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hodec2: fix candidate choices in congestion check 

Fix flaws in picking a candidate for congestion resolution, shown in
recently added tests.

- For TCH/H->TCH/F upgrading, do not favor moving to a weaker neighbor
  cell.

- When comparing dynamic timeslots on the same cell, favor a dynamic
  timeslot that frees an entire dyn TS even though the target rxlev
  differs.

Do not separate the passes for inter-cell and intra-cell candidates:
before, the inter-cell pass would already pick a candidate and start
handover, even though the subsequent intra-cell pass would have revealed
a better candidate. Join the intra-cell considerations into
pick_better_lchan_to_move().

The intra-cell pass was separate, because it would find the *weakest*
current rxlev, to give a TCH/H to TCH/F upgrade to the currently weakest
lchan.

Instead of the separate pass for weakest rxlev, in addition to the
target cell's rxlev, also consider the rxlev *change* in
pick_better_lchan_to_move(): For candidates that do not change the rxlev
(usually those that stay in the same cell) and that upgrade to TCH/F,
favor a candidate with weaker current rxlev.

Completely revisit the conditions in pick_better_lchan_to_move() to
yield the desired prioritization of candidate preferences.

In three handover tests, remove the "FAIL" comments and adjust to now
expect the actually desired behavior.

Related: SYS#5032
Change-Id: I2704899c85c35dfd4eba43468452483f40016ca2
This commit is contained in:
Neels Hofmeyr 2020-12-24 12:48:16 +01:00 committed by laforge
parent f447c4f3e3
commit 3f1c43e74b
4 changed files with 81 additions and 169 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

