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[Xen-devel] [PATCHv2 0/3] Implement per-cpu reader-writer locks

This patch series adds per-cpu reader-writer locks as a generic lock
implementation and then converts the grant table and p2m rwlocks to
use the percpu rwlocks, in order to improve multi-socket host performance.

CPU profiling has revealed the rwlocks themselves suffer from severe cache
line bouncing due to the cmpxchg operation used even when taking a read lock.
Multiqueue paravirtualised I/O results in heavy contention of the grant table
and p2m read locks of a specific domain and so I/O throughput is bottlenecked
by the overhead of the cache line bouncing itself.

Per-cpu read locks avoid lock cache line bouncing by using a per-cpu data
area to record a CPU has taken the read lock. Correctness is enforced for the 
write lock by using a per lock barrier which forces the per-cpu read lock 
to revert to using a standard read lock. The write lock then polls all 
the percpu data area until active readers for the lock have exited.

Removing the cache line bouncing on a multi-socket Haswell-EP system 
dramatically improves performance, with 16 vCPU network IO performance going 
from 15 gb/s to 64 gb/s! The host under test was fully utilising all 40 
logical CPU's at 64 gb/s, so a bigger logical CPU host may see an even better
IO improvement.

Note: Benchmarking of the these performance improvements should be done with 
the non debug version of the hypervisor otherwise the map_domain_page spinlock
is the main bottleneck.

Changes in V2:
- Add Cover letter
- Convert p2m rwlock to percpu rwlock
- Improve percpu rwlock to safely handle simultaneously holding 2 or more 
- Move percpu rwlock barrier from global to per lock
- Move write lock cpumask variable to a percpu variable
- Add macros to help initialise and use percpu rwlocks
- Updated IO benchmark results to cover revised locking implementation

 xen/arch/arm/mm.c             |   4 +-
 xen/arch/x86/mm.c             |   4 +-
 xen/arch/x86/mm/mm-locks.h    |  12 +++--
 xen/arch/x86/mm/p2m.c         |   1 +
 xen/common/grant_table.c      | 112 +++++++++++++++++++++---------------------
 xen/common/spinlock.c         |  38 ++++++++++++++
 xen/include/asm-arm/percpu.h  |   5 ++
 xen/include/asm-x86/mm.h      |   2 +-
 xen/include/asm-x86/percpu.h  |   6 +++
 xen/include/xen/grant_table.h |   4 +-
 xen/include/xen/percpu.h      |   4 ++
 xen/include/xen/spinlock.h    |  87 ++++++++++++++++++++++++++++++++
 12 files changed, 211 insertions(+), 68 deletions(-)


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