[RFC PATCH] dm: fix excessive dm-mq context switching

[Mike Snitzer]

On Tue, Feb 09 2016 at 10:32am -0500,
Hannes Reinecke <hare at suse.de> wrote:

> On 02/09/2016 03:55 PM, Mike Snitzer wrote:
> > On Tue, Feb 09 2016 at  2:50am -0500,
> > Hannes Reinecke <hare at suse.de> wrote:
> > 
> >> On 02/07/2016 06:20 PM, Mike Snitzer wrote:
> >>> On Sun, Feb 07 2016 at 11:54am -0500,
> >>> Sagi Grimberg <sagig at dev.mellanox.co.il> wrote:
> >>>
> >>>>
> >>>>>> If so, can you check with e.g.
> >>>>>> perf record -ags -e LLC-load-misses sleep 10 && perf report whether this
> >>>>>> workload triggers perhaps lock contention ? What you need to look for in
> >>>>>> the perf output is whether any functions occupy more than 10% CPU time.
> >>>>>
> >>>>> I will, thanks for the tip!
> >>>>
> >>>> The perf report is very similar to the one that started this effort..
> >>>>
> >>>> I'm afraid we'll need to resolve the per-target m->lock in order
> >>>> to scale with NUMA...
> >>>
> >>> Could be.  Just for testing, you can try the 2 topmost commits I've put
> >>> here (once applied both __multipath_map and multipath_busy won't have
> >>> _any_ locking.. again, very much test-only):
> >>>
> >>> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=devel2
> >>>
> >> So, I gave those patches a spin.
> >> Sad to say, they do _not_ resolve the issue fully.
> >>
> >> My testbed (2 paths per LUN, 40 CPUs, 4 cores) yields 505k IOPs with
> >> those patches.
> > 
> > That isn't a surprise.  We knew the m->lock spinlock contention to be a
> > problem.  And NUMA makes it even worse.
> > 
> >> Using a single path (without those patches, but still running
> >> multipath on top of that path) the same testbed yields 550k IOPs.
> >> Which very much smells like a lock contention ...
> >> We do get a slight improvement, though; without those patches I
> >> could only get about 350k IOPs. But still, I would somehow expect 2
> >> paths to be faster than just one ..
> > 
> > https://www.redhat.com/archives/dm-devel/2016-February/msg00036.html
> > 
> > hint hint...
> > 
> I hoped they wouldn't be needed with your patches.
> Plus perf revealed that I first need to address a spinlock
> contention in the lpfc driver before that even would make sense.
> 
> So more debugging to follow.

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?

Mike