[dm-devel] [PATCH V3 0/5] dm-rq: improve sequential I/O performance

[Mike Snitzer]

On Fri, Jan 12 2018 at  1:54pm -0500,
Bart Van Assche <Bart.VanAssche at wdc.com> wrote:

> On Fri, 2018-01-12 at 13:06 -0500, Mike Snitzer wrote:
> > OK, you have the stage: please give me a pointer to your best
> > explaination of the several.
> 
> Since the previous discussion about this topic occurred more than a month
> ago it could take more time to look up an explanation than to explain it
> again. Anyway, here we go. As you know a block layer request queue needs to
> be rerun if one or more requests are waiting and a previous condition that
> prevented the request to be executed has been cleared. For the dm-mpath
> driver, examples of such conditions are no tags available, a path that is
> busy (see also pgpath_busy()), path initialization that is in progress
> (pg_init_in_progress) or a request completes with status, e.g. if the
> SCSI core calls __blk_mq_end_request(req, error) with error != 0. For some
> of these conditions, e.g. path initialization completes, a callback
> function in the dm-mpath driver is called and it is possible to explicitly
> rerun the queue. I agree that for such scenario's a delayed queue run should
> not be triggered. For other scenario's, e.g. if a SCSI initiator submits a
> SCSI request over a fabric and the SCSI target replies with "BUSY" then the
> SCSI core will end the I/O request with status BLK_STS_RESOURCE after the
> maximum number of retries has been reached (see also scsi_io_completion()).
> In that last case, if a SCSI target sends a "BUSY" reply over the wire back
> to the initiator, there is no other approach for the SCSI initiator to
> figure out whether it can queue another request than to resubmit the
> request. The worst possible strategy is to resubmit a request immediately
> because that will cause a significant fraction of the fabric bandwidth to
> be used just for replying "BUSY" to requests that can't be processed
> immediately.

The thing is, the 2 scenarios you are most concerned about have
_nothing_ to do with dm_mq_queue_rq() at all.  They occur as an error in
the IO path _after_ the request is successfully retrieved with
blk_get_request() and then submitted.
 
> The intention of commit 6077c2d706097c0 was to address the last mentioned
> case.

So it really makes me question why you think commit 6077c2d706097c0
addresses the issue you think it does.  And gives me more conviction to
remove 6077c2d706097c0.

It may help just by virtue of blindly kicking the queue if
blk_get_request() fails (regardless of the target is responding with
BUSY or not).  Very unsatisfying to say the least.

I think it'd be much more beneficial for dm-mpath.c:multipath_end_io()
to be trained to be respond more intelligently to BLK_STS_RESOURCE.

E.g. set BLK_MQ_S_SCHED_RESTART if requests are known to be outstanding
on the path.  This is one case where Ming said the queue would be
re-run, as detailed in this header:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=5b18cff4baedde77e0d69bd62a13ae78f9488d89

And Jens has reinforced to me that BLK_MQ_S_SCHED_RESTART is a means to
kicking the queue more efficiently.  _BUT_ I'm not seeing any external
blk-mq interface that exposes this capability to a blk-mq driver.  As is
BLK_MQ_S_SCHED_RESTART gets set very much in the bowels of blk-mq
(blk_mq_sched_mark_restart_hctx).

SO I have to do more homework here...

Ming or Jens: might you be able to shed some light on how dm-mpath
would/could set BLK_MQ_S_SCHED_RESTART?  A new function added that can
be called from q->softirq_done_fn in response to BLK_STS_RESOURCE?  Or
is this level of control (and blk-mq detail) something that a blk-mq
driver should need to care about?

Anyway, we may need to get to that level of blk-mq driver controlled
retry optimization.

But Bart: again this is a vastly different feedback loop than the
stabbing in the dark your commit 6077c2d706097c0 is doing.  

Mike