RCU-ified dm-mpath for testing/review

[Mike Snitzer]

On Fri, Feb 12 2016 at 10:18am -0500,
Hannes Reinecke <hare at suse.de> wrote:

> On 02/11/2016 04:34 PM, Mike Snitzer wrote:
> > On Wed, Feb 10 2016 at  8:50pm -0500,
> > Mike Snitzer <snitzer at redhat.com> wrote:
> > 
> >> On Tue, Feb 09 2016 at  7:45pm -0500,
> >> Mike Snitzer <snitzer at redhat.com> wrote:
> >>
> >>>
> >>> OK, I took a crack at embracing RCU.  Only slightly better performance
> >>> on my single NUMA node testbed.  (But I'll have to track down a system
> >>> with multiple NUMA nodes to do any justice to the next wave of this
> >>> optimization effort)
> >>>
> >>> This RCU work is very heavy-handed and way too fiddley (there could
> >>> easily be bugs).  Anyway, please see:
> >>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=devel2&id=d80a7e4f8b5be9c81e4d452137623b003fa64745
> >>>
> >>> But this might give you something to build on to arrive at something
> >>> more scalable?
> >>
> >> I've a bit more polished version of this work (broken up into multiple
> >> commits, with some fixes, etc) here:
> >> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel3
> >>
> >> Hannes and/or Sagi, if you get a chance to try this on your NUMA system
> >> please let me know how it goes.
> > 
> > Initial review has uncovered some locking problems with the current code
> > (nothing that caused crashes or hangs in my testing but...) so please
> > hold off on testing until you hear from me (hopefully tomorrow).
> > 
> Good news is that I've managed to hit the roof for my array with the
> devel2 version of those patches. (And a _heavily_ patched-up lpfc
> driver :-)
> So from that perspective everything's fine now; we've reached the
> hardware limit for my setup.
> Which in itself is quite impressive; beating Intel P3700 with 16FC
> is not bad methinks :-)
> 
> So thanks for all your work here.

Ah, that's really good news!  But devel2 is definitely _not_ destined
for upstream.  'devel3' is much closer to "ready".  But your testing and
review would really push it forward.

Please see/test:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel3

Also, please read this header:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=devel3&id=65a01b76502dd68e8ca298ee6614c0151b677f4a

Even with devel2 I hacked it such that repeat_count > 1 is effectively
broken.  I'm now _seriously_ considering deprecating repeat_count
completely (adding a DMWARN that will inform the user. e.g.:
"repeat_count > 1 is no longer supported").  I see no point going to
great lengths to maintain a dm-mpath feature that was only a hack for
when dm-mpath was bio-based.  What do you think?

Thanks,
Mike