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>]

On 2/20/24 12:06 PM, Huang, Ying wrote:
> Donet Tom <donettom@linux.ibm.com> writes:
> 
>> On 2/19/24 17:37, Michal Hocko wrote:
>>> On Sat 17-02-24 01:31:35, Donet Tom wrote:
>>>> 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.
>>> The feature makes sense to me. How has this been tested? Do you have any
>>> numbers to present?
>>
>> Hi Michal
>>
>> I have a test program which allocate memory on a specified node and
>> trigger the promotion or migration (Keep accessing the pages).
>>
>> Without this patch if we set MPOL_PREFERRED_MANY promotion or migration was not happening
>> with this patch I could see pages are getting  migrated or promoted.
>>
>> My system has 2 CPU+DRAM node (Tier 1) and 1 PMEM node(Tier 2). Below
>> are my test results.
>>
>> In below table N0 and N1 are Tier1 Nodes. N6 is the Tier2 Node.
>> Exec_Node is the execution node, Policy is the nodes in nodemask and
>> "Curr Location Pages" is the node where pages present before migration
>> or promotion start.
>>
>> Tests Results
>> ------------------
>> Scenario 1:  if the executing node is in the policy node mask
>> ================================================================================
>> Exec_Node    Policy           Curr Location Pages       Observations
>> ================================================================================
>> N0           N0 N1 N6             N1                Pages Migrated from N1 to N0
>> N0           N0 N1 N6             N6                Pages Promoted from N6 to N0
>> N0           N0 N1                N1                Pages Migrated from N1 to N0
>> N0           N0 N1                N6                Pages Promoted from N6 to N0
>>
>> Scenario 2: If the folio node is in policy node mask and Exec node not in policy  node mask
>> ================================================================================
>> Exec_Node    Policy       Curr Location Pages       Observations
>> ================================================================================
>> N0           N1 N6             N1               Pages are not Migrating to N0
>> N0           N1 N6             N6               Pages are not migration to N0
>> N0           N1                N1               Pages are not Migrating to N0
>>
>> Scenario 3: both the folio node and executing node are outside the policy nodemask
>> ==============================================================================
>> Exec_Node    Policy         Curr Location Pages       Observations
>> ==============================================================================
>> N0            N1                     N6          Pages Promoted from N6 to N0
>> N0            N6                     N1          Pages Migrated from N1 to N0
>>
> 
> Please use some benchmarks (e.g., redis + memtier) and show the
> proc-vmstat stats and benchamrk score.


Without this change numa fault migration is not supported with MPOL_PREFERRED_MANY
policy. So there is no performance comparison with and without patch. W.r.t effectiveness of numa
fault migration, that is a different topic from this patch


-aneesh