scheduler: little refinement

[lance6716]

What problem does this PR solve?

reduce code repeat. In future PRs we will check about relay worker bugs.

What is changed and how it works?

little refinement

Check List

Tests

• pass original tests

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• GMHDBJD
• glorv

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[lance6716]

/cc @GMHDBJD @lichunzhu a short PR

[ti-chi-bot]

@lance6716: GitHub didn't allow me to request PR reviews from the following users: a, short, PR.

Note that only pingcap members and repo collaborators can review this PR, and authors cannot review their own PRs.

In response to [this](https://github.com/pingcap/dm/pull/2211#issuecomment-940924773): >/cc @GMHDBJD @lichunzhu a short PR Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[lance6716]

/cc @GMHDBJD @lichunzhu

a short PR

[lance6716]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: e38ea8a4b38b3c3dd63372fba8671421193c26ac

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2212.