Re: [PATCH] blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()

[Ming Lei <ming.lei@redhat.com>]

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