tikv: add dynamic config item 'readpool.unified.max-tasks-per-worker' (…

…pingcap#15551)

dynamic-config.md

@@ -166,6 +166,7 @@ The following TiKV configuration items can be modified dynamically:
 | `raftstore.store-io-pool-size` | The number of threads that process Raft I/O tasks, which is also the size of the StoreWriter thread pool (**DO NOT** modify this value from a non-zero value to 0 or from 0 to a non-zero value) |
 | `raftstore.periodic-full-compact-start-max-cpu` | The CPU usage threshold at which TiKV performs periodic full compaction if full compaction is enabled |
 | `readpool.unified.max-thread-count` | The maximum number of threads in the thread pool that uniformly processes read requests, which is the size of the UnifyReadPool thread pool |
+| `readpool.unified.max-tasks-per-worker` | The maximum number of tasks allowed for a single thread in the unified read pool. `Server Is Busy` error is returned when the value is exceeded. |
 | `readpool.unified.auto-adjust-pool-size` | Determines whether to automatically adjust the UnifyReadPool thread pool size |
 | `coprocessor.split-region-on-table` | Enables to split Region by table |
 | `coprocessor.batch-split-limit` | The threshold of Region split in batches |