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

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Alistair Popple <apopple@nvidia.com> writes:

> "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?

The synchronize_rcu ensures that the lockless read of pgdat->memtier
either see value NULL or a stable memtier which got current numa node in
its nodelist. IIUC what you are suggesting is we should move the
node_clear after synchronize_rcu?. I am also wondering whether I need
a smp_wmb()?

pgdat->memtier = NULL;
synchronize_rcu
remove node from memtier;
set node in new memtier
smp_wmb();
pgdat->memtier = new memtier;


>
>> +	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()?

Will fix this. This is early during boot. So the ordering won't impact
correctness. Hence i can skip the smp_wmb()? 

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