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scheduler: fix a bug where we subtract reserved node resources twice #23386

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merged 4 commits into from
Jun 21, 2024
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scheduler: fix a bug where we subtract reserved node resources twice #23386

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a0a7173
scheduler: fix a bug where we subtract reserved node resources twice
pkazmierczak Jun 19, 2024
e0dcac3
nodeResources.Comparable() should in fact return total resources
pkazmierczak Jun 19, 2024
10131ea
cl
pkazmierczak Jun 19, 2024
a73965f
mocked Node correction
pkazmierczak Jun 21, 2024
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3 changes: 3 additions & 0 deletions .changelog/23386.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
```release-note:bug
scheduler: Fix a bug where reserved resources are not calculated correctly
```
2 changes: 1 addition & 1 deletion nomad/structs/structs.go
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Expand Up @@ -3191,7 +3191,7 @@ func (n *NodeResources) Comparable() *ComparableResources {
c := &ComparableResources{
Flattened: AllocatedTaskResources{
Cpu: AllocatedCpuResources{
CpuShares: int64(n.Processors.Topology.UsableCompute()),
CpuShares: int64(n.Processors.Topology.TotalCompute()),
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The comment in the caller in AllocsFit seems to agree that this should be TotalCompute:

	// Check that the node resources (after subtracting reserved) are a
	// super set of those that are being allocated
	available := node.NodeResources.Comparable()
	available.Subtract(node.ReservedResources.Comparable())
	if superset, dimension := available.Superset(used); !superset {
		return false, dimension, used, nil
	}

But it bothers me that changing this wouldn't also cause some tests to fail. Are we missing tests on AllocsFit that exercise this code path?

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Right, so this is very tricky to test.

I found this bug while chasing AllocsFit and initially just removed the available.Subtract(node.ReservedResources.Comparable()) line, because I thought: ah, this just subtracts what's already been subtracted. It solves the issue of course, but making this change is what breaks a lot of tests in the plan applier and scheduler. The reason is: we mock nodes and their resources. The only way to properly test this is an e2e test, but that's also tricky: we'd have to create a client with reserved resources, run a job that requests all the remaining available resources and make sure it succeeds. To do that, we'd have to manipulate client config in the e2e (that is tricky), and dynamically create the jobspec based on the available resources of the e2e node. Can be done, but it's a lot of work I think. Unless I'm missing something?

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I suspect we could set reserved resources on the mocked nodes on some of those existing scheduler and plan applier tests such that they pass with this change and fail without it.

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ok. finally found the culprit. every node that has reserved resources set by client conf has an additional field OverrideWitholdCompute set in its Topology on "legacy" systems:

nomad/nomad/structs/cpucompat_generic.go

Line 45 in a73965f

OverrideWitholdCompute: withheld,

This field was missing from the mocked node in our unit tests. After adding it, tests fail without this PR's change.

ReservedCores: reservableCores,
},
Memory: AllocatedMemoryResources{
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