Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on explicit memory tiers

Ying Huang
2022-06-08T07:22:16Z

On Tue, 2022-06-07 at 15:51 -0700, Tim Chen wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
> >
> > +int next_demotion_node(int node)
> > +{
> > + struct demotion_nodes *nd;
> > + int target, nnodes, i;
> > +
> > + if (!node_demotion)
> > + return NUMA_NO_NODE;
> > +
> > + nd = &node_demotion[node];
> > +
> > + /*
> > + * node_demotion[] is updated without excluding this
> > + * function from running.
> > + *
> > + * Make sure to use RCU over entire code blocks if
> > + * node_demotion[] reads need to be consistent.
> > + */
> > + rcu_read_lock();
> > +
> > + nnodes = nodes_weight(nd->preferred);
> > + if (!nnodes)
> > + return NUMA_NO_NODE;
> > +
> > + /*
> > + * If there are multiple target nodes, just select one
> > + * target node randomly.
> > + *
> > + * In addition, we can also use round-robin to select
> > + * target node, but we should introduce another variable
> > + * for node_demotion[] to record last selected target node,
> > + * that may cause cache ping-pong due to the changing of
> > + * last target node. Or introducing per-cpu data to avoid
> > + * caching issue, which seems more complicated. So selecting
> > + * target node randomly seems better until now.
> > + */
> > + nnodes = get_random_int() % nnodes;
> > + target = first_node(nd->preferred);
> > + for (i = 0; i < nnodes; i++)
> > + target = next_node(target, nd->preferred);
>
> We can simplify the above 4 lines.
>
> target = node_random(nd->preferred);
>
> There's still a loop overhead though :(

To avoid loop overhead, we can use the original implementation of
next_demotion_node. The performance is much better for the most common
cases, the number of preferred node is 1.

Best Regards,
Huang, Ying

> >
> >

> > +
> > + rcu_read_unlock();
> > +
> > + return target;
> > +}
> > +
> >
> > + */
> > +static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
> > + unsigned long action, void *_arg)
> > +{
> > + struct memory_notify *arg = _arg;
> > +
> > + /*
> > + * Only update the node migration order when a node is
> > + * changing status, like online->offline.
> > + */
> > + if (arg->status_change_nid < 0)
> > + return notifier_from_errno(0);
> > +
> > + switch (action) {
> > + case MEM_OFFLINE:
> > + /*
> > + * In case we are moving out of N_MEMORY. Keep the node
> > + * in the memory tier so that when we bring memory online,
> > + * they appear in the right memory tier. We still need
> > + * to rebuild the demotion order.
> > + */
> > + mutex_lock(&memory_tier_lock);
> > + establish_migration_targets();
> > + mutex_unlock(&memory_tier_lock);
> > + break;
> > + case MEM_ONLINE:
> > + /*
> > + * We ignore the error here, if the node already have the tier
> > + * registered, we will continue to use that for the new memory
> > + * we are adding here.
> > + */
> > + node_set_memory_tier(arg->status_change_nid, DEFAULT_MEMORY_TIER);
>
> Should establish_migration_targets() be run here? Otherwise what are the
> demotion targets for this newly onlined node?
>
> > + break;
> > + }
> > +
> > + return notifier_from_errno(0);
> > +}
> > +
>
> Tim
>