191
src/osmo-bsc/handover_decision_2.c
View File

@ -1384,11 +1384,6 @@ static unsigned int ts_usage_count(struct gsm_bts_trx_ts *ts)
return count;
}
static bool is_intra_cell(const struct ho_candidate *c)
{
return c->bts && (c->lchan->ts->trx->bts == c->bts);
}
static bool is_upgrade_to_tchf(const struct ho_candidate *c, uint8_t for_requirement)
{
return c->lchan
@ -1399,46 +1394,78 @@ static bool is_upgrade_to_tchf(const struct ho_candidate *c, uint8_t for_require
/* Given two candidates, pick the one that should rather be moved during handover.
* Return the better candidate in out-parameters best_cand and best_avg_db.
*/
static struct ho_candidate *pick_better_lchan_to_move(bool want_highest_db,
struct ho_candidate *a,
struct ho_candidate *b)
static struct ho_candidate *pick_better_lchan_to_move(struct ho_candidate *a,
struct ho_candidate *b,
uint8_t for_requirement)
{
int a_rxlev;
int b_rxlev;
int a_rxlev_change;
int b_rxlev_change;
struct ho_candidate *ret = a;
if (!a)
return b;
if (!b)
return a;
a_rxlev = a->rxlev_target + a->rxlev_afs_bias;
b_rxlev = b->rxlev_target + b->rxlev_afs_bias;
a_rxlev_change = a->rxlev_target - a->rxlev_current;
b_rxlev_change = b->rxlev_target - b->rxlev_current;
if (want_highest_db && a_rxlev < b_rxlev)
return b;
if (!want_highest_db && a_rxlev > b_rxlev)
/* Typically, a congestion related handover reduces RXLEV. If there is a candidate that actually improves RXLEV,
* prefer that, because it pre-empts a likely handover due to measurement results later. Also favor unchanged
* RXLEV over a loss of RXLEV (favor staying within the same cell over moving to a worse cell). */
if (a_rxlev_change >= 0 && a_rxlev_change > b_rxlev_change)
return a;
if (b_rxlev_change >= 0 && b_rxlev_change > a_rxlev_change)
return b;
/* The two lchans have identical ratings, prefer picking a dynamic timeslot: free PDCH and allow more timeslot
* type flexibility for further congestion resolution. */
if (lchan_is_on_dynamic_ts(b->lchan)) {
/* If both are dynamic, prefer one that completely (or to a higher degree) frees its timeslot. */
if (lchan_is_on_dynamic_ts(a->lchan)
&& ts_usage_count(a->lchan->ts) < ts_usage_count(b->lchan->ts))
if (a_rxlev_change < 0 && b_rxlev_change < 0) {
/* For handover that reduces RXLEV, favor the highest resulting RXLEV, AFS bias applied. */
int a_rxlev = a->rxlev_target + a->rxlev_afs_bias;
int b_rxlev = b->rxlev_target + b->rxlev_afs_bias;
if (a_rxlev > b_rxlev)
return a;
/* If both equally satisfy these preferences, it does not matter which one is picked.
* Give slight preference to moving later dyn TS, so that a free dyn TS may group with following static
* PDCH, though this depends on how the user configured the TS -- not harmful to do so anyway. */
return b;
if (b_rxlev > a_rxlev)
return b;
/* There is no target RXLEV difference between the two candidates. Let other factors influence the
* choice. */
}
return a;
/* Prefer picking a dynamic timeslot: free PDCH and allow more timeslot type flexibility for further
* congestion resolution. */
if (lchan_is_on_dynamic_ts(b->lchan)) {
unsigned int ac, bc;
if (!lchan_is_on_dynamic_ts(a->lchan))
return b;
/* Both are dynamic timeslots. Prefer one that completely (or to a higher degree) frees its
* timeslot. */
ac = ts_usage_count(a->lchan->ts);
bc = ts_usage_count(b->lchan->ts);
if (bc < ac)
return b;
if (ac < bc)
return a;
/* (If both are dynamic timeslots, favor moving the later dynamic timeslot. That is a vague preference
* for later dynamic TS to become PDCH and join up with plain PDCH that follow it -- not actually clear
* whether that helps, and depends on user's TS config. No harm done either way.) */
ret = b;
}
/* When upgrading TCH/H to TCH/F, favor moving a TCH/H with lower current rxlev, because presumably that
* one benefits more from a higher bandwidth. */
if (is_upgrade_to_tchf(a, for_requirement) && is_upgrade_to_tchf(b, for_requirement)) {
if (b->rxlev_current < a->rxlev_current)
return b;
if (a->rxlev_current < b->rxlev_current)
return a;
}
return ret;
}
static struct ho_candidate *pick_best_candidate(struct ho_candidate *clist, int clist_len,
bool want_highest_db,
bool omit_intra_cell_upgrade_to_tchf,
bool only_intra_cell_upgrade_to_tchf,
uint8_t for_requirement)
{
struct ho_candidate *result = NULL;
@ -1446,8 +1473,6 @@ static struct ho_candidate *pick_best_candidate(struct ho_candidate *clist, int
for (i = 0; i < clist_len; i++) {
struct ho_candidate *c = &clist[i];
bool intra_cell;
bool upgrade_to_tch_f;
/* For multiple passes of congestion resolution, already handovered candidates are marked by lchan =
* NULL. (though at the time of writing, multiple passes of congestion resolution are DISABLED.) */
