Web page is stuck when task is running

Web sometimes hang when task is running, the stack is below:
```
Current thread 0x00007fce86d99740 (most recent call first):
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1069 in flush
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1089 in emit
File "/opt/conda/lib/python3.8/logging/__init__.py", line 954 in handle
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1661 in callHandlers
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1599 in handle
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1589 in _log
File "/opt/conda/lib/python3.8/logging/__init__.py", line 1434 in debug
File "/home/admin/work/_public-mars-hks.zip/mars/services/scheduling/supervisor/manager.py", line 271 in submit_subtask_to_band
File "/opt/conda/lib/python3.8/asyncio/events.py", line 81 in _run
File "/opt/conda/lib/python3.8/asyncio/base_events.py", line 1859 in _run_once
File "/opt/conda/lib/python3.8/asyncio/base_events.py", line 570 in run_forever
File "/opt/conda/lib/python3.8/asyncio/base_events.py", line 603 in run_until_complete
File "/opt/conda/lib/python3.8/asyncio/runners.py", line 44 in run
File "/home/admin/work/_public-mars-hks.zip/mars/oscar/backends/mars/pool.py", line 196 in _start_sub_pool
File "/opt/conda/lib/python3.8/multiprocessing/process.py", line 108 in run
File "/opt/conda/lib/python3.8/multiprocessing/process.py", line 315 in _bootstrap
File "/opt/conda/lib/python3.8/multiprocessing/spawn.py", line 129 in _main
File "/opt/conda/lib/python3.8/multiprocessing/spawn.py", line 116 in spawn_main
File "", line 1 in
```

Start by reading mars/services/scheduling/supervisor/manager.py around submit_subtask_to_band and mars/oscar/backends/mars/pool.py around _start_sub_pool, using the supplied stack trace as the entry point. Reproduce a task run that causes the web page to hang, then verify that the page remains responsive while the task is running.

Written by the indexing model from the issue text.