virtio_balloon: introduce oom-kill invocations

When the guest OS runs under critical memory pressure, the guest
starts to kill processes. A guest monitor agent may scan 'oom_kill'
from /proc/vmstat, and reports the OOM KILL event. However, the agent
may be killed and we will loss this critical event(and the later
events).

For now we can also grep for magic words in guest kernel log from host
side. Rather than this unstable way, virtio balloon reports OOM-KILL
invocations instead.

Acked-by: David Hildenbrand <[e-mail]>
Signed-off-by: zhenwei pi <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/virtio/virtio_balloon.c

···
 		    pages_to_bytes(events[PSWPOUT]));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
+	update_stat(vb, idx++, VIRTIO_BALLOON_S_OOM_KILL, events[OOM_KILL]);
 
 #ifdef CONFIG_HUGETLB_PAGE
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_HTLB_PGALLOC,

include/uapi/linux/virtio_balloon.h

···
 #define VIRTIO_BALLOON_S_CACHES   7   /* Disk caches */
 #define VIRTIO_BALLOON_S_HTLB_PGALLOC  8  /* Hugetlb page allocations */
 #define VIRTIO_BALLOON_S_HTLB_PGFAIL   9  /* Hugetlb page allocation failures */
-#define VIRTIO_BALLOON_S_NR       10
+#define VIRTIO_BALLOON_S_OOM_KILL      10 /* OOM killer invocations */
+#define VIRTIO_BALLOON_S_NR       11
 
 #define VIRTIO_BALLOON_S_NAMES_WITH_PREFIX(VIRTIO_BALLOON_S_NAMES_prefix) { \
 	VIRTIO_BALLOON_S_NAMES_prefix "swap-in", \
···
 	VIRTIO_BALLOON_S_NAMES_prefix "available-memory", \
 	VIRTIO_BALLOON_S_NAMES_prefix "disk-caches", \
 	VIRTIO_BALLOON_S_NAMES_prefix "hugetlb-allocations", \
-	VIRTIO_BALLOON_S_NAMES_prefix "hugetlb-failures" \
+	VIRTIO_BALLOON_S_NAMES_prefix "hugetlb-failures", \
+	VIRTIO_BALLOON_S_NAMES_prefix "oom-kills" \
 }
 
 #define VIRTIO_BALLOON_S_NAMES VIRTIO_BALLOON_S_NAMES_WITH_PREFIX("")