FUSE: fixes to improve scalability on NUMA systems

[Srinivas Eeda <srinivas.eeda@oracle.com>]

Hi Miklos and all,

I would like to submit following fixes for review which enhance FUSE
scalability on NUMA systems. The changes add new mount option 'numa' and
involve kernel and user library changes. I am currently forwarding kernel fixes
only and will forward library changes once the kernel fixes seem ok to you :)

In our internal tests, we noticed that FUSE was not scaling well when multiple
users access same mount point. The contention is on fc->lock spinlock, but the
real problem is not the spinlock itself but because of the latency involved
in accessing single spinlock from multiple NUMA nodes. 

This fix groups various fields in fuse_conn and creates a set for each NUMA
node to reduce contention. A spinlock is created for each NUMA node which will
synchronize access to node local set. All processes will now access nodes local
spinlock thus reducing latency. To get this behavior users(fuse library) or
the file system implementers should pass 'numa' mount option. If 'numa' option
is not specified during mount, FUSE will create single set of the grouped
fields and behavior is similar to current. File systems that support NUMA
option should listen on /dev/fuse from all NUMA nodes to serve
incoming/outgoing requests. If File systems are using fuse library then the
library will do that for them.

Please let me know any suggestions you may have to fix this better. I am open
to any ideas that improve the performance :)

TESTS:

We did some simple dd tests to read/write to FUSE based fs and also ran some
database workloads. All tests show that these fixes improve performance.
 
1) Writes to single mount
 dd process   throughput(without numa)  throughput(with numa)
    4                 1254 M                  1200 M
    8                  764 M                  1931 M
    16                 430 M                  1931 M
    32                 454 M                  2641 M
    64                 448 M                  2891 M
    128                435 M                  7414 M

2) Reading from single mount
# dd process   throughput(without numa)  throughput(with numa)
    4                 550 M                    581 M
    8                1013 M                   1071 M
    16                430 M                   1767 M
    32                314 M                   2114 M
    64                300 M                   3872 M
    128               297 M                   4563 M

3) Other workloads

   a) Database load tables application which used to take 47 min
      now finishes in 8 minutes.

   b) Data generation application which takes 2hr 50min now
      finishes in 1hr 21min.

Thanks,
--Srini