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

[Ying Huang <ying.huang@intel.com>]

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
>