global sort split range take a long time

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
global sort add index for a 120T table
you can see after step 1,2 , there is only 1 tidb node works for a long time

### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)
master https://github.com/pingcap/tidb/commit/6db1d6b79c2855cf022f04ced86af3e6bc8b0dcf

Start by reproducing the global sort add-index operation on a 120T table at commit 6db1d6b79c2855cf022f04ced86af3e6bc8b0dcf. Observe the TiDB nodes during the split-range steps and determine why only one node remains active; the issue needs expected behavior and a precise completion condition before implementation can begin.

Written by the indexing model from the issue text.