PD not schedules down peers

[BusyJay]

Bug Report

A TiKV is killed by OOM. PD schedules

What did you do?

Setup a cluster with 4 nodes. One TiKV is killed by OOM. And balance-region-scheduler is removed.

What did you expect to see?

PD schedules all replicas from the down TiKV to other nodes.

What did you see instead?

PD schedules most of replicas but still leave some.

What version of PD are you using (pd-server -V)?

Release Version: v5.1.1
Edition: Community
Git Commit Hash: 7cba1912b317a533e18b16ea2ba9a14ed2891129
Git Branch: heads/refs/tags/v5.1.1
UTC Build Time:  2021-07-23 01:16:18

[BusyJay]

After restarting PD server, it starts scheduling again, and down-peer reaches 0 at the end.

[mayjiang0203]

/severity major

[HunDunDM]

fixed by #4078