While some parts of the core kernel reached a relatively stable “done”
state years ago, others never really seem to be finished. One of the
latter variety is undoubtedly the kernel’s implementation of spinlocks,
which arbitrate access to data at the lowest levels of the kernel. Lock
performance can have a significant effect on the performance of the system
as a whole, so optimization work can pay back big dividends. Lest one
think that this work is finally done, the NUMA-aware
qspinlock patch set shows how some more performance can be squeezed out
of the kernel’s spinlock implementation.

Source: LWN.net – [$] NUMA-aware qspinlocks