pdcp,rlc: increase number of PDCP SDUs for notification from RLC

the current value of 256 limits the number of PDCP SDUs that can be
notified from RLC. The limit is quickly hit when too many SDUs
are in flight. This can cause unwanted log entries and weird PDCP
behaviour.

the patch increases the value to 1024, which still can be too few if
many smaller SDUs are traveling.

The patch also set the log level to warning to quicker spot
misconfigs in logs.

Fixes #2616

lib/include/srsran/interfaces/pdcp_interface_types.h

@@ -170,8 +170,9 @@ struct pdcp_lte_state_t {
 };
 
 // Custom type for interface between PDCP and RLC to convey SDU delivery status
-#define MAX_SDUS_PER_RLC_PDU (256) // default to RLC SDU queue length
-#define MAX_SDUS_TO_NOTIFY (MAX_SDUS_PER_RLC_PDU) // Arbitrarily chosen limit
+// Arbitrarily chosen limit, optimal value depends on the RLC (pollPDU) and PDCP config, channel BLER,
+// traffic characterisitcs, etc. The chosen value has been tested with 100 PRB bi-dir TCP
+#define MAX_SDUS_TO_NOTIFY (1024)
 typedef srsran::bounded_vector<uint32_t, MAX_SDUS_TO_NOTIFY> pdcp_sn_vector_t;
 
 } // namespace srsran

lib/include/srsran/upper/pdcp_entity_lte.h

@@ -83,7 +83,7 @@ private:
 
   uint32_t                                   count = 0;
   uint32_t                                   bytes = 0;
-  uint32_t                                   fms   = 0;
+  uint32_t                                   fms   = 0; // SN of the first missing PDCP SDU
   uint32_t                                   lms   = 0;
   srsran::circular_array<sdu_data, capacity> sdus;
 };

lib/include/srsran/upper/rlc_am_lte.h

@@ -345,6 +345,9 @@ private:
 
     // Mutexes
     std::mutex mutex;
+
+    // default to RLC SDU queue length
+    const uint32_t MAX_SDUS_PER_RLC_PDU = RLC_TX_QUEUE_LEN;
   };
 
   // Receiver sub-class

lib/src/upper/pdcp_entity_lte.cc

@@ -148,7 +148,7 @@ void pdcp_entity_lte::write_sdu(unique_byte_buffer_t sdu, int upper_sn)
   if (!rlc->rb_is_um(lcid) and is_drb()) {
     if (not store_sdu(used_sn, sdu)) {
       // Could not store the SDU, discarding
-      logger.info("Could not store SDU. Discarding %d\n", used_sn);
+      logger.warning("Could not store SDU. Discarding SN=%d", used_sn);
       return;
     }
   }
@@ -688,7 +688,7 @@ bool pdcp_entity_lte::store_sdu(uint32_t sn, const unique_byte_buffer_t& sdu)
 // Discard Timer Callback (discardTimer)
 void pdcp_entity_lte::discard_callback::operator()(uint32_t timer_id)
 {
-  parent->logger.debug("Discard timer expired for PDU with SN = %d", discard_sn);
+  parent->logger.info("Discard timer for SN=%d expired", discard_sn);
 
   // Notify the RLC of the discard. It's the RLC to actually discard, if no segment was transmitted yet.
   parent->rlc->discard_sdu(parent->lcid, discard_sn);