skip calculate available replicas when no need

[zhzhuang-zju]

What type of PR is this?
/kind feature

What this PR does / why we need it:
When Karmada is scheduling, it will always invoke calAvailableReplicas to calculate available replicas, regardless of necessity, thereby increasing CPU overhead.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
NONE
Does this PR introduce a user-facing change?:

NONE

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 53.07%. Comparing base (d508417) to head (8ef8ca3).
Report is 240 commits behind head on master.

Files	Patch %	Lines
.../scheduler/core/spreadconstraint/group_clusters.go	0.00%	4 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4574      +/-   ##
==========================================
+ Coverage   51.89%   53.07%   +1.17%     
==========================================
  Files         246      251       +5     
  Lines       24329    20395    -3934     
==========================================
- Hits        12625    10824    -1801     
+ Misses      11017     8857    -2160     
- Partials      687      714      +27     

Flag	Coverage Δ
unittests	53.07% <58.33%> (+1.17%)	⬆️

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[Garrybest]

When Karmada is scheduling, it will always invoke calAvailableReplicas to calculate available replicas, regardless of necessity, thereby increasing CPU overhead.

Well, I'm afraid not. This func will only be invoked when strategy is Aggregated or DynamicWeight which is necessay indeed.

[zhzhuang-zju]

Well, I'm afraid not. This func will only be invoked when strategy is Aggregated or DynamicWeight which is necessay indeed.

refer to

karmada/pkg/scheduler/core/common.go

Lines 32 to 38 in d508417

	func SelectClusters(clustersScore framework.ClusterScoreList,
	placement *policyv1alpha1.Placement, spec *workv1alpha2.ResourceBindingSpec) ([]*clusterv1alpha1.Cluster, error) {
	startTime := time.Now()
	defer metrics.ScheduleStep(metrics.ScheduleStepSelect, startTime)
	groupClustersInfo := spreadconstraint.GroupClustersWithScore(clustersScore, placement, spec, calAvailableReplicas)
	return spreadconstraint.SelectBestClusters(placement, groupClustersInfo, spec.Replicas)

as you can see, the function GroupClustersWithScore is executed first, and then the function SelectBestClusters. And function GroupClustersWithScore will execute function calAvailableReplicas to calculate available replicas, no matter what the strategy is.

[jwcesign]

Other lgtm

[RainbowMango]

/retest

[zhzhuang-zju]

cc @Garrybest