this patch refactors the SDU queuing and dropping policy of the RLC and PDCP layer.
the previous design had issues when packets have been generated at a higher
rate above the PDCP than they could be consumed below the RLC.
When the RLC SDU queues were full, we allowed two policies, one to block on the write
and the other to drop the SDU. Both options are not ideal because they either
lead to a blocking stack thread or to lost PDCP PDUs.
To avoid this, this patch makes the following changes:
* PDCP monitors RLC's SDU queue and drops packets on its north-bound SAP if queues are full
* a new method sdu_queue_is_full() has been added to the RLC interface for PDCP
* remove blocking write from pdcp and rlc write_sdu() interface
* all writes into queues need to be non-blocking
* if Tx queues are overflowing, SDUs are dropped above PDCP, not RLC
* log warning if RLC still needs to drop SDUs
* this case should be avoided with the monitoring mechanism
Here is the test output with the fix disabled:
13:47:42.679774 [RLC_UM_1] [D] MAC opportunity - 14 bytes
13:47:42.679784 [RLC_UM_1] [D] pdu_space=14, head_len=2
13:47:42.679790 [RLC_UM_1] [D] adding new SDU segment - 10 bytes of 10 remaining
13:47:42.679834 [RLC_UM_1] [D] Complete SDU scheduled for tx. Stack latency: 0 us
13:47:42.679909 [RLC_UM_1] [D] pdu_space=4, head_len=2
13:47:42.679922 [RLC_UM_1] [D] adding new SDU segment - 0 bytes of 10 remaining
13:47:42.679928 [RLC_UM_1] [I] Tx PDU SN=0 (14 B)
13:47:42.679974 [RLC_UM_1] [D] vt_us = 1
The test checks the correct packing and of the two PDUs at the end.
this allows to create a queue that has a non-default capacity
currently this is 8192, but the value might now be suitable in some
cases like for TTI sync events