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>

1 files changed, 110 insertions, 0 deletions

diff --git a/Documentation/cgroups/cgroup_event_listener.c b/Documentation/cgroups/cgroup_event_listener.c
new file mode 100644
index 000000000000..8c2bfc4a6358
--- /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);
+}