Re: [PATCH 3/3] mm/numa_balancing:Allow migrate on protnone reference with MPOL_PREFERRED_MANY policy

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

"Huang, Ying" <ying.huang@intel.com> writes:

> Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:
>
>> "Huang, Ying" <ying.huang@intel.com> writes:
>>
>>> Donet Tom <donettom@linux.ibm.com> writes:
>>>
>>>> commit bda420b98505 ("numa balancing: migrate on fault among multiple bound
>>>> nodes") added support for migrate on protnone reference with MPOL_BIND
>>>> memory policy. This allowed numa fault migration when the executing node
>>>> is part of the policy mask for MPOL_BIND. This patch extends migration
>>>> support to MPOL_PREFERRED_MANY policy.
>>>>
>>>> Currently, we cannot specify MPOL_PREFERRED_MANY with the mempolicy flag
>>>> MPOL_F_NUMA_BALANCING. This causes issues when we want to use
>>>> NUMA_BALANCING_MEMORY_TIERING. To effectively use the slow memory tier,
>>>> the kernel should not allocate pages from the slower memory tier via
>>>> allocation control zonelist fallback. Instead, we should move cold pages
>>>> from the faster memory node via memory demotion. For a page allocation,
>>>> kswapd is only woken up after we try to allocate pages from all nodes in
>>>> the allocation zone list. This implies that, without using memory
>>>> policies, we will end up allocating hot pages in the slower memory tier.
>>>>
>>>> MPOL_PREFERRED_MANY was added by commit b27abaccf8e8 ("mm/mempolicy: add
>>>> MPOL_PREFERRED_MANY for multiple preferred nodes") to allow better
>>>> allocation control when we have memory tiers in the system. With
>>>> MPOL_PREFERRED_MANY, the user can use a policy node mask consisting only
>>>> of faster memory nodes. When we fail to allocate pages from the faster
>>>> memory node, kswapd would be woken up, allowing demotion of cold pages
>>>> to slower memory nodes.
>>>>
>>>> With the current kernel, such usage of memory policies implies we can't
>>>> do page promotion from a slower memory tier to a faster memory tier
>>>> using numa fault. This patch fixes this issue.
>>>>
>>>> For MPOL_PREFERRED_MANY, if the executing node is in the policy node
>>>> mask, we allow numa migration to the executing nodes. If the executing
>>>> node is not in the policy node mask but the folio is already allocated
>>>> based on policy preference (the folio node is in the policy node mask),
>>>> we don't allow numa migration. If both the executing node and folio node
>>>> are outside the policy node mask, we allow numa migration to the
>>>> executing nodes.
>>>>
>>>> Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
>>>> Signed-off-by: Donet Tom <donettom@linux.ibm.com>
>>>> ---
>>>>  mm/mempolicy.c | 28 ++++++++++++++++++++++++++--
>>>>  1 file changed, 26 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>>>> index 73d698e21dae..8c4c92b10371 100644
>>>> --- a/mm/mempolicy.c
>>>> +++ b/mm/mempolicy.c
>>>> @@ -1458,9 +1458,10 @@ static inline int sanitize_mpol_flags(int *mode, unsigned short *flags)
>>>>  	if ((*flags & MPOL_F_STATIC_NODES) && (*flags & MPOL_F_RELATIVE_NODES))
>>>>  		return -EINVAL;
>>>>  	if (*flags & MPOL_F_NUMA_BALANCING) {
>>>> -		if (*mode != MPOL_BIND)
>>>> +		if (*mode == MPOL_BIND || *mode == MPOL_PREFERRED_MANY)
>>>> +			*flags |= (MPOL_F_MOF | MPOL_F_MORON);
>>>> +		else
>>>>  			return -EINVAL;
>>>> -		*flags |= (MPOL_F_MOF | MPOL_F_MORON);
>>>>  	}
>>>>  	return 0;
>>>>  }
>>>> @@ -2463,6 +2464,23 @@ static void sp_free(struct sp_node *n)
>>>>  	kmem_cache_free(sn_cache, n);
>>>>  }
>>>>  
>>>> +static inline bool mpol_preferred_should_numa_migrate(int exec_node, int folio_node,
>>>> +					    struct mempolicy *pol)
>>>> +{
>>>> +	/* if the executing node is in the policy node mask, migrate */
>>>> +	if (node_isset(exec_node, pol->nodes))
>>>> +		return true;
>>>> +
>>>> +	/* If the folio node is in policy node mask, don't migrate */
>>>> +	if (node_isset(folio_node, pol->nodes))
>>>> +		return false;
>>>> +	/*
>>>> +	 * both the folio node and executing node are outside the policy nodemask,
>>>> +	 * migrate as normal numa fault migration.
>>>> +	 */
>>>> +	return true;
>>>
>>> Why?  This may cause some unexpected result.  For example, pages may be
>>> distributed among multiple sockets unexpectedly.  So, I prefer the more
>>> conservative policy, that is, only migrate if this node is in
>>> pol->nodes.
>>>
>>
>> This will only have an impact if the user specifies
>> MPOL_F_NUMA_BALANCING. This means that the user is explicitly requesting
>> for frequently accessed memory pages to be migrated. Memory policy
>> MPOL_PREFERRED_MANY is able to allocate pages from nodes outside of
>> policy->nodes. For the specific use case that I am interested in, it
>> should be okay to restrict it to policy->nodes. However, I am wondering
>> if this is too restrictive given the definition of MPOL_PREFERRED_MANY.
>
> IMHO, we can start with some consecutive way and expand it if it's
> proved necessary.
>

