Re: [PATCH V3 5/5] blk-mq: issue request directly for blk_insert_cloned_request

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Jan 11 2018 at  1:01am -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> blk_insert_cloned_request() is called in fast path of dm-rq driver, and
> in this function we append request to hctx->dispatch_list of the underlying
> queue directly.
> 
> 1) This way isn't efficient enough because hctx lock is always required
> 
> 2) With blk_insert_cloned_request(), we bypass underlying queue's IO scheduler
> totally, and depend on DM rq driver to do IO schedule completely. But DM
> rq driver can't get underlying queue's dispatch feedback at all, and this
> information is extreamly useful for IO merge. Without that IO merge can't
> be done basically by blk-mq, and causes very bad sequential IO performance.
> 
> This patch makes use of blk_mq_try_issue_directly() to dispatch rq to
> underlying queue and provides disptch result to dm-rq and blk-mq, and
> improves the above situations very much.
> 
> With this patch, seqential IO is improved by 3X in my test over
> mpath/virtio-scsi.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

Tested-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>