qga: implement a 'guest-get-load' command

Provide a way to report the process load average, via a new
'guest-get-load' command.

This is only implemented for POSIX platforms providing 'getloadavg'.

Example illustrated with qmp-shell:

(QEMU) guest-get-load
{
    "return": {
        "load15m": 1.546875,
        "load1m": 1.669921875,
        "load5m": 1.9306640625
    }
}

Windows has no native equivalent API, but it would be possible to
simulate it as illustrated here (BSD-3-Clause):

  https://github.com/giampaolo/psutil/pull/1485

This is left as an exercise for future contributors.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-ID: <20241202121927.864335-1-berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>

meson.build

@@ -2646,6 +2646,7 @@ config_host_data.set('CONFIG_SETNS', cc.has_function('setns') and cc.has_functio
 config_host_data.set('CONFIG_SYNCFS', cc.has_function('syncfs'))
 config_host_data.set('CONFIG_SYNC_FILE_RANGE', cc.has_function('sync_file_range'))
 config_host_data.set('CONFIG_TIMERFD', cc.has_function('timerfd_create'))
+config_host_data.set('CONFIG_GETLOADAVG', cc.has_function('getloadavg'))
 config_host_data.set('HAVE_COPY_FILE_RANGE', cc.has_function('copy_file_range'))
 config_host_data.set('HAVE_GETIFADDRS', cc.has_function('getifaddrs'))
 config_host_data.set('HAVE_GLIB_WITH_SLICE_ALLOCATOR', glib_has_gslice)

qga/commands-posix.c

@@ -1368,3 +1368,23 @@ char *qga_get_host_name(Error **errp)
 
     return g_steal_pointer(&hostname);
 }
+
+#ifdef CONFIG_GETLOADAVG
+GuestLoadAverage *qmp_guest_get_load(Error **errp)
+{
+    double loadavg[3];
+    GuestLoadAverage *ret = NULL;
+
+    if (getloadavg(loadavg, G_N_ELEMENTS(loadavg)) < 0) {
+        error_setg_errno(errp, errno,
+                         "cannot query load average");
+        return NULL;
+    }
+
+    ret = g_new0(GuestLoadAverage, 1);
+    ret->load1m = loadavg[0];
+    ret->load5m = loadavg[1];
+    ret->load15m = loadavg[2];
+    return ret;
+}
+#endif

qga/qapi-schema.json

@@ -1843,6 +1843,43 @@
   'if': 'CONFIG_LINUX'
 }
 
+
+##
+# @GuestLoadAverage:
+#
+# Statistics about process load information
+#
+# @load1m: 1-minute load avage
+#
+# @load5m: 5-minute load avage
+#
+# @load15m: 15-minute load avage
+#
+# Since: 10.0
+##
+{ 'struct': 'GuestLoadAverage',
+  'data': {
+      'load1m': 'number',
+      'load5m': 'number',
+      'load15m': 'number'
+  },
+  'if': 'CONFIG_GETLOADAVG'
+}
+
+##
+# @guest-get-load:
+#
+# Retrieve CPU process load information
+#
+# Returns: load information
+#
+# Since: 10.0
+##
+{ 'command': 'guest-get-load',
+  'returns': 'GuestLoadAverage',
+  'if': 'CONFIG_GETLOADAVG'
+}
+
 ##
 # @GuestNetworkRoute:
 #