core: account for linux systems with no reservable cores (#19458)

* core: account for linux systems with no reservable cores

* cl: add cl

* core: remove condition on reservable cores for legacy empty check

.changelog/19458.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core: Fixed a bug where linux nodes with no reservable cores would panic the scheduler
+```

nomad/structs/cpucompat_default.go

@@ -6,9 +6,7 @@
 package structs
 
 import (
-	"github.com/hashicorp/nomad/client/lib/idset"
 	"github.com/hashicorp/nomad/client/lib/numalib"
-	"github.com/hashicorp/nomad/client/lib/numalib/hw"
 )
 
 // Compatibility will translate the LegacyNodeCpuResources into NodeProcessor
@@ -40,41 +38,5 @@ func (n *NodeResources) Compatibility() {
 }
 
 func topologyFromLegacy(old LegacyNodeCpuResources) *numalib.Topology {
-	coreCount := old.TotalCpuCores
+	return topologyFromLegacyGeneric(old)
-
-	// interpret per-core frequency given total compute and total core count
-	frequency := hw.MHz(old.CpuShares / (int64(coreCount)))
-
-	// synthesize a set of cores that abstractly matches the legacy cpu specs
-	cores := make([]numalib.Core, 0, coreCount)
-
-	for i := 0; i < int(coreCount); i++ {
-		cores = append(cores, numalib.Core{
-			ID:         hw.CoreID(i),
-			SocketID:   0,                   // no numa support on non-linux
-			NodeID:     0,                   // no numa support on non-linux
-			Grade:      numalib.Performance, // assume P-cores
-			Disable:    false,               // no reservable cores on non-linux
-			GuessSpeed: frequency,
-		})
-	}
-
-	withheld := (frequency * hw.MHz(coreCount)) - hw.MHz(old.CpuShares)
-
-	return &numalib.Topology{
-		// legacy: assume one node with id 0
-		NodeIDs: idset.From[hw.NodeID]([]hw.NodeID{0}),
-
-		// legacy: with one node the distance matrix is 1-D
-		Distances: numalib.SLIT{{10}},
-
-		// legacy: a pseudo representation of each actual core profile
-		Cores: cores,
-
-		// legacy: set since we have the value
-		OverrideTotalCompute: hw.MHz(old.CpuShares),
-
-		// legacy: set since we can compute the value
-		OverrideWitholdCompute: withheld,
-	}
 }

nomad/structs/cpucompat_generic.go

@@ -0,0 +1,50 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: BUSL-1.1
+
+package structs
+
+import (
+	"github.com/hashicorp/nomad/client/lib/idset"
+	"github.com/hashicorp/nomad/client/lib/numalib"
+	"github.com/hashicorp/nomad/client/lib/numalib/hw"
+)
+
+func topologyFromLegacyGeneric(old LegacyNodeCpuResources) *numalib.Topology {
+	coreCount := old.TotalCpuCores
+
+	// interpret per-core frequency given total compute and total core count
+	frequency := hw.MHz(old.CpuShares / (int64(coreCount)))
+
+	// synthesize a set of cores that abstractly matches the legacy cpu specs
+	cores := make([]numalib.Core, 0, coreCount)
+
+	for i := 0; i < int(coreCount); i++ {
+		cores = append(cores, numalib.Core{
+			ID:         hw.CoreID(i),
+			SocketID:   0,                   // no numa support on non-linux
+			NodeID:     0,                   // no numa support on non-linux
+			Grade:      numalib.Performance, // assume P-cores
+			Disable:    false,               // no reservable cores on non-linux
+			GuessSpeed: frequency,
+		})
+	}
+
+	withheld := (frequency * hw.MHz(coreCount)) - hw.MHz(old.CpuShares)
+
+	return &numalib.Topology{
+		// legacy: assume one node with id 0
+		NodeIDs: idset.From[hw.NodeID]([]hw.NodeID{0}),
+
+		// legacy: with one node the distance matrix is 1-D
+		Distances: numalib.SLIT{{10}},
+
+		// legacy: a pseudo representation of each actual core profile
+		Cores: cores,
+
+		// legacy: set since we have the value
+		OverrideTotalCompute: hw.MHz(old.CpuShares),
+
+		// legacy: set since we can compute the value
+		OverrideWitholdCompute: withheld,
+	}
+}

nomad/structs/cpucompat_linux.go

@@ -45,6 +45,13 @@ func (n *NodeResources) Compatibility() {
 }
 
 func topologyFromLegacy(old LegacyNodeCpuResources) *numalib.Topology {
+	if len(old.ReservableCpuCores) == 0 {
+		return topologyFromLegacyGeneric(old)
+	}
+	return topologyFromLegacyLinux(old)
+}
+
+func topologyFromLegacyLinux(old LegacyNodeCpuResources) *numalib.Topology {
 	// interpret per-core frequency given total compute and total core count
 	frequency := hw.MHz(old.CpuShares / (int64(len(old.ReservableCpuCores))))
 

nomad/structs/numa.go

@@ -82,7 +82,7 @@ type LegacyNodeCpuResources struct {
 // partial struct serialization / copy / merge sadness means this struct can
 // exist with no data, which is a condition we must detect during the upgrade path
 func (r LegacyNodeCpuResources) empty() bool {
-	return r.CpuShares == 0 || r.TotalCpuCores == 0 || len(r.ReservableCpuCores) == 0
+	return r.CpuShares == 0 || r.TotalCpuCores == 0
 }
 
 // NomadProcessorResources captures the CPU hardware resources of the Nomad node.