On 1/25/19 4:56 PM, Florian Stecker wrote:

On 1/25/19 4:49 AM, jianchao.wang wrote:

quoted

It sounds like not so easy to trigger.

blk_mq_dispatch_rq_list
   scsi_queue_rq
      if (atomic_read(&sdev->device_busy) ||
        scsi_device_blocked(sdev))
        ret = BLK_STS_DEV_RESOURCE;             scsi_end_request
                                                  __blk_mq_end_request
                                                    blk_mq_sched_restart // clear RESTART
                                                      blk_mq_run_hw_queue
                                                  blk_mq_run_hw_queues
     list_splice_init(list, &hctx->dispatch)
   needs_restart = blk_mq_sched_needs_restart(hctx)

The 'needs_restart' will be false, so the queue would be rerun.

Thanks
Jianchao

Good point. So the RESTART flag is supposed to protect against this? Now I see, this is also sort of what the lengthy comment in blk_mq_dispatch_rq_list is saying.

May I complain that this is very unintuitive (the queue gets rerun when the RESTART flag is _not_ set) and also unreliable, as not every caller of blk_mq_dispatch_rq_list seems to set the flag, and also it does not always get cleared in __blk_mq_end_request?

__blk_mq_end_request does the following:

    if (rq->end_io) {
        rq_qos_done(rq->q, rq);
        rq->end_io(rq, error);
    } else {
        if (unlikely(blk_bidi_rq(rq)))
            blk_mq_free_request(rq->next_rq);
        blk_mq_free_request(rq);
    }

and blk_mq_free_request then calls blk_mq_sched_restart, which clears the flag. But in my case, rq->end_io != 0, so blk_mq_free_request is never called.

So what is your rq->end_io ?
flush_end_io ? or mq_flush_data_end_io ? or other ?
In normal case, the blk_mq_end_request should be finally invoked.

Did you ever try the bfq io scheduler instead of mq-deadline ?

Can you share your dmesg and config file here ?

Thanks
Jianchao