@ -1461,23 +1486,13 @@ static struct ho_candidate *pick_best_candidate(struct ho_candidate *clist, int
if (!(c->requirements & for_requirement))
continue;
intra_cell = is_intra_cell(c);
upgrade_to_tch_f = is_upgrade_to_tchf(c, for_requirement);
if (only_intra_cell_upgrade_to_tchf
&& !(intra_cell && upgrade_to_tch_f))
continue;
if (omit_intra_cell_upgrade_to_tchf
&& (intra_cell && upgrade_to_tch_f))
continue;
/* improve AHS */
if (upgrade_to_tch_f)
if (is_upgrade_to_tchf(c, for_requirement))
c->rxlev_afs_bias = ho_get_hodec2_afs_bias_rxlev(c->bts->ho);
else
c->rxlev_afs_bias = 0;
result = pick_better_lchan_to_move(want_highest_db, result, c);
result = pick_better_lchan_to_move(result, c, for_requirement);
}
return result;
@ -1643,11 +1658,7 @@ next_b1:
/* TODO: attempt inter-BSC HO if no local cells qualify, and rely on the remote BSS to
* deny receiving the handover if it also considers itself congested. Maybe do that only
* when the cell is absolutely full, i.e. not only min-free-slots. (x) */
best_cand = pick_best_candidate(clist, candidates,
/* want_highest_db */ true,
/* omit_intra_cell_upgrade_to_tchf */ true,
/* only_intra_cell_upgrade_to_tchf */ false,
REQUIREMENT_B_MASK);
best_cand = pick_best_candidate(clist, candidates, REQUIREMENT_B_MASK);
if (best_cand) {
any_ho = 1;
LOGPHOCAND(best_cand, LOGL_DEBUG, "Best candidate: RX level %d%s\n",
@ -1675,55 +1686,13 @@ next_b1:
goto exit;
}
#if 0
next_b2:
#endif
/* For upgrading TCH/H to TCH/F within the same cell, we want to pick the *lowest* average rxlev: for staying
* within the same cell, give the MS with the worst service more bandwidth. When staying within the same cell,
* the target avg rxlev is identical to the source lchan rxlev, so it is fine to use the same avg rxlev value,
* but simply pick the lowest one.
* Upgrading TCH/H channels obviously only applies when TCH/H is actually congested. */
if (tchh_congestion > 0)
best_cand = pick_best_candidate(clist, candidates,
/* want_highest_db */ false,
/* omit_intra_cell_upgrade_to_tchf */ false,
/* only_intra_cell_upgrade_to_tchf */ true,
REQUIREMENT_B_MASK);
if (best_cand) {
any_ho = 1;
LOGPHOCAND(best_cand, LOGL_INFO, "Worst candidate: RX level %d from TCH/H -> TCH/F%s\n",
rxlev2dbm(best_cand->rxlev_target),
best_cand->rxlev_afs_bias ? " (applied AHS -> AFS rxlev bias)" : "");
trigger_ho(best_cand, best_cand->requirements & REQUIREMENT_B_MASK);
#if 0
/* if there is still congestion, mark lchan as deleted
* and redo this process */
tchh_congestion--;
if (tchh_congestion > 0) {
delete_lchan = best_cand->lchan;
best_cand = NULL;
goto next_b2;
}
#else
/* must exit here, because triggering handover/assignment
* will cause change in requirements. more check for this
* bts is performed in the next iteration.
*/
#endif
goto exit;
}
#if 0
next_c1:
#endif
/* Select best candidate that balances congestion.
* Again no remote BSS.
* Again no TCH/H -> F upgrades within the same cell. */
best_cand = pick_best_candidate(clist, candidates,
/* want_highest_db */ true,
/* omit_intra_cell_upgrade_to_tchf */ true,
/* only_intra_cell_upgrade_to_tchf */ false,
REQUIREMENT_C_MASK);
best_cand = pick_best_candidate(clist, candidates, REQUIREMENT_C_MASK);
if (best_cand) {
any_ho = 1;
LOGPHOCAND(best_cand, LOGL_INFO, "Best candidate: RX level %d%s\n",
@ -1754,46 +1723,6 @@ next_c1:
LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a best candidate that fulfills requirement C"
" (omitting change from AHS to AFS)\n");
#if 0
next_c2:
#endif
/* Look for upgrading TCH/H to TCH/F within the same cell, which balances congestion, again upgrade the TCH/H
* lchan that has the worst reception: */
if (tchh_congestion > 0)
best_cand = pick_best_candidate(clist, candidates,
/* want_highest_db */ false,
/* omit_intra_cell_upgrade_to_tchf */ false,
/* only_intra_cell_upgrade_to_tchf */ true,
REQUIREMENT_C_MASK);
/* perform handover, if there is a candidate */
if (best_cand) {
any_ho = 1;
LOGPHOCAND(best_cand, LOGL_INFO, "Worst candidate: RX level %d from TCH/H -> TCH/F%s\n",
rxlev2dbm(best_cand->rxlev_target),
best_cand->rxlev_afs_bias ? " (applied AHS -> AFS rxlev bias)" : "");
trigger_ho(best_cand, best_cand->requirements & REQUIREMENT_C_MASK);
#if 0
/* if there is still congestion, mark lchan as deleted
* and redo this process */
tchh_congestion--;
if (tchh_congestion > 0) {
delete_lchan = best_cand->lchan;
best_cand = NULL;
goto next_c2;
}
#else
/* must exit here, because triggering handover/assignment
* will cause change in requirements. more check for this
* bts is performed in the next iteration.
*/
#endif
goto exit;
}
LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a worst candidate that fulfills requirement C,"
" selecting candidates that change from AHS to AFS only\n");
exit:
/* free array */
talloc_free(clist);

