[dm-devel] [PATCH RESEND] blk-mq: insert request not through ->queue_rq into sw/scheduler queue

[Mike Snitzer]

On Tue, Aug 18 2020 at 10:50am -0400,
Jens Axboe <axboe at kernel.dk> wrote:

> On 8/18/20 2:07 AM, Ming Lei wrote:
> > c616cbee97ae ("blk-mq: punt failed direct issue to dispatch list") supposed
> > to add request which has been through ->queue_rq() to the hw queue dispatch
> > list, however it adds request running out of budget or driver tag to hw queue
> > too. This way basically bypasses request merge, and causes too many request
> > dispatched to LLD, and system% is unnecessary increased.
> > 
> > Fixes this issue by adding request not through ->queue_rq into sw/scheduler
> > queue, and this way is safe because no ->queue_rq is called on this request
> > yet.
> > 
> > High %system can be observed on Azure storvsc device, and even soft lock
> > is observed. This patch reduces %system during heavy sequential IO,
> > meantime decreases soft lockup risk.
> 
> Applied, thanks Ming.

Hmm, strikes me as strange that this is occurring given the direct
insertion into blk-mq queue (bypassing scheduler) is meant to avoid 2
layers of IO merging when dm-mulipath is stacked on blk-mq path(s).  The
dm-mpath IO scheduler does all merging and underlying paths' blk-mq
request_queues are meant to just dispatch the top-level's requests.

So this change concerns me.  Feels like this design has broken down.

Could be that some other entry point was added for the
__blk_mq_try_issue_directly() code?  And it needs to be untangled away
from the dm-multipath use-case?

Apologies for not responding to this patch until now.

Mike