Comment 4 for bug 2104040

Reviewed: https://review.opendev.org/c/openstack/placement/+/945465
Committed: https://opendev.org/openstack/placement/commit/5ac4c70b8ad63a0fd7cea9f66e925569eaf0612c
Submitter: "Zuul (22348)"
Branch: master

commit 5ac4c70b8ad63a0fd7cea9f66e925569eaf0612c
Author: Dan Smith <email address hidden>
Date: Mon Mar 24 12:24:35 2025 -0700

    Fix placement allocate while over-capacity

    There is a bug in the placement over-capacity check that prevents
    updating existing allocations if a resource provider is over capacity,
    even if that change makes no change to the amount of resources that
    are allocated, or even if the allocation makes the situation better.
    This can happen when the reserved or allocation_ratio is changed on
    a provider with existing allocations. The bug prevents escaping this
    situation by deallocating resources unless a single atomic allocation
    can be performed to result in an under-capacity outcome. That is not
    something consumers like Nova can orchestrate and thus operators get
    stuck in a situation where they must un-reserve some resource, or
    temporarily increase the allocation_ratio in order to be able to
    escape. That could result in a worsening situation.

    This change makes us allow allocation changes if they improve or
    otherwise do not worsen an over-capacity situation, which is clearly
    what operators expect based on the multiple bugs opened against Nova
    for this issue.

    Closes-Bug: #2104040
    Related-Bug: #1924123
    Related-Bug: #1941892
    Related-Bug: #1943191
    Change-Id: If7264d3ce679b6f9b604a3bcd2a417cc175e3793