[PG] add checking for increasing ingress PG drop packet counters (#4176)

Add ability to check PG dropped packet counters

- How did you do it?
Use existing QoS infrastructure to check dropped packets per PG per Interface

- How did you verify/test it?
run test:
py.test qos/test_qos_sai.py::TestQosSai::testQosSaiPfcXoffLimit --inventory "../ansible/inventory, ../ansible/veos" --host-pattern (dut)-ptf32 --module-path ../ansible/library/ --testbed (dut)-ptf32 --testbed_file ../ansible/testbed.csv

tests/saitests/sai_qos_tests.py

@@ -25,6 +25,7 @@
                     port_list,
                     sai_thrift_read_port_watermarks,
                     sai_thrift_read_pg_counters,
+                    sai_thrift_read_pg_drop_counters,
                     sai_thrift_read_pg_shared_watermark,
                     sai_thrift_read_buffer_pool_watermark,
                     sai_thrift_read_headroom_pool_watermark,
@@ -674,6 +675,13 @@ def runTest(self):
         # get counter names to query
         ingress_counters, egress_counters = get_counter_names(sonic_version)
 
+        # get a snapshot of PG drop packets counter
+        if '201811' not in sonic_version:
+            # According to SONiC configuration lossless dscps are classified as follows:
+            # dscp  3 -> pg 3
+            # dscp  4 -> pg 4
+            pg_dropped_cntrs_old = sai_thrift_read_pg_drop_counters(self.client, port_list[src_port_id])
+
         # Prepare IP packet data
         ttl = 64
         default_packet_length = 64
@@ -795,6 +803,12 @@ def runTest(self):
             for cntr in egress_counters:
                 assert(xmit_counters[cntr] == xmit_counters_base[cntr])
 
+            if '201811' not in sonic_version:
+                pg_dropped_cntrs = sai_thrift_read_pg_drop_counters(self.client, port_list[src_port_id])
+                logging.info("Dropped packet counters on port #{} :{} packets, current dscp: {}".format(src_port_id, pg_dropped_cntrs[dscp], dscp))
+                # Check that counters per lossless PG increased
+                assert pg_dropped_cntrs[dscp] > pg_dropped_cntrs_old[dscp]
+
         finally:
             sai_thrift_port_tx_enable(self.client, asic_type, [dst_port_id])
 

tests/saitests/switch.py

@@ -747,6 +747,28 @@ def sai_thrift_read_pg_counters(client, port_id):
 
     return pg_cntrs
 
+def sai_thrift_read_pg_drop_counters(client, port_id):
+    pg_cntr_ids = [
+        SAI_INGRESS_PRIORITY_GROUP_STAT_DROPPED_PACKETS
+    ]
+
+    # fetch pg ids under port id
+    pg_ids = []
+    port_attrs = client.sai_thrift_get_port_attribute(port_id)
+    attrs = port_attrs.attr_list
+    for attr in attrs:
+        if attr.id == SAI_PORT_ATTR_INGRESS_PRIORITY_GROUP_LIST:
+            for pg_id in attr.value.objlist.object_id_list:
+                pg_ids.append(pg_id)
+
+    # get counter values of counter ids of interest under each pg
+    pg_cntrs = []
+    for pg_id in pg_ids:
+        cntr_vals = client.sai_thrift_get_pg_stats(pg_id, pg_cntr_ids, len(pg_cntr_ids))
+        pg_cntrs.append(cntr_vals[0])
+
+    return pg_cntrs
+
 def sai_thrift_read_pg_shared_watermark(client, port_id):
     pg_cntr_ids=[
         SAI_INGRESS_PRIORITY_GROUP_STAT_SHARED_WATERMARK_BYTES