code polish

[loongs-zhang]

// Describe your PR here; eg. Fixes #issueNo

Make sure that:

• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed.

[codecov]

Codecov Report

Attention: Patch coverage is 65.85366% with 14 lines in your changes missing coverage. Please review.

Project coverage is 77.27%. Comparing base (561b7e5) to head (4918ed8).

Files with missing lines	Patch %	Lines
core/src/net/event_loop.rs	67.56%	12 Missing ⚠️
core/src/co_pool/mod.rs	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
- Coverage   77.31%   77.27%   -0.04%     
==========================================
  Files          45       45              
  Lines        5149     5162      +13     
==========================================
+ Hits         3981     3989       +8     
- Misses       1168     1173       +5     

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