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

["Huang, Ying" <ying.huang@intel.com>]

Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:

> "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;

IMO, the above deletion should be put in another patch?

--
Best Regards,
Huang, Ying

> -
> -	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.
>
> [back]
> .