8
tests/handover/test_amr_tch_h_to_f_congestion_two_cells.ho_vty
View File

@ -12,12 +12,6 @@ set-ts-use trx 1 0 states * - - - TCH/H- - - *
meas-rep repeat 10 lchan 1 0 4 0 rxlev 30 rxqual 0 ta 0 neighbors 20
expect-no-chan
congestion-check
# FAIL: bts 1 has better rxlev, so the call should stay in bts 1. Instead, a handover to bts 0 happens.
expect-ho from lchan 1 0 4 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - - - *
expect-ts-use trx 1 0 states * - - - - - - *
# the fail continues: later the better rxqual does *another* ho back to the original cell
meas-rep lchan 0 0 1 0 rxlev 20 rxqual 0 ta 0 neighbors 30
expect-ho from lchan 0 0 1 0 to lchan 1 0 1 0
expect-ho from lchan 1 0 4 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - - - *
expect-ts-use trx 1 0 states * TCH/F - - - - - *

46
tests/handover/test_congestion_intra_vs_inter_cell.ho_vty
View File

@ -12,11 +12,9 @@ meas-rep lchan 0 0 6 0 rxlev 30 rxqual 0 ta 0 neighbors 20
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
# FAIL! should favor upgrading the weaker TS 6.
expect-ho from lchan 0 0 5 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - - TCH/H- *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 6 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - TCH/H- - *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *
@ -29,10 +27,9 @@ meas-rep lchan 0 0 6 0 rxlev 31 rxqual 0 ta 0 neighbors 20
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
expect-ho from lchan 0 0 5 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - - TCH/H- *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 5 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - - TCH/H- *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *
@ -45,11 +42,9 @@ meas-rep lchan 0 0 6 0 rxlev 31 rxqual 0 ta 0 neighbors 21
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
# FAIL! should favor upgrading the weaker TS 5.
expect-ho from lchan 0 0 6 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - TCH/H- - *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 5 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - - TCH/H- *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *
@ -62,10 +57,9 @@ meas-rep lchan 0 0 6 0 rxlev 31 rxqual 0 ta 0 neighbors 20
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
expect-ho from lchan 0 0 5 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - - TCH/H- *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 5 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - - TCH/H- *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *
@ -78,11 +72,9 @@ meas-rep lchan 0 0 6 0 rxlev 31 rxqual 0 ta 0 neighbors 31
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
# FAIL! should favor upgrading the weaker TS 5.
expect-ho from lchan 0 0 6 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - TCH/H- - *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 5 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - - TCH/H- *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *
@ -95,11 +87,9 @@ meas-rep lchan 0 0 6 0 rxlev 30 rxqual 0 ta 0 neighbors 30
expect-no-chan
congestion-check
# FAIL! moving to a weaker neighbor, should move to TCH/F in current cell instead.
# FAIL! should favor upgrading the weaker TS 6.
expect-ho from lchan 0 0 5 0 to lchan 1 0 1 0
expect-ts-use trx 0 0 states * - - - - - TCH/H- *
expect-ts-use trx 1 0 states * TCH/F - - - - - *
expect-ho from lchan 0 0 6 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F - - - TCH/H- - *
expect-ts-use trx 1 0 states * - - - - - - *
# clear measurements for next run
set-ts-use trx 0 0 states * - - - - - - *

5
tests/handover/test_dyn_ts_favor_moving_half_used_tch_h.ho_vty
View File

@ -41,6 +41,5 @@ meas-rep lchan 0 0 3 0 rxlev 33 rxqual 0 ta 0
meas-rep lchan 0 0 4 0 rxlev 32 rxqual 0 ta 0
meas-rep lchan 0 0 4 1 rxlev 31 rxqual 0 ta 0
congestion-check
# FAIL: should still pick lchan 0 0 3 0!
expect-ho from lchan 0 0 4 1 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F TCH/HH TCH/H- TCH/H- pdch - -
expect-ho from lchan 0 0 3 0 to lchan 0 0 1 0
expect-ts-use trx 0 0 states * TCH/F TCH/HH pdch TCH/HH pdch - -