Re: [for-4.16 PATCH v6 2/3] blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jan 17 2018 at 10:25pm -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> Hi Mike,
> 
> On Wed, Jan 17, 2018 at 11:25:57AM -0500, Mike Snitzer wrote:
> > From: Ming Lei <ming.lei@redhat.com>
> > 
> > blk_insert_cloned_request() is called in the fast path of a dm-rq driver
> > (e.g. blk-mq request-based DM mpath).  blk_insert_cloned_request() uses
> > blk_mq_request_bypass_insert() to directly append the request to the
> > blk-mq hctx->dispatch_list of the underlying queue.
> > 
> > 1) This way isn't efficient enough because the hctx spinlock is always
> > used.
> > 
> > 2) With blk_insert_cloned_request(), we completely bypass underlying
> > queue's elevator and depend on the upper-level dm-rq driver's elevator
> > to schedule IO.  But dm-rq currently can't get the underlying queue's
> > dispatch feedback at all.  Without knowing whether a request was issued
> > or not (e.g. due to underlying queue being busy) the dm-rq elevator will
> > not be able to provide effective IO merging (as a side-effect of dm-rq
> > currently blindly destaging a request from its elevator only to requeue
> > it after a delay, which kills any opportunity for merging).  This
> > obviously causes very bad sequential IO performance.
> > 
> > Fix this by updating blk_insert_cloned_request() to use
> > blk_mq_request_direct_issue().  blk_mq_request_direct_issue() allows a
> > request to be issued directly to the underlying queue and returns the
> > dispatch feedback (blk_status_t).  If blk_mq_request_direct_issue()
> > returns BLK_SYS_RESOURCE the dm-rq driver will now use DM_MAPIO_REQUEUE
> > to _not_ destage the request.  Whereby preserving the opportunity to
> > merge IO.
> > 
> > With this, request-based DM's blk-mq sequential IO performance is vastly
> > improved (as much as 3X in mpath/virtio-scsi testing).
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > [blk-mq.c changes heavily influenced by Ming Lei's initial solution, but
> > they were refactored to make them less fragile and easier to read/review]
> > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
...
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index c117c2baf2c9..f5f0d8456713 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -1731,15 +1731,19 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
> >  
> >  static void __blk_mq_fallback_to_insert(struct blk_mq_hw_ctx *hctx,
> >  					struct request *rq,
> > -					bool run_queue)
> > +					bool run_queue, bool bypass_insert)
> >  {
> > -	blk_mq_sched_insert_request(rq, false, run_queue, false,
> > -					hctx->flags & BLK_MQ_F_BLOCKING);
> > +	if (!bypass_insert)
> > +		blk_mq_sched_insert_request(rq, false, run_queue, false,
> > +					    hctx->flags & BLK_MQ_F_BLOCKING);
> > +	else
> > +		blk_mq_request_bypass_insert(rq, run_queue);
> >  }
> 
> If 'bypass_insert' is true, we don't need to insert the request into
> hctx->dispatch_list for dm-rq, then it causes the issue(use after free)
> reported by Bart and Laurence.
> 
> Also this way is the exact opposite of the idea of the improvement,
> we do not want to dispatch request if underlying queue is busy.

Yeap, please see the patch I just posted to fix it.

But your v4 does fallback to using blk_mq_request_bypass_insert() as
well, just in a much narrower case -- specifically:
       if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))

Thanks,
Mike