Subject: [PATCH v5 0/5] Add NUMA-awareness to qspinlock

From: Alex Kogan <[email-masked]>
Date: Wed, 16 Oct 2019 00:28:58 -0400

Changes from v4:
----------------

- Switch to a deterministic bound on the number of intra-node handoffs,
as suggested by Longman.

- Scan the main queue after acquiring the MCS lock and before acquiring 
the spinlock (pre-scan), as suggested by Longman. If no thread is found 
in pre-scan, try again after acquiring the spinlock, resuming from the
same place where pre-scan stopped.

- Convert the secondary queue to a cyclic list such that the tail’s @next
points to the head of the queue. Store the pointer to the secondary queue
tail (rather than head) in @locked. This eliminates the need for the @tail
field in CNA nodes, making space for fields required by the two changes
above.

- Change arch_mcs_spin_lock_contended() to arch_mcs_spin_lock(), and
fix misuse of old macro names, as suggested by Hanjun.

Summary
-------

Lock throughput can be increased by handing a lock to a waiter on the
same NUMA node as the lock holder, provided care is taken to avoid
starvation of waiters on other NUMA nodes. This patch introduces CNA
(compact NUMA-aware lock) as the slow path for qspinlock. It is
enabled through a configuration option (NUMA_AWARE_SPINLOCKS).

CNA is a NUMA-aware version of the MCS lock. Spinning threads are
organized in two queues, a main queue for threads running on the same
node as the current lock holder, and a secondary queue for threads
running on other nodes. Threads store the ID of the node on which
they are running in their queue nodes. After acquiring the MCS lock and
before acquiring the spinlock, the lock holder scans the main queue
looking for a thread running on the same node (pre-scan). If found (call
it thread T), all threads in the main queue between the current lock
holder and T are moved to the end of the secondary queue.  If such T
is not found, we make another scan of the main queue after acquiring 
the spinlock when unlocking the MCS lock (post-scan), starting at the
node where pre-scan stopped. If both scans fail to find such T, the
MCS lock is passed to the first thread in the secondary queue. If the
secondary queue is empty, the MCS lock is passed to the next thread in the
main queue. To avoid starvation of threads in the secondary queue, those
threads are moved back to the head of the main queue after a certain
number of intra-node lock hand-offs.

More details are available at https://arxiv.org/abs/1810.05600.

We have done some performance evaluation with the locktorture module
as well as with several benchmarks from the will-it-scale repo.
The following locktorture results are from an Oracle X5-4 server
(four Intel Xeon E7-8895 v3 @ 2.60GHz sockets with 18 hyperthreaded
cores each). Each number represents an average (over 25 runs) of the
total number of ops (x10^7) reported at the end of each run. The 
standard deviation is also reported in (), and in general is about 3%
from the average. The 'stock' kernel is v5.4.0-rc1,
commit d90f2df63c5c, compiled in the default configuration. 
'patch-CNA' is the modified kernel with NUMA_AWARE_SPINLOCKS set; 
the speedup is calculated dividing 'patch-CNA' by 'stock'.

#thr  	 stock        patch-CNA   speedup (patch-CNA/stock)
  1  2.674 (0.118)  2.736 (0.119)  1.023
  2  2.588 (0.141)  2.603 (0.108)  1.006
  4  4.230 (0.120)  4.220 (0.127)  0.998
  8  5.362 (0.181)  6.679 (0.182)  1.246
 16  6.639 (0.133)  8.050 (0.200)  1.213
 32  7.359 (0.149)  8.792 (0.168)  1.195
 36  7.443 (0.142)  8.873 (0.230)  1.192
 72  6.554 (0.147)  9.317 (0.158)  1.421
108  6.156 (0.093)  9.404 (0.191)  1.528
142  5.659 (0.093)  9.361 (0.184)  1.654

The following tables contain throughput results (ops/us) from the same
setup for will-it-scale/open1_threads: 

