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

[Donet Tom]

Hi

On 4/2/26 8:57 AM, Huang, Ying wrote:
> Donet Tom <donettom@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxx/
> > ---
> >   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