Commit f983622a authored Apr 23, 2020 by Ming Lei Committed by Martin K. Petersen Apr 24, 2020

scsi: core: Avoid calling synchronize_rcu() for each device in scsi_host_block()

scsi_host_block() calls scsi_internal_device_block() for each scsi_device and
scsi_internal_device_block() calls blk_mq_quiesce_queue() for each LUN.

Since synchronize_rcu() is called from blk_mq_quiesce_queue(), this can cause
substantial slowdowns on systems with many LUNs.

Use scsi_internal_device_block_nowait() to implement scsi_host_block() so it
is sufficient to run synchronize_rcu() once. This is safe since SCSI does not
set the BLK_MQ_F_BLOCKING flag.

[mkp: commit desc and comment tweaks]

Link: https://lore.kernel.org/r/20200423020713.332743-1-ming.lei@redhat.com


Cc: Steffen Maier <maier@linux.ibm.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

parent 3fa65812

drivers/scsi/scsi_lib.c

+17 −1

Original line number	Diff line number	Diff line
		@@ -2841,11 +2841,27 @@ scsi_host_block(struct Scsi_Host *shost)
		struct scsi_device *sdev;
		int ret = 0;

		/*
		* Call scsi_internal_device_block_nowait so we can avoid
		* calling synchronize_rcu() for each LUN.
		*/
		shost_for_each_device(sdev, shost) {
		ret = scsi_internal_device_block(sdev);
		mutex_lock(&sdev->state_mutex);
		ret = scsi_internal_device_block_nowait(sdev);
		mutex_unlock(&sdev->state_mutex);
		if (ret)
		break;
		}

		/*
		* SCSI never enables blk-mq's BLK_MQ_F_BLOCKING flag so
		* calling synchronize_rcu() once is enough.
		*/
		WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);

		if (!ret)
		synchronize_rcu();

		return ret;
		}
		EXPORT_SYMBOL_GPL(scsi_host_block);

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