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:
>
>> 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.

--
Best Regards,
Huang, Ying