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
apply same change that we've done on the eNB also on the UE
to avoid the PHY processing TTIs faster than the stack.
Without that, we see lots of those in the logs:
...
08:39:17.580325 [STCK] [W] Detected slow task processing (sync_queue_len=7).
...
before entering RRC idle, after receiving a RRC connection release for example,
we need to wait until the RLC for SRB1 or SRB2 have been flushed, i.e.
the RLC has acknowledged the reception of the message.
Previously we have only waited for SRB1 but the message can also be received on SRB2
and in this case both bearers need to be checked.
The method is now streamlined to check both SRBs and is also used when
checking the msg transmission of an detach request.