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

[Tim Chen]

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 :(

> +
> +	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