On Fri, Jun 22, 2018 at 08:47:35AM -0600, Jens Axboe wrote: > 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? Looks a good idea, I guess it can be triggered on scsi_debug too, will cook a patch later. thanks, Ming
