On 6/19/18 8:55 PM, Ming Lei wrote:

SCSI probing may synchronously create and destroy a lot of request_queues
for non-existent devices. Any synchronize_rcu() in queue creation or
destroy path may introduce long latency during booting, see detailed
description in comment of blk_register_queue().

This patch removes two synchronize_rcu() inside blk_cleanup_queue()
for this case:

1) commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue)
need synchronize_rcu() for implementing blk_mq_quiesce_queue(), but
when queue isn't initialized, it isn't necessary to do that since
only pass-through requests are involved, no original issue in
scsi_execute() at all.

2) when only one request queue is attached to tags, no necessary to
call synchronize_rcu() too.

Without this patch, it may take more 20+ seconds for virtio-scsi to
complete disk probe. With this patch, the time becomes less than 100ms.

Looks reasonable to me. But this is something that we've been breaking
multiple times over the years, any chance you could add a blktests
test for it?

-- 
Jens Axboe