grant_table: convert grant table rwlock to percpu rwlock

grant_table: convert grant table rwlock to percpu rwlock

The per domain grant table read lock suffers from significant contention when
performance multi-queue block or network IO due to the parallel
grant map/unmaps/copies occurring on the DomU's grant table.

On multi-socket systems, the contention results in the locked compare swap
operation failing frequently which results in a tight loop of retries of the
compare swap operation. As the coherency fabric can only support a specific
rate of compare swap operations for a particular data location then taking
the read lock itself becomes a bottleneck for grant operations.

Standard rwlock performance of a single VIF VM-VM transfer with 16 queues
configured was limited to approximately 15 gbit/s on a 2 socket Haswell-EP
host.

Percpu rwlock performance with the same configuration is approximately
48 gbit/s.

Oprofile was used to determine the initial overhead of the read-write locks
and to confirm the overhead was dramatically reduced by the percpu rwlocks.

Signed-off-by: Malcolm Crossley <malcolm.crossley@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>