Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

Peter Zijlstra
2020-01-23T15:29:40Z

On Thu, Jan 23, 2020 at 09:15:55AM -0500, Waiman Long wrote:
> On 1/14/20 10:59 PM, Alex Kogan wrote:
> > +static int __init cna_init_nodes(void)
> > +{
> > + unsigned int cpu;
> > +
> > + /*
> > + * this will break on 32bit architectures, so we restrict
> > + * the use of CNA to 64bit only (see arch/x86/Kconfig)
> > + */
> > + BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
> > + /* we store an ecoded tail word in the node's @locked field */
> > + BUILD_BUG_ON(sizeof(u32) > sizeof(unsigned int));
> > +
> > + for_each_possible_cpu(cpu)
> > + cna_init_nodes_per_cpu(cpu);
> > +
> > + return 0;
> > +}
> > +early_initcall(cna_init_nodes);
> > +
>
> I just realized that you shouldn't call cna_init_nodes as an
> early_initcall. Instead,
>
> > +/*
> > + * Switch to the NUMA-friendly slow path for spinlocks when we have
> > + * multiple NUMA nodes in native environment, unless the user has
> > + * overridden this default behavior by setting the numa_spinlock flag.
> > + */
> > +void __init cna_configure_spin_lock_slowpath(void)
> > +{
> > + if ((numa_spinlock_flag == 1) ||
> > + (numa_spinlock_flag == 0 && nr_node_ids > 1 &&
> > + pv_ops.lock.queued_spin_lock_slowpath ==
> > + native_queued_spin_lock_slowpath)) {
> > + pv_ops.lock.queued_spin_lock_slowpath =
> > + __cna_queued_spin_lock_slowpath;
> > +
> > + pr_info("Enabling CNA spinlock\n");
> > + }
> > +}
>
> call it when it is sure that CNA spinlock is going to be used. At this
> point, the system is still in UP mode and the slowpath will not be called.

Indeed, let me go fix that!