Is this good?

1 file changed, 14 insertions(+), 34 deletions(-)
mm/mempolicy.c | 48 ++++++++++++++----------------------------------

modified   mm/mempolicy.c
@@ -2464,23 +2464,6 @@ static void sp_free(struct sp_node *n)
 	kmem_cache_free(sn_cache, n);
 }
 
-static inline bool mpol_preferred_should_numa_migrate(int exec_node, int folio_node,
-					    struct mempolicy *pol)
-{
-	/* if the executing node is in the policy node mask, migrate */
-	if (node_isset(exec_node, pol->nodes))
-		return true;
-
-	/* If the folio node is in policy node mask, don't migrate */
-	if (node_isset(folio_node, pol->nodes))
-		return false;
-	/*
-	 * both the folio node and executing node are outside the policy nodemask,
-	 * migrate as normal numa fault migration.
-	 */
-	return true;
-}
-
 /**
  * mpol_misplaced - check whether current folio node is valid in policy
  *
@@ -2533,29 +2516,26 @@ int mpol_misplaced(struct folio *folio, struct vm_fault *vmf,
 		break;
 
 	case MPOL_BIND:
-		/* Optimize placement among multiple nodes via NUMA balancing */
+	case MPOL_PREFERRED_MANY:
+		/*
+		 * Even though MPOL_PREFERRED_MANY can allocate pages outside
+		 * policy nodemask we don't allow numa migration to nodes
+		 * outside policy nodemask for now. This is done so that if we
+		 * want demotion to slow memory to happen, before allocating
+		 * from some DRAM node say 'x', we will end up using a
+		 * MPOL_PREFERRED_MANY mask excluding node 'x'. In such scenario
+		 * we should not promote to node 'x' from slow memory node.
+		 */
 		if (pol->flags & MPOL_F_MORON) {
+			/*
+			 * Optimize placement among multiple nodes
+			 * via NUMA balancing
+			 */
 			if (node_isset(thisnid, pol->nodes))
 				break;
 			goto out;
 		}
 
-		if (node_isset(curnid, pol->nodes))
-			goto out;
-		z = first_zones_zonelist(
-				node_zonelist(thisnid, GFP_HIGHUSER),
-				gfp_zone(GFP_HIGHUSER),
-				&pol->nodes);
-		polnid = zone_to_nid(z->zone);
-		break;
-
-	case MPOL_PREFERRED_MANY:
-		if (pol->flags & MPOL_F_MORON) {
-			if (!mpol_preferred_should_numa_migrate(thisnid, curnid, pol))
-				goto out;
-			break;
-		}
-
 		/*
 		 * use current page if in policy nodemask,
 		 * else select nearest allowed node, if any.

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