linux-kernel - Re: [PATCH v9 5/8] mm/demotion: Build demotion targets based on explicit memory tiers

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Date:   Fri, 15 Jul 2022 12:51:51 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v9 5/8] mm/demotion: Build demotion targets based on
 explicit memory tiers

Alistair Popple <apopple@...dia.com> writes:
....

> + */
>> +static void establish_migration_targets(void)
>> +{
>> +	struct memory_tier *memtier;
>> +	struct demotion_nodes *nd;
>> +	int target = NUMA_NO_NODE, node;
>> +	int distance, best_distance;
>> +	nodemask_t used;
>> +
>> +	if (!node_demotion || !IS_ENABLED(CONFIG_MIGRATION))
>
> Does it make sense to include the memory tiering/demotion code if
> CONFIG_MIGRATION isn't enabled? From what I can tell none of the
> information established here is used if CONFIG_MIGRATION isn't enabled,
> so it would be better to remove the IS_ENABLED checks and not include
> the code at all.

We use the same function/codepath for updating top_tier details. We
would want to get node_is_toptier() to work even with CONFIG_MIGRATION
disabled? 

>
>> +		return;
>> +
>> +	disable_all_migrate_targets();
>> +
>> +	for_each_node_state(node, N_MEMORY) {
>> +		best_distance = -1;
>> +		nd = &node_demotion[node];
>> +
>> +		memtier = __node_get_memory_tier(node);
>> +		if (!memtier || list_is_last(&memtier->list, &memory_tiers))
>> +			continue;
>> +		/*
>> +		 * Get the next memtier to find the  demotion node list.
>> +		 */
>> +		memtier = list_next_entry(memtier, list);
>> +
>> +		/*
>> +		 * find_next_best_node, use 'used' nodemask as a skip list.
>> +		 * Add all memory nodes except the selected memory tier
>> +		 * nodelist to skip list so that we find the best node from the
>> +		 * memtier nodelist.
>> +		 */
>> +		nodes_andnot(used, node_states[N_MEMORY], memtier->nodelist);
>> +
>> +		/*
>> +		 * Find all the nodes in the memory tier node list of same best distance.
>> +		 * add them to the preferred mask. We randomly select between nodes
>> +		 * in the preferred mask when allocating pages during demotion.
>> +		 */
>> +		do {
>> +			target = find_next_best_node(node, &used);
>> +			if (target == NUMA_NO_NODE)
>> +				break;
>> +
>> +			distance = node_distance(node, target);
>> +			if (distance == best_distance || best_distance == -1) {
>> +				best_distance = distance;
>> +				node_set(target, nd->preferred);
>> +			} else {
>> +				break;
>> +			}
>> +		} while (1);
>> +	}
>> +}
>> +

.....

-aneesh