = true;
+}
+
+static int __cmd_start(struct daemon *daemon, struct option parent_options[],
+		       int argc, const char **argv)
+{
+	struct option start_options[] = {
+		OPT_PARENT(parent_options),
+		OPT_END()
+	};
+	int err = 0;
+
+	argc = parse_options(argc, argv, start_options, daemon_usage, 0);
+	if (argc)
+		usage_with_options(daemon_usage, start_options);
+
+	debug_set_file(daemon->out);
+	debug_set_display_time(true);
+
+	pr_info("daemon started (pid %d)\n", getpid());
+
+	signal(SIGINT, sig_handler);
+	signal(SIGTERM, sig_handler);
+
+	while (!done && !err) {
+		sleep(1);
+	}
+
+	pr_info("daemon exited\n");
+	fclose(daemon->out);
+	return err;
+}
+
+int cmd_daemon(int argc, const char **argv)
+{
+	struct option daemon_options[] = {
+		OPT_INCR('v', "verbose", &verbose, "be more verbose"),
+		OPT_END()
+	};
+
+	perf_exe(__daemon.perf, sizeof(__daemon.perf));
+	__daemon.out = stdout;
+
+	argc = parse_options(argc, argv, daemon_options, daemon_usage,
+			     PARSE_OPT_STOP_AT_NON_OPTION);
+
+	if (argc) {
+		if (!strcmp(argv[0], "start"))
+			return __cmd_start(&__daemon, daemon_options, argc, argv);
+
+		pr_err("failed: unknown command '%s'\n", argv[0]);
+		return -1;
+	}
+
+	return -1;
+}

tools/perf/builtin.h

···
 int cmd_mem(int argc, const char **argv);
 int cmd_data(int argc, const char **argv);
 int cmd_ftrace(int argc, const char **argv);
+int cmd_daemon(int argc, const char **argv);
 
 int find_scripts(char **scripts_array, char **scripts_path_array, int num,
 		 int pathlen);

tools/perf/command-list.txt

···
 perf-top			mainporcelain common
 perf-trace			mainporcelain audit
 perf-version			mainporcelain common
+perf-daemon			mainporcelain common

tools/perf/perf.c

···
 	{ "mem",	cmd_mem,	0 },
 	{ "data",	cmd_data,	0 },
 	{ "ftrace",	cmd_ftrace,	0 },
+	{ "daemon",	cmd_daemon,	0 },
 };
 
 struct pager_config {