Add support for node down and softirq alert

roles/kube_prometheus_stack/files/jsonnet/legacy.libsonnet

@@ -174,6 +174,14 @@
                 severity: 'P5',
               },
             },
+            {
+              alert: 'NodeDown',
+              expr: 'up{job="node-exporter"} == 0',
+              'for': '2m',
+              labels: {
+                severity: 'P1',
+              },
+            },
           ],
         },
         {
@@ -226,6 +234,23 @@
               alertRule('dropped', '0', '0.75'),
             ],
         },
+        {
+          name: 'softirq',
+          rules: [
+            {
+              alert: 'NodeSoftirqRcu',
+              expr: 'rate(node_softirqs_total{vector="rcu"}[1m]) > 10000',
+              'for': '5m',
+              labels: {
+                severity: 'warning',
+              },
+              annotations: {
+                summary: 'High softirq rcu on node {{ $labels.instance }}: {{ $value }} ',
+                description: 'This can result in high software interrupt load on the node which can bring system performance down.',
+              },
+            },
+          ],
+        },
       ],
     },
   },