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

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Date:   Mon, 18 Jul 2022 15:41:39 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.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


"Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:

> 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?

Why though? As far as I can tell node_is_toptier() only makes a
difference if CONFIG_MIGRATION is enabled, so it could just return a
static value if CONFIG_MIGRATION isn't enabled.

 - Alistair

>>
>>> +		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