Added cg-oom-killed meta entry

It is hard to judge solely from the meta output whether a program has
been SIGKILL by the OOM killer or something else.

Since Linux 4.15, the cgroup API provides us with a memory.oom_control
file that contains a key "oom_kill" with the number of times the cgroup
has received an OOM kill. If this value is greater than 0, we report that
there was an OOM kill in the meta file with the cg-oom-killed entry.

Based on work by Antoine Pietri.

cg.c

@@ -105,7 +105,7 @@ cg_read(cg_controller controller, const char *attr, char *buf)
   buf[n] = 0;
 
   if (verbose > 1)
-    msg("CG: Read %s = %s\n", attr, buf);
+    msg("CG: Read %s = <%s>\n", attr, buf);
 
   result = 1;
 fail_close:
@@ -280,6 +280,24 @@ cg_stats(void)
     }
   if (mem)
     meta_printf("cg-mem:%lld\n", mem >> 10);
+
+  // OOM kill detection
+  if (cg_read(CG_MEMORY, "?memory.oom_control", buf))
+    {
+      int oom_killed = 0;
+      char *s = buf;
+      while (s)
+	{
+	  if (sscanf(s, "oom_kill %d", &oom_killed) == 1 && oom_killed)
+	    {
+	      meta_printf("cg-oom-killed:1\n");
+	      break;
+	    }
+	  s = strchr(s, '\n');
+	  if (s)
+	    s++;
+	}
+    }
 }
 
 void

isolate.1.txt

@@ -235,6 +235,10 @@ of format 'key'*:*'value'. The following keys are defined:
 *cg-mem*::
 	When control groups are enabled, this is the total memory use
 	by the whole control group (in kilobytes).
+*cg-oom-killed*::
+	Present when the program was killed by the out-of-memory killer
+	(e.g., because it has exceeded the memory limit of its control group).
+	This is reported only on Linux 4.13 and later.
 *csw-forced*::
 	Number of context switches forced by the kernel.
 *csw-voluntary*::