paging: Avoid queueing more than 60 second estimated requests

Reaching this point will only make system load (CPU, mem) grow, making
it hard for the process to keep up with work to do, with no benefit
since the requests will anyway be scheduled too late.

Related: SYS#5922
Change-Id: I6523c6816a4d16b71084d004e979be40cf0aeeb0

include/osmocom/bsc/bts.h

@@ -55,6 +55,7 @@ enum bts_counter_id {
 	BTS_CTR_PAGING_EXPIRED,
 	BTS_CTR_PAGING_NO_ACTIVE_PAGING,
 	BTS_CTR_PAGING_MSC_FLUSH,
+	BTS_CTR_PAGING_OVERLOAD,
 	BTS_CTR_CHAN_ACT_TOTAL,
 	BTS_CTR_CHAN_ACT_SDCCH,
 	BTS_CTR_CHAN_ACT_TCH,

include/osmocom/bsc/paging.h

@@ -101,6 +101,8 @@ struct gsm_paging_request {
 struct gsm_bts_paging_state {
 	/* pending requests */
 	struct llist_head pending_requests;
+	/* Number of requests in pending_requests_len */
+	unsigned int pending_requests_len;
 	struct gsm_bts *bts;
 
 	struct osmo_timer_list work_timer;

src/osmo-bsc/bts.c

@@ -1083,6 +1083,9 @@ const struct rate_ctr_desc bts_ctr_description[] = {
 	[BTS_CTR_PAGING_MSC_FLUSH] = \
 		{ "paging:msc_flush",
 		  "Paging flushed due to MSC Reset BSSMAP message" },
+	[BTS_CTR_PAGING_OVERLOAD] = \
+		{ "paging:overload",
+		  "Paging dropped due to BSC Paging queue overload" },
 	[BTS_CTR_CHAN_ACT_TOTAL] = \
 		{ "chan_act:total",
 		  "Total number of Channel Activations" },

src/osmo-bsc/paging.c

@@ -61,6 +61,8 @@ void *tall_paging_ctx = NULL;
 /* How many paging requests to Tx on RSL at max before going back to main loop */
 #define MAX_PAGE_REQ_PER_ITER 10
 
+#define MAX_TX_DELAY_TIME_SEC 60
+
 /* How often to attempt sending new paging requests (initial, not retrans): 250ms */
 static const struct timespec initial_period = {
 	.tv_sec = 0,
@@ -87,6 +89,7 @@ static void paging_remove_request(struct gsm_bts_paging_state *paging_bts,
 	to_be_deleted->bsub->active_paging_requests--;
 	osmo_timer_del(&to_be_deleted->T3113);
 	llist_del(&to_be_deleted->entry);
+	paging_bts->pending_requests_len--;
 	bsc_subscr_put(to_be_deleted->bsub, BSUB_USE_PAGING_REQUEST);
 	talloc_free(to_be_deleted);
 	if (llist_empty(&paging_bts->pending_requests))
@@ -400,6 +403,12 @@ static int _paging_request(const struct bsc_paging_params *params, struct gsm_bt
 
 	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 MAX_TX_DELAY_TIME_SEC seconds */
+	if (paging_pending_requests_nr(bts) > paging_estimate_available_slots(bts, MAX_TX_DELAY_TIME_SEC)) {
+		rate_ctr_inc(rate_ctr_group_get_ctr(bts->bts_ctrs, BTS_CTR_PAGING_OVERLOAD));
+		return -ENOSPC;
+	}
+
 	/* Iterate list of pending requests to find if we already have one for
 	 * the given subscriber. While on it, find the last
 	 * not-yet-ever-once-transmitted request; the new request will be added
@@ -440,6 +449,7 @@ static int _paging_request(const struct bsc_paging_params *params, struct gsm_bt
 	req->msc = params->msc;
 	osmo_timer_setup(&req->T3113, paging_T3113_expired, req);
 
+	bts_entry->pending_requests_len++;
 	/* there's no initial req (attempts==0), add to the start of the list */
 	if (last_initial_req == NULL)
 		llist_add(&req->entry, &bts_entry->pending_requests);
@@ -611,13 +621,7 @@ void paging_update_buffer_space(struct gsm_bts *bts, uint16_t free_slots)
 /*! Count the number of pending paging requests on given BTS */
 unsigned int paging_pending_requests_nr(struct gsm_bts *bts)
 {
-	unsigned int requests = 0;
-	struct gsm_paging_request *req;
-
-	llist_for_each_entry(req, &bts->paging.pending_requests, entry)
-		++requests;
-
-	return requests;
+	return bts->paging.pending_requests_len;
 }
 
 /*! Flush all paging requests at a given BTS for a given MSC (or NULL if all MSC should be flushed). */