Re: [PATCH v2] memory tiering: Do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled

[Donet Tom <donettom@linux.ibm.com>]

Hi

On 4/2/26 8:57 AM, Huang, Ying wrote:
> Donet Tom <donettom@linux.ibm.com> writes:
>
>> In the current implementation, if NUMA_BALANCING_MEMORY_TIERING is
>> disabled and the pages are on the lower tier, the pages may still be
>> promoted.
>>
>> This happens because task_numa_work() updates the last_cpupid field to
>> record the last access time only when NUMA_BALANCING_MEMORY_TIERING is
>> enabled and the folio is on the lower tier. If
>> NUMA_BALANCING_MEMORY_TIERING is disabled, the last_cpupid field
>> can retains a valid last CPU id.
>>
>> In should_numa_migrate_memory(), the decision checks whether
>> NUMA_BALANCING_MEMORY_TIERING is disabled, the folio is on the lower
>> tier, and last_cpupid is invalid. However, the last_cpupid can be
>> valid when NUMA_BALANCING_MEMORY_TIERING is disabled, the condition
>> evaluates to false and migration is allowed.
>>
>> This patch prevents promotion when NUMA_BALANCING_MEMORY_TIERING is
>> disabled and the folio is on the lower tier.
>>
>> Behavior before this change:
>> ============================
>>    - If NUMA_BALANCING_NORMAL is enabled, migration occurs between
>>      nodes within the same memory tier, and promotion from lower
>>      tier to higher tier may also happen.
>>
>>    - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from
>>      lower tier to higher tier nodes is allowed.
>>
>> Behavior after this change:
>> ===========================
>>    - If NUMA_BALANCING_NORMAL is enabled, migration will occur only
>>      between nodes within the same memory tier.
>>
>>    - If NUMA_BALANCING_MEMORY_TIERING is enabled, promotion from lower
>>      tier to higher tier nodes will be allowed.
>>
>>    - If both NUMA_BALANCING_MEMORY_TIERING and NUMA_BALANCING_NORMAL are
>>      enabled, both migration (same tier) and promotion (cross tier) are
>>      allowed.
>>
>> Fixes: 33024536bafd ("memory tiering: hot page selection with hint page fault latency")
>> Signed-off-by: Donet Tom <donettom@linux.ibm.com>
>> ---
>> v1 -> v2
>> ========
>> 1. Dropped changes in task_numa_fault() since the original changes
>>     already handle runtime disabling of NUMA_BALANCING_MEMORY_TIERING.
>>
>> v1 -> https://lore.kernel.org/all/20260320092251.1290207-1-donettom@linux.ibm.com/
>> ---
>>   kernel/sched/fair.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index bf948db905ed..4b43809a3fb1 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -2024,8 +2024,12 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>   	this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
>>   	last_cpupid = folio_xchg_last_cpupid(folio, this_cpupid);
>>   
>> +	/*
>> +	 * Do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled
>> +	 * and the pages are on the lower tier.
>> +	 */
>>   	if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) &&
>> -	    !node_is_toptier(src_nid) && !cpupid_valid(last_cpupid))
>> +	    !node_is_toptier(src_nid))
>>   		return false;
>>   
>>   	/*
> No.  Even if NUMA_BALANCING_MEMORY_TIERING is disabled, we should still
> allow migrate pages from lower tier to higher tier via
> NUMA_BALANCING_NORMAL.  If we have precious DDR, why waste it?  This
> follows the semantics of NUMA_BALANCING_NORMAL before introducing
> NUMA_BALANCING_MEMORY_TIERING.

Thank you for the review comments.

One thing I am trying to understand is that page promotion
appears to happen regardless of whether
NUMA_BALANCING_MEMORY_TIERING is enabled or disabled. In that
case, what is the specific role of
NUMA_BALANCING_MEMORY_TIERING? Do we get better performance
when it is enabled?

My initial understanding was that disabling
NUMA_BALANCING_MEMORY_TIERING could be used to turn off
promotion. However, it seems that currently we cannot control
promotion independently. If NUMA_BALANCING_NORMAL is disabled,
neither migration nor promotion happens, and if it is enabled,
both migration and promotion can occur.

I was under the impression that:
- NUMA_BALANCING_NORMAL would handle migration within the same tier,
- NUMA_BALANCING_MEMORY_TIERING would handle promotion across tiers,
- and enabling both would allow both migration and promotion.

This would provide more fine-grained control. Is my
understanding correct, or am I missing something here?


>
> ---
> Best Regards,
> Huang, Ying