diff --git a/services/core/java/com/android/server/am/ActivityManagerService.java b/services/core/java/com/android/server/am/ActivityManagerService.java
index f98327b3c1c18..0a80d4505d47b 100644
--- a/services/core/java/com/android/server/am/ActivityManagerService.java
+++ b/services/core/java/com/android/server/am/ActivityManagerService.java
@@ -3969,11 +3969,12 @@ public class ActivityManagerService extends IActivityManager.Stub
         final long callingId = Binder.clearCallingIdentity();
         try {
             synchronized (this) {
-                mProcessList.killAllBackgroundProcessesLocked();
-
+                // Allow memory level to go down (the flag needs to be set before updating oom adj)
+                // because this method is also used to simulate low memory.
                 mAllowLowerMemLevel = true;
+                mProcessList.killPackageProcessesLocked(null /* packageName */, -1 /* appId */,
+                        UserHandle.USER_ALL, ProcessList.CACHED_APP_MIN_ADJ, "kill all background");
 
-                updateOomAdjLocked();
                 doLowMemReportIfNeededLocked(null);
             }
         } finally {
diff --git a/services/core/java/com/android/server/am/ProcessList.java b/services/core/java/com/android/server/am/ProcessList.java
index 620fb2cd8553c..d02fd7387f598 100644
--- a/services/core/java/com/android/server/am/ProcessList.java
+++ b/services/core/java/com/android/server/am/ProcessList.java
@@ -2085,25 +2085,6 @@ public final class ProcessList {
         }
     }
 
-    void killAllBackgroundProcessesLocked() {
-        final ArrayList<ProcessRecord> procs = new ArrayList<>();
-        final int NP = mProcessNames.getMap().size();
-        for (int ip = 0; ip < NP; ip++) {
-            final SparseArray<ProcessRecord> apps = mProcessNames.getMap().valueAt(ip);
-            final int NA = apps.size();
-            for (int ia = 0; ia < NA; ia++) {
-                final ProcessRecord app = apps.valueAt(ia);
-                if (app.isPersistent()) {
-                    // We don't kill persistent processes.
-                    continue;
-                }
-                if (app.removed || app.setAdj >= ProcessList.CACHED_APP_MIN_ADJ) {
-                    procs.add(app);
-                }
-            }
-        }
-    }
-
     @GuardedBy("mService")
     boolean killPackageProcessesLocked(String packageName, int appId, int userId, int minOomAdj,
             String reason) {
@@ -2140,6 +2121,11 @@ public final class ProcessList {
 
                 // Skip process if it doesn't meet our oom adj requirement.
                 if (app.setAdj < minOomAdj) {
+                    // Note it is still possible to have a process with oom adj 0 in the killed
+                    // processes, but it does not mean misjudgment. E.g. a bound service process
+                    // and its client activity process are both in the background, so they are
+                    // collected to be killed. If the client activity is killed first, the service
+                    // may be scheduled to unbind and become an executing service (oom adj 0).
                     continue;
                 }