Change in osmo-bsc[master]: hodec2: to balance congestion, use overload percent

[neels]

Hello Jenkins Builder, 

I'd like you to reexamine a change. Please visit

    https://gerrit.osmocom.org/c/osmo-bsc/+/22085

to look at the new patch set (#3).

Change subject: hodec2: to balance congestion, use overload percent
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hodec2: to balance congestion, use overload percent

For balancing load across congested cells and across congested TCH/*
kinds, instead of comparing the number of lchans above the configured
congestion threshold, compare the percent of lchans of overload.

In short, using a percentage prevents cells with less min-free-slots to
fill up 100% while neighbor cells still may have several free lchans
available.

An obvious example of why this is desirable is illustrated by
test_balance_congestion_by_percentage.ho_vty:

Cell A has min-free-slots 2, and has all slots occupied.
Cell B has min-free-slots 4, and has 2 slots remaining free.

If we count congested lchans as in current master: cell A has a
congestion count of 2: two more lchans in use than "allowed". If we move
one lchan over to cell B, it ends up with a congestion count of 3, which
is worse than 2. So when counting lchans, we decide that cell A should
remain full.

Instead, when comparing percentage of remaining lchans, we would see
that cell A is loaded 100% above congestion (2 of 2 remaining lchans in
use), but when moving one lchan to cell B, that would only be 75% loaded
above its treshold (3 of 4 remaining lchans in use). So a percentage
comparison would cause a handover to cell B.

Related: SYS#5259
Change-Id: I55234c6c99eb02ceee52be0d7388bea14304930f
---
M src/osmo-bsc/handover_decision_2.c
M tests/handover/test_balance_congestion_by_percentage.ho_vty
M tests/handover/test_balance_congestion_tchf_tchh.ho_vty
3 files changed, 51 insertions(+), 19 deletions(-)


  git pull ssh://gerrit.osmocom.org:29418/osmo-bsc refs/changes/85/22085/3
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Gerrit-Project: osmo-bsc
Gerrit-Branch: master
Gerrit-Change-Id: I55234c6c99eb02ceee52be0d7388bea14304930f
Gerrit-Change-Number: 22085
Gerrit-PatchSet: 3
Gerrit-Owner: neels <nhofmeyr at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-MessageType: newpatchset
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