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:

> 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()?

rcu_assign_pointer() includes the appropriate barriers to ensure any
initialisation will be visible, so I don't believe you need smp_wmb().

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

Yeah, that's what I was suggesting. Although to be clear I don't think
there was actually a correctness issue with what you had, because
memtier->nodelist is protected by memory_tier_lock anyway and not
accessed under the rcu_read_lock().

It just looked a little odd IMHO to be updating something that was still
potentially being used prior to synchronize_rcu() completing.

>>
>>> +	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()?

Yeah, rcu_assign_pointer() should include appropriate barriers anyway.

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