lmkd: Kill cached apps when thrashing or below low watermark

With Android U removing AMS kills, lmkd has additional duty to kill
cached apps which previously were killed by AMS. The former logic is
not proactive enough and leads to too many cached apps contributing
to memory pressure.
Implement additional logic to kill cached apps (excluding previous
foreground apps) when low watermark is breached or when device is
thrashing.

Bug: 300660611
Change-Id: I356eac1fe6d44dad292a7ea2fadee69a5be61479
Signed-off-by: Suren Baghdasaryan <surenb@google.com>

lmkd.cpp

@@ -93,6 +93,7 @@ static inline void trace_kill_end() {}
 #define PROC_STATUS_SWAP_FIELD "VmSwap:"
 
 #define PERCEPTIBLE_APP_ADJ 200
+#define PREVIOUS_APP_ADJ 700
 
 /* Android Logger event logtags (see event.logtags) */
 #define KILLINFO_LOG_TAG 10195355
@@ -2847,6 +2848,15 @@ static void mp_event_psi(int data, uint32_t events, struct polling_params *poll_
         }
     }
 
+    /* Check if a cached app should be killed */
+    if (kill_reason == NONE && wmark < WMARK_HIGH) {
+        /* TODO: introduce a new kill reason */
+        kill_reason = LOW_MEM_AND_SWAP;
+        snprintf(kill_desc, sizeof(kill_desc), "%s watermark is breached",
+            wmark < WMARK_LOW ? "min" : "low");
+        min_score_adj = PREVIOUS_APP_ADJ + 1;
+    }
+
     /* Kill a process if necessary */
     if (kill_reason != NONE) {
         struct kill_info ki = {