enb_mac: pre-allocate UE object for quicker PRACH/RAR handling

we've seen long delays in handling PRACH on low-power devices
such as the RPi4. This was mainly caused by long delays
in creating the UE object on the fly during PRACH handling.

This patch pre-allocates one UE object that is then used
during the PRACH.

srsenb/hdr/stack/mac/mac.h

@@ -136,7 +136,10 @@ private:
 
   /* Map of active UEs */
   std::map<uint16_t, std::unique_ptr<ue> > ue_db;
-  uint16_t                                 last_rnti = 0;
+  uint16_t                                 last_rnti = 70;
+
+  srslte::block_queue<std::unique_ptr<ue> > ue_pool; ///< Pool of pre-allocated UE objects
+  void                                      prealloc_ue(uint32_t nof_ue);
 
   uint8_t* assemble_rar(sched_interface::dl_sched_rar_grant_t* grants,
                         uint32_t                               nof_grants,

srsenb/hdr/stack/mac/ue.h

@@ -57,6 +57,8 @@ public:
 
   void set_tti(uint32_t tti);
 
+  uint16_t get_rnti() { return rnti; }
+
   uint32_t set_ta(int ta) override;
 
   void     start_ta() { ta_fsm.start(); };

srsenb/src/stack/mac/mac.cc

@@ -37,7 +37,6 @@ using namespace asn1::rrc;
 namespace srsenb {
 
 mac::mac() :
-  last_rnti(0),
   rar_pdu_msg(sched_interface::MAX_RAR_LIST),
   rar_payload(),
   common_buffers(SRSLTE_MAX_CARRIERS)
@@ -93,6 +92,9 @@ bool mac::init(const mac_args_t&        args_,
 
     reset();
 
+    // Pre-alloc UE objects for first attaching users
+    prealloc_ue(10);
+
     started = true;
   }
 
@@ -483,10 +485,12 @@ void mac::rach_detected(uint32_t tti, uint32_t enb_cc_idx, uint32_t preamble_idx
   log_h->step(tti);
   auto rach_tprof_meas = rach_tprof.start();
 
-  uint16_t rnti = allocate_rnti();
-
-  // Create new UE
-  std::unique_ptr<ue> ue_ptr{new ue(rnti, args.nof_prb, &scheduler, rrc_h, rlc_h, phy_h, log_h, cells.size())};
+  // Get pre-allocated UE object
+  if (ue_pool.empty()) {
+    Error("Ignoring RACH attempt. UE pool empty.\n");
+  }
+  auto     ue_ptr = ue_pool.wait_pop();
+  uint16_t rnti = ue_ptr->get_rnti();
 
   // Set PCAP if available
   if (pcap != nullptr) {
@@ -535,6 +539,18 @@ void mac::rach_detected(uint32_t tti, uint32_t enb_cc_idx, uint32_t preamble_idx
     log_h->info("RACH:  tti=%d, preamble=%d, offset=%d, temp_crnti=0x%x\n", tti, preamble_idx, time_adv, rnti);
     log_h->console("RACH:  tti=%d, preamble=%d, offset=%d, temp_crnti=0x%x\n", tti, preamble_idx, time_adv, rnti);
   });
+
+  // Allocate one new UE object in advance
+  prealloc_ue(1);
+}
+
+void mac::prealloc_ue(uint32_t nof_ue)
+{
+  for (uint32_t i = 0; i < nof_ue; i++) {
+    std::unique_ptr<ue> ptr = std::unique_ptr<ue>(
+        new ue(allocate_rnti(), args.nof_prb, &scheduler, rrc_h, rlc_h, phy_h, log_h, cells.size()));
+    ue_pool.push(std::move(ptr));
+  }
 }
 
 int mac::get_dl_sched(uint32_t tti_tx_dl, dl_sched_list_t& dl_sched_res_list)