schedule: revise region_scatter distribution for multi groups (#3422)

[ti-srebot]

cherry-pick #3422 to release-5.0
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# In pd repo:
git pr https://github.com/tikv/pd/pull/3621

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Signed-off-by: Song Gao disxiaofei@163.com

What problem does this PR solve?

ref pingcap/tidb#22788

Currently region-scatter didn't work well for multi regions in different groups.

For example, there are 50 regions and 6 stores in the cluster. Each region belong to a dedicated group.

region scatter may locate them in the store1/store2/store3. Thus from each region in the group, the distribution is well. And for the whole cluster, the distribution is not well.

What is changed and how it works?

During region scatter, we should consider whether the target store already have the most peer or leader, if it is, we should skip it.

Check List

Tests

• Unit test

I also tested with the testcases in this document: https://docs.google.com/document/d/1cGZU2DCm0UuIueqAQP8TGqTivyavCvB_QiwIzqvP_rA/edit

All the test case results didn't change.

Release note

• No release note

[ti-srebot]

@Yisaer you're already a collaborator in bot's repo.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• nolouch

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[nolouch]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 1ff15bc

[codecov]

Codecov Report

Merging #3621 (71218b8) into release-5.0 (04ed1ff) will decrease coverage by 0.06%.
The diff coverage is 94.28%.

@@               Coverage Diff               @@
##           release-5.0    #3621      +/-   ##
===============================================
- Coverage        74.65%   74.58%   -0.07%     
===============================================
  Files              245      245              
  Lines            24149    24178      +29     
===============================================
+ Hits             18028    18034       +6     
- Misses            4509     4523      +14     
- Partials          1612     1621       +9     

Flag	Coverage Δ
unittests	74.58% <94.28%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/schedule/operator/builder.go	87.98% <ø> (ø)
server/schedule/region_scatterer.go	88.37% <92.85%> (+1.32%)	⬆️
pkg/cache/ttl.go	97.77% <100.00%> (+0.15%)	⬆️
server/schedule/operator/create_operator.go	89.76% <100.00%> (+0.08%)	⬆️
server/region_syncer/server.go	83.33% <0.00%> (-6.07%)	⬇️
server/member/member.go	65.21% <0.00%> (-4.90%)	⬇️
server/election/leadership.go	83.52% <0.00%> (-2.36%)	⬇️
client/base_client.go	83.78% <0.00%> (-1.63%)	⬇️
server/tso/tso.go	72.39% <0.00%> (-1.23%)	⬇️
server/grpc_service.go	46.60% <0.00%> (-0.80%)	⬇️
... and 10 more

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