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

[Mike Snitzer]

On Tue, Aug 18 2020 at  7:52pm -0400,
Ming Lei <ming.lei at redhat.com> wrote:

> On Tue, Aug 18, 2020 at 11:20:22AM -0400, Mike Snitzer wrote:
> > 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.
> > 
> 
> 'bypass_insert' is 'true' when blk_insert_cloned_request() is
> called from device mapper code, so this patch doesn't affect dm.

Great.
 
> > 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?
> 
> __blk_mq_try_issue_directly() can be called from blk-mq directly, that
> is the case this patch is addressing, if one request can't be queued to
> LLD because of running out of budget or driver tag, it should be added to
> scheduler queue for improving io merge, meantime we can avoid too many
> requests dispatched to hardware.

I see, so if retry is needed best to attempt merge again.

Thanks for the explanation.

Mike