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

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

Donet Tom <donettom@linux.ibm.com> writes:

> Hi

Hi, Donet,

> 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?

You can search NUMA_BALANCING_MEMORY_TIERING to find out what it does.
We can get better performance as the original commit message says.

When NUMA_BALANCING_MEMORY_TIERING is introduced, we didn't change the
original behavior of NUMA_BALANCING_MEMORY_NORMAL because we had no good
reason to do that.  In fact, you change its behavior, so you should
provide some supporting data or bug report to justify the change.

> 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?

You can change this, if you have some supporting data or bug report.

---
Best Regards,
Huang, Ying