Commit 52d7f1b5 authored Oct 28, 2016 by Bart Van Assche Committed by Jens Axboe Nov 02, 2016

blk-mq: Avoid that requeueing starts stopped queues



Since blk_mq_requeue_work() starts stopped queues and since
execution of this function can be scheduled after a queue has
been stopped it is not possible to stop queues without using
an additional state variable to track whether or not the queue
has been stopped. Hence modify blk_mq_requeue_work() such that it
does not start stopped queues. My conclusion after a review of
the blk_mq_stop_hw_queues() and blk_mq_{delay_,}kick_requeue_list()
callers is as follows:
* In the dm driver starting and stopping queues should only happen
  if __dm_suspend() or __dm_resume() is called and not if the
  requeue list is processed.
* In the SCSI core queue stopping and starting should only be
  performed by the scsi_internal_device_block() and
  scsi_internal_device_unblock() functions but not by any other
  function. Although the blk_mq_stop_hw_queue() call in
  scsi_queue_rq() may help to reduce CPU load if a LLD queue is
  full, figuring out whether or not a queue should be restarted
  when requeueing a command would require to introduce additional
  locking in scsi_mq_requeue_cmd() to avoid a race with
  scsi_internal_device_block(). Avoid this complexity by removing
  the blk_mq_stop_hw_queue() call from scsi_queue_rq().
* In the NVMe core only the functions that call
  blk_mq_start_stopped_hw_queues() explicitly should start stopped
  queues.
* A blk_mq_start_stopped_hwqueues() call must be added in the
  xen-blkfront driver in its blkif_recover() function.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: James Bottomley <jejb@linux.vnet.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

parent 2253efc8

block/blk-mq.c

+1 −5

Original line number	Diff line number	Diff line
		@@ -501,11 +501,7 @@ static void blk_mq_requeue_work(struct work_struct *work)
		blk_mq_insert_request(rq, false, false, false);
		}

		/*
		* Use the start variant of queue running here, so that running
		* the requeue work will kick stopped queues.
		*/
		blk_mq_start_hw_queues(q);
		blk_mq_run_hw_queues(q, false);
		}

		void blk_mq_add_to_requeue_list(struct request *rq, bool at_head)

drivers/block/xen-blkfront.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -2045,6 +2045,7 @@ static int blkif_recover(struct blkfront_info *info)
		BUG_ON(req->nr_phys_segments > segs);
		blk_mq_requeue_request(req);
		}
		blk_mq_start_stopped_hw_queues(info->rq, true);
		blk_mq_kick_requeue_list(info->rq);

		while ((bio = bio_list_pop(&info->bio_list)) != NULL) {

drivers/md/dm-rq.c

+1 −6

Original line number	Diff line number	Diff line
		@@ -338,12 +338,7 @@ static void dm_old_requeue_request(struct request *rq)

		static void __dm_mq_kick_requeue_list(struct request_queue *q, unsigned long msecs)
		{
		unsigned long flags;

		spin_lock_irqsave(q->queue_lock, flags);
		if (!blk_queue_stopped(q))
		blk_mq_delay_kick_requeue_list(q, msecs);
		spin_unlock_irqrestore(q->queue_lock, flags);
		}

		void dm_mq_kick_requeue_list(struct mapped_device *md)

drivers/scsi/scsi_lib.c

+0 −1

Original line number	Diff line number	Diff line
		@@ -1952,7 +1952,6 @@ out_put_device:
		out:
		switch (ret) {
		case BLK_MQ_RQ_QUEUE_BUSY:
		blk_mq_stop_hw_queue(hctx);
		if (atomic_read(&sdev->device_busy) == 0 &&
		!scsi_device_blocked(sdev))
		blk_mq_delay_queue(hctx, SCSI_QUEUE_DELAY);

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