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)
