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

[Mike Snitzer]

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