#thr  	 stock        patch-CNA   speedup (patch-CNA/stock)
  1  0.532 (0.002)  0.532 (0.003)  1.000
  2  0.785 (0.024)  0.779 (0.025)  0.992
  4  1.426 (0.018)  1.409 (0.021)  0.988
  8  1.779 (0.101)  1.711 (0.127)  0.962
 16  1.761 (0.093)  1.671 (0.104)  0.949
 32  0.935 (0.063)  1.619 (0.093)  1.731
 36  0.936 (0.082)  1.591 (0.086)  1.699
 72  0.839 (0.043)  1.667 (0.097)  1.988
108  0.842 (0.035)  1.701 (0.091)  2.021
142  0.830 (0.037)  1.714 (0.098)  2.066

and will-it-scale/lock2_threads:

#thr  	 stock        patch-CNA   speedup (patch-CNA/stock)
  1  1.555 (0.009)  1.577 (0.002)  1.014
  2  2.644 (0.060)  2.682 (0.062)  1.014
  4  5.159 (0.205)  5.197 (0.231)  1.007
  8  4.302 (0.221)  4.279 (0.318)  0.995
 16  4.259 (0.111)  4.087 (0.163)  0.960
 32  2.583 (0.112)  4.077 (0.120)  1.578
 36  2.499 (0.106)  4.076 (0.106)  1.631
 72  1.979 (0.085)  4.077 (0.123)  2.061
108  2.096 (0.090)  4.043 (0.130)  1.929
142  1.913 (0.109)  3.984 (0.108)  2.082

Our evaluation shows that CNA also improves performance of user 
applications that have hot pthread mutexes. Those mutexes are 
blocking, and waiting threads park and unpark via the futex 
mechanism in the kernel. Given that kernel futex chains, which
are hashed by the mutex address, are each protected by a 
chain-specific spin lock, the contention on a user-mode mutex 
translates into contention on a kernel level spinlock. 

Here are the results for the leveldb ‘readrandom’ benchmark:

#thr  	 stock        patch-CNA   speedup (patch-CNA/stock)
  1  0.532 (0.007)  0.535 (0.015)  1.006
  2  0.665 (0.030)  0.673 (0.034)  1.011
  4  0.715 (0.023)  0.716 (0.026)  1.002
  8  0.686 (0.023)  0.686 (0.024)  1.001
 16  0.719 (0.030)  0.737 (0.025)  1.025
 32  0.740 (0.034)  0.959 (0.105)  1.296
 36  0.730 (0.024)  1.079 (0.112)  1.478
 72  0.652 (0.018)  1.160 (0.024)  1.778
108  0.622 (0.016)  1.157 (0.028)  1.860
142  0.600 (0.015)  1.145 (0.035)  1.908

Additional performance numbers are available in previous revisions
of the series.

Further comments are welcome and appreciated.

Alex Kogan (5):
  locking/qspinlock: Rename mcs lock/unlock macros and make them more
    generic
  locking/qspinlock: Refactor the qspinlock slow path
  locking/qspinlock: Introduce CNA into the slow path of qspinlock
  locking/qspinlock: Introduce starvation avoidance into CNA
  locking/qspinlock: Introduce the shuffle reduction optimization into
    CNA

 arch/arm/include/asm/mcs_spinlock.h |   6 +-
 arch/x86/Kconfig                    |  19 +++
 arch/x86/include/asm/qspinlock.h    |   4 +
 arch/x86/kernel/alternative.c       |  41 +++++
 include/asm-generic/mcs_spinlock.h  |   4 +-
 kernel/locking/mcs_spinlock.h       |  20 +--
 kernel/locking/qspinlock.c          |  77 ++++++++-
 kernel/locking/qspinlock_cna.h      | 312 ++++++++++++++++++++++++++++++++++++
 kernel/locking/qspinlock_paravirt.h |   2 +-
 9 files changed, 462 insertions(+), 23 deletions(-)
 create mode 100644 kernel/locking/qspinlock_cna.h

-- 
2.11.0 (Apple Git-81)

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