Re: [PATCH v9 6/8] mm/demotion: Add pg_data_t member to track node memory tier details

[Alistair Popple <apopple@nvidia.com>]

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

> Also update different helpes to use NODE_DATA()->memtier. Since
> node specific memtier can change based on the reassignment of
> NUMA node to a different memory tiers, accessing NODE_DATA()->memtier
> needs to happen under an rcu read lock or memory_tier_lock.
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  include/linux/mmzone.h |  3 ++
>  mm/memory-tiers.c      | 64 +++++++++++++++++++++++++++++++-----------
>  2 files changed, 50 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aab70355d64f..353812495a70 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -928,6 +928,9 @@ typedef struct pglist_data {
>  	/* Per-node vmstats */
>  	struct per_cpu_nodestat __percpu *per_cpu_nodestats;
>  	atomic_long_t		vm_stat[NR_VM_NODE_STAT_ITEMS];
> +#ifdef CONFIG_NUMA
> +	struct memory_tier __rcu *memtier;
> +#endif
>  } pg_data_t;
>
>  #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index e951f54ce56c..bab4700bf58d 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -7,6 +7,7 @@
>  #include <linux/moduleparam.h>
>  #include <linux/memory.h>
>  #include <linux/random.h>
> +#include <linux/rcupdate.h>
>  #include <linux/memory-tiers.h>
>
>  #include "internal.h"
> @@ -124,18 +125,23 @@ static struct memory_tier *register_memory_tier(unsigned int tier)
>  static void unregister_memory_tier(struct memory_tier *memtier)
>  {
>  	list_del(&memtier->list);
> -	kfree(memtier);
> +	kfree_rcu(memtier);
>  }
>
>  static struct memory_tier *__node_get_memory_tier(int node)
>  {
> -	struct memory_tier *memtier;
> +	pg_data_t *pgdat;
>
> -	list_for_each_entry(memtier, &memory_tiers, list) {
> -		if (node_isset(node, memtier->nodelist))
> -			return memtier;
> -	}
> -	return NULL;
> +	pgdat = NODE_DATA(node);
> +	if (!pgdat)
> +		return NULL;
> +	/*
> +	 * Since we hold memory_tier_lock, we can avoid
> +	 * RCU read locks when accessing the details. No
> +	 * parallel updates are possible here.
> +	 */
> +	return rcu_dereference_check(pgdat->memtier,
> +				     lockdep_is_held(&memory_tier_lock));
>  }
>
>  static struct memory_tier *__get_memory_tier_from_id(int id)
> @@ -149,6 +155,33 @@ static struct memory_tier *__get_memory_tier_from_id(int id)
>  	return NULL;
>  }
>
> +/*
> + * Called with memory_tier_lock. Hence the device references cannot
> + * be dropped during this function.
> + */
> +static void memtier_node_set(int node, struct memory_tier *memtier)
> +{
> +	pg_data_t *pgdat;
> +	struct memory_tier *current_memtier;
> +
> +	pgdat = NODE_DATA(node);
> +	if (!pgdat)
> +		return;
> +	/*
> +	 * Make sure we mark the memtier NULL before we assign the new memory tier
> +	 * to the NUMA node. This make sure that anybody looking at NODE_DATA
> +	 * finds a NULL memtier or the one which is still valid.
> +	 */
> +	current_memtier = rcu_dereference_check(pgdat->memtier,
> +						lockdep_is_held(&memory_tier_lock));
> +	rcu_assign_pointer(pgdat->memtier, NULL);
> +	if (current_memtier)
> +		node_clear(node, current_memtier->nodelist);

It seems odd to me that you would update the current memtier prior to
the synchronize_rcu(). I suppose it's really memory_tier_lock that
protects the details like ->nodelist, but is there any reason not do the
update after anyway?

> +	synchronize_rcu();
> +	node_set(node, memtier->nodelist);
> +	rcu_assign_pointer(pgdat->memtier, memtier);
> +}
> +
>  static int __node_create_and_set_memory_tier(int node, int tier)
>  {
>  	int ret = 0;
> @@ -162,7 +195,7 @@ static int __node_create_and_set_memory_tier(int node, int tier)
>  			goto out;
>  		}
>  	}
> -	node_set(node, memtier->nodelist);
> +	memtier_node_set(node, memtier);
>  out:
>  	return ret;
>  }
> @@ -184,14 +217,7 @@ int node_create_and_set_memory_tier(int node, int tier)
>  	if (current_tier->id == tier)
>  		goto out;
>
> -	node_clear(node, current_tier->nodelist);
> -
>  	ret = __node_create_and_set_memory_tier(node, tier);
> -	if (ret) {
> -		/* reset it back to older tier */
> -		node_set(node, current_tier->nodelist);
> -		goto out;
> -	}
>  	if (nodes_empty(current_tier->nodelist))
>  		unregister_memory_tier(current_tier);
>
> @@ -213,7 +239,7 @@ static int __node_set_memory_tier(int node, int tier)
>  		ret = -EINVAL;
>  		goto out;
>  	}
> -	node_set(node, memtier->nodelist);
> +	memtier_node_set(node, memtier);
>  out:
>  	return ret;
>  }
> @@ -428,6 +454,7 @@ static void __init migrate_on_reclaim_init(void)
>
>  static int __init memory_tier_init(void)
>  {
> +	int node;
>  	struct memory_tier *memtier;
>
>  	/*
> @@ -444,7 +471,10 @@ static int __init memory_tier_init(void)
>  		      __func__, PTR_ERR(memtier));
>
>  	/* CPU only nodes are not part of memory tiers. */
> -	memtier->nodelist = node_states[N_MEMORY];
> +	for_each_node_state(node, N_MEMORY) {
> +		rcu_assign_pointer(NODE_DATA(node)->memtier, memtier);
> +		node_set(node, memtier->nodelist);

Similar comment here - the order seems opposite to what I'd expect.
Shouldn't memtier->nodelist be fully initialised prior to making it
visible with rcu_assign_pointer()?

> +	}
>  	mutex_unlock(&memory_tier_lock);
>
>  	migrate_on_reclaim_init();