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blk-mq host_tag IO hang - Coggle Diagram

- - - - max_queue=1 host_max_queue=1 submit_queues=4
        
        host_max_queue=1 is must
        
        all hctx share single tags
        
        can be reproduced with max_queue=4
        
        single lun
    - - --direct=1 --rw=randrw --bs=4k --iodepth=1 --runtime=100 --numjobs=40 --time_based
  - - - one request stays in ->dispatch
      - tags sbitmap bits is free
    - - ws_active is 1
        
        not wakeup
      - wake_batch is 1
      - completion_cnt == wakeup_cnt
      - hctx wait is linked to ws1
      - wake_index = 0
    - - wait_index is 3
      - multiple hctx are added to single tags wait head