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paging: Replace reqs waiting for retransmission with new incoming inital req if queue is full 

If queue size (in transmit delay of requests) is too long (above
threshold) when a new initial incoming request arrives, instead of
directly discarding it, see if we can drop a pending retransmission and
insert the new one instead, in order to avoid losing initial requests.

This is done under the assumption that is it more important to transmit
intial requests than to retransmit already transmitted ones. The
rationale is that there's lower chances than an MS which didn't answer
lately will answer now (aka being reachable at the cell), so it's better
to allocate resources for new requests (new MS) which may be available
in the cell.

Related: OS#5552
Change-Id: Idfd93254ae456b1ee08416e05479488299dd063d
This commit is contained in:
Pau Espin 2022-11-28 13:10:21 +01:00
parent 671811f4fc
commit 4ba57df5b2
1 changed files with 41 additions and 7 deletions
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48
src/osmo-bsc/paging.c
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@ -416,16 +416,10 @@ static int _paging_request(const struct bsc_paging_params *params, struct gsm_bt
unsigned int reqs_before = 0, reqs_before_same_pgroup = 0;
uint8_t pgroup = gsm0502_calc_paging_group(&bts->si_common.chan_desc,
str_to_imsi(params->bsub->imsi));
bool queue_full = false;
rate_ctr_inc(rate_ctr_group_get_ctr(bts->bts_ctrs, BTS_CTR_PAGING_ATTEMPTED));
/* Don't try to queue more requests than we can realistically handle within X3113 seconds,
* see PAGING_THRESHOLD_X3113_DEFAULT_SEC. */
if (paging_pending_requests_nr(bts) > paging_estimate_available_slots(bts, x3113_s)) {
rate_ctr_inc(rate_ctr_group_get_ctr(bts->bts_ctrs, BTS_CTR_PAGING_OVERLOAD));
return -ENOSPC;
}
/* Find if we already have one for the given subscriber on this BTS: */
if (bsc_subscr_find_req_by_bts(params->bsub, bts)) {
LOG_PAGING_BTS(params, bts, DPAG, LOGL_INFO, "Paging request already pending for this subscriber\n");
@ -433,6 +427,21 @@ static int _paging_request(const struct bsc_paging_params *params, struct gsm_bt
return -EEXIST;
}
/* Don't try to queue more requests than we can realistically handle within X3113 seconds,
* see PAGING_THRESHOLD_X3113_DEFAULT_SEC. */
if (paging_pending_requests_nr(bts) > paging_estimate_available_slots(bts, x3113_s)) {
struct gsm_paging_request *last_req;
rate_ctr_inc(rate_ctr_group_get_ctr(bts->bts_ctrs, BTS_CTR_PAGING_OVERLOAD));
OSMO_ASSERT(!llist_empty(&bts_entry->pending_requests));
last_req = llist_last_entry(&bts_entry->pending_requests, struct gsm_paging_request, entry);
if (last_req->attempts == 0) {
/* Queue is full of initial requests, discard new one */
return -ENOSPC;
}
/* There's at least 1 retrans. Flag to remove first retrans in queue once found below. */
queue_full = true;
}
/* Find the last not-yet-ever-once-transmitted request; the new request
* will be added immediately after it, giving higher prio to initial
* transmissions (no retrans). This avoids new subscribers being paged to
@ -464,6 +473,31 @@ static int _paging_request(const struct bsc_paging_params *params, struct gsm_bt
reqs_before_same_pgroup++;
}
/* Need to drop a retrans from the queue if possible, in order to make space for the new initial req. */
if (queue_full) {
if (!last_initial_req) {
/* Queue is full of retransmissions only, drop first one to free some space: */
struct gsm_paging_request *first_retrans_req;
first_retrans_req = llist_first_entry_or_null(&bts_entry->pending_requests, struct gsm_paging_request, entry);
OSMO_ASSERT(first_retrans_req);
reqs_before--;
if (first_retrans_req->pgroup == pgroup)
reqs_before_same_pgroup--;
paging_remove_request(first_retrans_req);
} else {
struct gsm_paging_request *last_req;
last_req = llist_last_entry(&bts_entry->pending_requests, struct gsm_paging_request, entry);
OSMO_ASSERT(last_req);
if (&last_initial_req->entry == llist_last(&bts_entry->pending_requests)) {
/* Queue is full of initial requests only, discard the current new req: */
return -ENOSPC;
}
/* We checked beforehand (set queue_full = true) that queue contains a retrans.
* Hence queue contains [1..N intials, 1..N retrans]: Drop first retransmission to free some space: */
paging_remove_request(llist_entry(last_initial_req->entry.next, struct gsm_paging_request, entry));
}
}
LOG_PAGING_BTS(params, bts, DPAG, LOGL_DEBUG, "Start paging\n");
req = talloc_zero(tall_paging_ctx, struct gsm_paging_request);
OSMO_ASSERT(req);