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:
>>
>>> 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
>>
>> IIUC, the goal of the patch is to optimize performance, right?  If so,
>> the benchmark score will help justify the change.
>>
>
> The objective is to enable the use of the MPOL_PREFERRED_MANY policy,
> which is essential for the correct functioning of memory demotion in
> conjunction with memory promotion. Once we can use memory promotion, we
> should be able to observe the same benefits as those provided by numa
> fault memory promotion. The actual benefit of numa fault migration is
> dependent on various factors such as the speed of the slower memory
> device, the access pattern of the application, etc. We are discussing
> its effectiveness and how to improve numa fault overhead in other
> forums. However, we believe that this discussion should not hinder the
> merging of this patch.
>
> This change is similar to commit bda420b98505 ("numa balancing: migrate
> on fault among multiple bound nodes")

We provide the performance data in the description of that commit :-)

--
Best Regards,
Huang, Ying