Re: [PATCH v8 0/5] Add NUMA-awareness to qspinlock

Shijith Thotton
2020-01-08T05:10:05Z

Hi Will,

On Mon, Dec 30, 2019 at 02:40:37PM -0500, Alex Kogan wrote:
> Minor changes from v7 based on feedback from Longman:
> -----------------------------------------------------
>
> - Move __init functions from alternative.c to qspinlock_cna.h
>
> - Introduce enum for return values from cna_pre_scan(), for better
> readability.
>
> - Add/revise a few comments to improve readability.
>
>
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_1810.05600&d=DwIDAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=G9w4KsPaQLACBfGCL35PtiRH996yqJDxAZwrWegU2qQ&m=AoOLTQlgNjtdBvY_yWd6ViBXrVM6o2wqXOdFA4B_F2A&s=yUjG9gfi3BtLKDEjgki86h52GVXMvDQ6ZClMvoIG034&e= .
>
> The series applies on top of v5.5.0-rc2, commit ea200dec51.
> 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
>
> .../admin-guide/kernel-parameters.txt | 18 +
> arch/arm/include/asm/mcs_spinlock.h | 6 +-
> arch/x86/Kconfig | 20 +
> arch/x86/include/asm/qspinlock.h | 4 +
> arch/x86/kernel/alternative.c | 4 +
> include/asm-generic/mcs_spinlock.h | 4 +-
> kernel/locking/mcs_spinlock.h | 20 +-
> kernel/locking/qspinlock.c | 82 +++-
> kernel/locking/qspinlock_cna.h | 400 ++++++++++++++++++
> kernel/locking/qspinlock_paravirt.h | 2 +-
> 10 files changed, 537 insertions(+), 23 deletions(-)
> create mode 100644 kernel/locking/qspinlock_cna.h
>
> --
> 2.21.0 (Apple Git-122.2)
>

Tried out queued spinlock slowpath improvements on arm64 (ThunderX2) by
hardwiring CNA APIs to queued_spin_lock_slowpath() and numbers are pretty
good with the CNA changes.

Speed-up on v5.5-rc4 kernel:

will-it-scale/open1_threads:
#thr speed-up
1 1.00
2 0.97
4 0.98
8 1.02
16 0.95
32 1.63
64 1.70
128 2.09
224 2.16

will-it-scale/lock2_threads:
#thr speed-up
1 0.98
2 0.99
4 0.90
8 0.98
16 0.99
32 1.52
64 2.31
128 2.25
224 2.04

#thr - number of threads
speed-up - number with CNA patch / number with stock kernel

Please share your thoughts on best way to enable this series on arm64.

Thanks,
Shijith