hodec2: fix congestion resolution on dyn TS

For handover algorithm 2, properly figure out what effects the target
cell will see for the *other* TCH kind when a handover would occupy a
dynamic timeslot.

Before this, only TCH/F or TCH/H would be regarded at a time. This
introduces detection of whether a dynamic timeslot would be occupied by
a handover, and how losing one unused dynamic timeslot affects the
congestion situation for the TCH kind that is not targeted by the
handover.

In other words, if a handover to TCH/F causes congestion in TCH/H
because of a dynamic timeslot becoming occupied, the handover will not
be performed. Before this, oscillation situations could occur.

A subsequent patch will do the same for congestion balancing.

Related: SYS#5297
Change-Id: I1536b60f03cb0aeb6ba14a72b518aec82fa572fe

src/osmo-bsc/handover_decision_2.c

@@ -40,6 +40,7 @@
 #include <osmocom/bsc/neighbor_ident.h>
 #include <osmocom/bsc/timeslot_fsm.h>
 #include <osmocom/bsc/bts.h>
+#include <osmocom/bsc/lchan_select.h>
 
 #define LOGPHOBTS(bts, level, fmt, args...) \
 	LOGP(DHODEC, level, "(BTS %u) " fmt, bts->nr, ## args)
@@ -118,6 +119,9 @@ struct ho_candidate {
 		int min_free_tchf;
 		int free_tchh;
 		int min_free_tchh;
+		/* Effects of occupying a dynamic timeslot: */
+		int next_tchf_reduces_tchh;
+		int next_tchh_reduces_tchf;
 	} target;
 };
 
@@ -637,11 +641,15 @@ static void check_requirements(struct ho_candidate *c)
 	/* the minimum free timeslots that are defined for this cell must
 	 * be maintained _after_ handover/assignment */
 	if (requirement & REQUIREMENT_A_TCHF) {
-		if (c->target.free_tchf - 1 >= c->target.min_free_tchf)
+		if ((c->target.free_tchf - 1) >= c->target.min_free_tchf
+		    && (!c->target.next_tchf_reduces_tchh
+			|| (c->target.free_tchh - c->target.next_tchf_reduces_tchh) >= c->target.min_free_tchh))
 			requirement |= REQUIREMENT_B_TCHF;
 	}
 	if (requirement & REQUIREMENT_A_TCHH) {
-		if (c->target.free_tchh - 1 >= c->target.min_free_tchh)
+		if ((c->target.free_tchh - 1) >= c->target.min_free_tchh
+		    && (!c->target.next_tchh_reduces_tchf
+			|| (c->target.free_tchf - c->target.next_tchh_reduces_tchf) >= c->target.min_free_tchf))
 			requirement |= REQUIREMENT_B_TCHH;
 	}
 
@@ -894,6 +902,8 @@ static inline void debug_candidate(struct ho_candidate *candidate)
 
 static void candidate_set_free_tch(struct ho_candidate *c)
 {
+	struct gsm_lchan *next_lchan;
+
 	c->current.free_tch = bts_count_free_ts(c->current.bts, c->current.lchan->ts->pchan_is);
 	switch (c->current.lchan->ts->pchan_is) {
 	case GSM_PCHAN_TCH_F:
@@ -909,6 +919,22 @@ static void candidate_set_free_tch(struct ho_candidate *c)
 	c->target.min_free_tchf = ho_get_hodec2_tchf_min_slots(c->target.bts->ho);
 	c->target.free_tchh = bts_count_free_ts(c->target.bts, GSM_PCHAN_TCH_H);
 	c->target.min_free_tchh = ho_get_hodec2_tchh_min_slots(c->target.bts->ho);
+
+	/* Would the next TCH/F lchan occupy a dynamic timeslot that currently counts for free TCH/H timeslots? */
+	next_lchan = lchan_avail_by_type(c->target.bts, GSM_LCHAN_TCH_F);
+	if (next_lchan && next_lchan->ts->pchan_on_init == GSM_PCHAN_TCH_F_TCH_H_PDCH)
+		c->target.next_tchf_reduces_tchh = 2;
+	else
+		c->target.next_tchf_reduces_tchh = 0;
+
+	/* Would the next TCH/H lchan occupy a dynamic timeslot that currently counts for free TCH/F timeslots?
+	 * Note that a dyn TS already in TCH/H mode (half occupied) would not reduce free TCH/F. */
+	next_lchan = lchan_avail_by_type(c->target.bts, GSM_LCHAN_TCH_H);
+	if (next_lchan && next_lchan->ts->pchan_on_init == GSM_PCHAN_TCH_F_TCH_H_PDCH
+	    && next_lchan->ts->pchan_is != GSM_PCHAN_TCH_H)
+		c->target.next_tchh_reduces_tchf = 1;
+	else
+		c->target.next_tchh_reduces_tchf = 0;
 }
 
 /* add candidate for re-assignment within the current cell */

tests/handover/test_dyn_ts_congestion_tch_f_vs_tch_h.ho_vty

@@ -20,12 +20,8 @@ meas-rep lchan * * * * rxlev 40 rxqual 0 ta 0 neighbors 30
 expect-no-chan
 
 meas-rep lchan 0 0 1 0 rxlev 20 rxqual 0 ta 0 neighbors 40
-# FAIL: should not handover because that results in congestion on TCH/H in bts 1
-expect-ho from lchan 0 0 1 0 to lchan 1 0 5 0
-
-expect-ts-use trx 0 0 states * - - - pdch pdch pdch *
-expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/HH TCH/F pdch *
-
+# no handover because that results in congestion on TCH/H in bts 1
+expect-no-chan
 
 ###
 
@@ -34,11 +30,9 @@ set-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/HH TCH/F pdch *
 meas-rep lchan * * * * rxlev 40 rxqual 0 ta 0 neighbors 30
 
 congestion-check
-# FAIL: really weird: handover one TCH/H to the same cell to TCH/F, taking up another dyn TS.
-# TCH/H congestion hence actually gets worse.
-expect-ho from lchan 1 0 4 1 to lchan 1 0 6 0
-expect-ts-use trx 0 0 states * - - - pdch pdch pdch *
-expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/H- TCH/F TCH/F *
+expect-ho from lchan 1 0 4 1 to lchan 0 0 4 0
+expect-ts-use trx 0 0 states * - - - TCH/H- pdch pdch *
+expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/H- TCH/F pdch *
 
 
 ###
@@ -65,12 +59,9 @@ meas-rep lchan 1 * * * rxlev 40 rxqual 0 ta 0 neighbors 30
 expect-no-chan
 
 meas-rep lchan 0 * * * rxlev 30 rxqual 0 ta 0 neighbors 40
-# FAIL: back to bts 1 because of rxlev, plus moving TCH/H to TCH/F!?
-expect-ho from lchan 0 0 4 0 to lchan 1 0 4 0
-
-expect-ts-use trx 0 0 states * - - - pdch pdch pdch *
-expect-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/F TCH/F TCH/F *
-
+# no HO because the target is congested on TCH/H. Moving to TCH/F would also
+# reduce TCH/H lchans because it would convert another dyn TS.
+expect-no-chan
 
 ###
 
@@ -78,5 +69,6 @@ set-ts-use trx 0 0 states * - - - pdch pdch pdch *
 set-ts-use trx 1 0 states * TCH/F TCH/F TCH/F TCH/F TCH/F TCH/F *
 
 congestion-check
+# FAIL: TCH/F occupy dynamic timeslots -- should hand over to bts 0 to free a
+# dyn TS and reduce TCH/H congestion.
 expect-no-chan
-# Stable situation now only because TCH/F has min-free-slots set to 0