alloc: Change tx_window optimization strategy

Currently each tx_window combination is checked only once by using a
set containing the sets of TX slots that have been checked already.
This approach does not ensure, that num_tx and ul_ts really match the
tx_window being tested. This does not make a difference with the
current test cases probably because num_tx starts with 1 and is
increased each iteration. Since the bitmap optimization is equivalent
to a cache optimization strategy that only uses tx_window as key. On
the other hand, ul_ts, num_tx, and rx_mask cannot be derived from
tx_window, but these values are also refered to after the call to
test_and_set_bit(). This makes it difficult to prove that correctness
of the caching. While this will not lead to a defect, the results
might be less optimal.

This commit changes the optimization strategy to skip all tx_window
where ul_ts and ul_ts+num_tx-1 are not both contained.  This provides
a similar degree of optimization like the set approach (only the
iteration with num_ts == 8 is not optimized, which only applies to to
ms class 18 and 29 MS) but ensures that the values of the related
variables have a clear relationship.

Note that the bitset based optimization for rx_window does not suffer
from a possible cache inconsistency, since only tx_window and
rx_window (tx_slot_count and rx_slot_count can be derived from the
windows and thus are covered by the cache key) are used after the call
to test_and_set_bit(). tx_window is constant over the whole lifetime
of the cache.

Sponsored-by: On-Waves ehf

1 files changed, 4 insertions, 5 deletions

diff --git a/src/gprs_rlcmac_ts_alloc.cpp b/src/gprs_rlcmac_ts_alloc.cpp
index d4cd9841..ffbbe2b8 100644
--- a/src/gprs_rlcmac_ts_alloc.cpp
+++ b/src/gprs_rlcmac_ts_alloc.cpp
@@ -481,8 +481,6 @@ static int find_multi_slots(struct gprs_rlcmac_bts *bts,
 	enum {MASK_TT, MASK_TR};
 	unsigned mask_sel;
 
-	uint32_t checked_tx[256/32] = {0};
-
 	if (ms->ms_class() >= 32) {
 		LOGP(DRLCMAC, LOGL_ERROR, "Multislot class %d out of range.\n",
 			ms->ms_class());
@@ -593,11 +591,12 @@ static int find_multi_slots(struct gprs_rlcmac_bts *bts,
 		/* Filter out unavailable slots */
 		tx_window &= *ul_slots;
 
-		/* Avoid repeated TX combination check */
-		if (test_and_set_bit(checked_tx, tx_window))
+		/* Skip if the the first TS (ul_ts) is not in the set */
+		if ((tx_window & (1 << ul_ts)) == 0)
 			continue;
 
-		if (!tx_window)
+		/* Skip if the the last TS (ul_ts+num_tx-1) is not in the set */
+		if ((tx_window & (1 << ((ul_ts+num_tx-1) % 8))) == 0)
 			continue;
 
 		tx_slot_count = bitcount(tx_window);