cgroups: add simple listener of cgroup events to documentation

An example of cgroup notification API usage.

Signed-off-by: Kirill A.  Shutemov <kirill@shutemov.name>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Dan Malek <dan@embeddedalley.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/cgroups/cgroup_event_listener.c b/Documentation/cgroups/cgroup_event_listener.c
new file mode 100644 (file)
index 0000000..8c2bfc4
--- /dev/null
+++ b/Documentation/cgroups/cgroup_event_listener.c
@@ -0,0 +1,110 @@
+/*
+ * cgroup_event_listener.c - Simple listener of cgroup events
+ *
+ * Copyright (C) Kirill A. Shutemov <kirill@shutemov.name>
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/eventfd.h>
+
+#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>\n"
+
+int main(int argc, char **argv)
+{
+       int efd = -1;
+       int cfd = -1;
+       int event_control = -1;
+       char event_control_path[PATH_MAX];
+       char line[LINE_MAX];
+       int ret;
+
+       if (argc != 3) {
+               fputs(USAGE_STR, stderr);
+               return 1;
+       }
+
+       cfd = open(argv[1], O_RDONLY);
+       if (cfd == -1) {
+               fprintf(stderr, "Cannot open %s: %s\n", argv[1],
+                               strerror(errno));
+               goto out;
+       }
+
+       ret = snprintf(event_control_path, PATH_MAX, "%s/cgroup.event_control",
+                       dirname(argv[1]));
+       if (ret >= PATH_MAX) {
+               fputs("Path to cgroup.event_control is too long\n", stderr);
+               goto out;
+       }
+
+       event_control = open(event_control_path, O_WRONLY);
+       if (event_control == -1) {
+               fprintf(stderr, "Cannot open %s: %s\n", event_control_path,
+                               strerror(errno));
+               goto out;
+       }
+
+       efd = eventfd(0, 0);
+       if (efd == -1) {
+               perror("eventfd() failed");
+               goto out;
+       }
+
+       ret = snprintf(line, LINE_MAX, "%d %d %s", efd, cfd, argv[2]);
+       if (ret >= LINE_MAX) {
+               fputs("Arguments string is too long\n", stderr);
+               goto out;
+       }
+
+       ret = write(event_control, line, strlen(line) + 1);
+       if (ret == -1) {
+               perror("Cannot write to cgroup.event_control");
+               goto out;
+       }
+
+       while (1) {
+               uint64_t result;
+
+               ret = read(efd, &result, sizeof(result));
+               if (ret == -1) {
+                       if (errno == EINTR)
+                               continue;
+                       perror("Cannot read from eventfd");
+                       break;
+               }
+               assert(ret == sizeof(result));
+
+               ret = access(event_control_path, W_OK);
+               if ((ret == -1) && (errno == ENOENT)) {
+                               puts("The cgroup seems to have removed.");
+                               ret = 0;
+                               break;
+               }
+
+               if (ret == -1) {
+                       perror("cgroup.event_control "
+                                       "is not accessable any more");
+                       break;
+               }
+
+               printf("%s %s: crossed\n", argv[1], argv[2]);
+       }
+
+out:
+       if (efd >= 0)
+               close(efd);
+       if (event_control >= 0)
+               close(event_control);
+       if (cfd >= 0)
+               close(cfd);
+
+       return (ret != 0);
+}