Keep track of the number of intra-node lock handoffs, and force
inter-node handoff once this number reaches a preset threshold.
The default value for the threshold can be overridden with
the new kernel boot command-line option "numa_spinlock_threshold".

Signed-off-by: Alex Kogan <redacted>
Reviewed-by: Steve Sistare <redacted>
---
 .../admin-guide/kernel-parameters.txt         |  8 ++++++
 arch/x86/kernel/alternative.c                 | 27 +++++++++++++++++++
 kernel/locking/qspinlock.c                    |  3 +++
 kernel/locking/qspinlock_cna.h                | 27 ++++++++++++++++---
 4 files changed, 62 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 904cb32f592d..887fbfce701d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -3185,6 +3185,14 @@
 			Not specifying this option is equivalent to
 			numa_spinlock=auto.
 
+	numa_spinlock_threshold=	[NUMA, PV_OPS]
+			Set the threshold for the number of intra-node
+			lock hand-offs before the NUMA-aware spinlock
+			is forced to be passed to a thread on another NUMA node.
+			Valid values are in the [0..31] range. Smaller values
+			result in a more fair, but less performant spinlock, and
+			vice versa. The default value is 16.
+
 	cpu0_hotplug	[X86] Turn on CPU0 hotplug feature when
 			CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
 			Some features depend on CPU0. Known dependencies are:

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 6a4ccbf4e09c..28552e0491b5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c

@@ -723,6 +723,33 @@ static int __init numa_spinlock_setup(char *str)
 
 __setup("numa_spinlock=", numa_spinlock_setup);
 
+/*
+ * Controls the threshold for the number of intra-node lock hand-offs before
+ * the NUMA-aware variant of spinlock is forced to be passed to a thread on
+ * another NUMA node. By default, the chosen value provides reasonable
+ * long-term fairness without sacrificing performance compared to a lock
+ * that does not have any fairness guarantees.
+ */
+int intra_node_handoff_threshold = 1 << 16;