Re: [PATCH v2 3/3] sched/numa: fix scan-period adjustment for tiering-only mode

[Li Zhe]

On 10/29/26 2:21 PM, Bharata B Rao wrote:
> On 28-Oct-26 9:42 AM, Li Zhe wrote:
>> When automatic NUMA balancing runs in memory-tiering-only mode, NUMA hint
>> faults identify slow-memory pages that are worth promoting. They are not
>> normal task-placement feedback.
>>
>> Without separate tiering feedback, successful tiering promotions are fed
>> to update_task_scan_period() as private remote faults. That leaves
>> local == 0 and shared == 0, so the function hits its early slow-down path
>> and doubles p->numa_scan_period. Repeated promotion feedback can therefore
>> drive the scan period towards the maximum.
>>
>> Track memory-tiering feedback separately from normal task-placement stats.
>> Successful free-space bypass promotions are not counted as tiering scan
>> feedback because they did not pass the hotness and rate-limit checks. Cold,
>> hot, and throttled feedback is generated only by the hotness/rate-limit
>> path, while migration failures slow scanning like the normal placement
>> heuristic does.
>>
>> Use the same 70% threshold style as the normal scan-period heuristic. A
>> hot-dominated window speeds scanning up, a cold-dominated window slows it
>> down, and a throttled ratio of at least 30% also slows it down. If a
>> window has no tiering feedback, for example because promotions used the
>> free-space bypass, double the scan period, following the same
>> no-useful-signal slow-down direction as normal scan-period adjustment.
>>
>> Evaluate the tiering window when a scan sequence completes as well as when
>> tiering faults are reported. This lets an empty window back off the scan
>> period after qualifying tiering feedback stops.
>>
>> On a CXL system with node0 as DRAM and node3 as slow-tier memory, a
>> STREAM Triad-derived OpenMP workload was used to model a
>> bandwidth-sensitive working-set shift. It first accessed one 100G range,
>> then switched to another 100G range. The workload was first-touch placed
>> on node3 and then run from node0 with memory-tiering-only mode enabled.
>>
>> With the free-space bypass disabled, so that promotions have to pass the
>> hotness and rate-limit checks, a kernel with only the bypass control was
>> used as the no-feedback baseline. Tiering scan-period feedback improved
>> the transition window after the active range switch:
>>
>>                                no feedback       tiering feedback
>>    first 80s after switch       122.4 GB/s             129.3 GB/s  (+5.7%)
>>    200-240s after switch        139.9 GB/s             140.6 GB/s  (+0.5%)
>>
>> Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
>> ---
>>   include/linux/sched.h                |  17 ++++
>>   include/linux/sched/numa_balancing.h |   9 +++
>>   kernel/sched/fair.c                  | 113 ++++++++++++++++++++++++++-
>>   3 files changed, 137 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 1c93da75226e..c4959747374b 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -833,6 +833,16 @@ struct task_ipi_mask {
>>   struct task_ipi_mask { };
>>   #endif
>>   
>> +#ifdef CONFIG_NUMA_BALANCING
>> +enum numa_tiering_fault_stats {
>> +	NUMA_TIERING_FAULT_COLD,
>> +	NUMA_TIERING_FAULT_HOT,
>> +	NUMA_TIERING_FAULT_THROTTLED,
>> +	NUMA_TIERING_FAULT_MIGRATE_FAIL,
>> +	NR_NUMA_TIERING_FAULT_STATS
>> +};
>> +#endif
>> +
>>   struct task_struct {
>>   #ifdef CONFIG_THREAD_INFO_IN_TASK
>>   	/*
>> @@ -1433,6 +1443,13 @@ struct task_struct {
>>   	 */
>>   	unsigned long			numa_faults_locality[3];
>>   
>> +	/*
>> +	 * Memory tiering faults are tracked separately from normal NUMA
>> +	 * placement statistics. In tiering-only mode, slow-memory hint faults
>> +	 * are used to decide promotion, not task placement.
>> +	 */
>> +	unsigned long			numa_tiering_faults[NR_NUMA_TIERING_FAULT_STATS];
>> +
>>   	unsigned long			numa_pages_migrated;
>>   #endif /* CONFIG_NUMA_BALANCING */
>>   
>> diff --git a/include/linux/sched/numa_balancing.h b/include/linux/sched/numa_balancing.h
>> index 200f50075d5c..12f31ed56d66 100644
>> --- a/include/linux/sched/numa_balancing.h
>> +++ b/include/linux/sched/numa_balancing.h
>> @@ -8,6 +8,7 @@
>>    */
>>   
>>   #include <linux/sched.h>
>> +#include <linux/sched/sysctl.h>
>>   
>>   #define TNF_MIGRATED	0x01
>>   #define TNF_NO_GROUP	0x02
>> @@ -16,6 +17,14 @@
>>   #define TNF_MIGRATE_FAIL 0x10
>>   #define TNF_ACCESS_TIME	0x20
>>   
>> +static inline bool numa_balancing_tiering_only(void)
>> +{
>> +	int mode = sysctl_numa_balancing_mode;
>> +
>> +	return (mode & NUMA_BALANCING_MEMORY_TIERING) &&
>> +	       !(mode & NUMA_BALANCING_NORMAL);
>> +}
>> +
>>   enum numa_vmaskip_reason {
>>   	NUMAB_SKIP_UNSUITABLE,
>>   	NUMAB_SKIP_SHARED_RO,
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 136207b62d87..1d9efc2b7427 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -2760,6 +2760,32 @@ static inline bool cpupid_valid(int cpupid)
>>   	return cpupid_to_cpu(cpupid) < nr_cpu_ids;
>>   }
>>   
>> +static void task_numa_tiering_placement(struct task_struct *p);
>> +
>> +static bool task_numa_tiering_update(struct task_struct *p)
>> +{
>> +	if (!numa_balancing_tiering_only() || !p->mm)
>> +		return false;
>> +
>> +	task_numa_tiering_placement(p);
>> +	return true;
>> +}
>> +
>> +static void task_numa_tiering_no_feedback(struct task_struct *p)
>> +{
>> +	task_numa_tiering_update(p);
>> +}
>> +
>> +static void task_numa_tiering_fault(struct task_struct *p,
>> +				    enum numa_tiering_fault_stats stat,
>> +				    unsigned long pages)
>> +{
>> +	if (!task_numa_tiering_update(p))
>> +		return;
>> +
>> +	p->numa_tiering_faults[stat] += pages;
>> +}
>> +
>>   /*
>>    * For memory tiering mode, if the target fast memory node has enough free
>>    * pages, recently accessed slow memory pages can bypass the normal hotness
>> @@ -2887,6 +2913,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>   		pgdat = NODE_DATA(dst_nid);
>>   		if (numa_balancing_promote_free_space_bypass &&
>>   		    pgdat_free_space_enough(pgdat)) {
>> +			task_numa_tiering_no_feedback(p);
>>   			/* workload changed, reset hot threshold */
>>   			pgdat->nbp_threshold = 0;
>>   			mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NRL, nr);
>> @@ -2899,10 +2926,18 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>   
>>   		th = pgdat->nbp_threshold ? : def_th;
>>   		latency = numa_hint_fault_latency(folio);
>> -		if (latency >= th)
>> +		if (latency >= th) {
>> +			task_numa_tiering_fault(p, NUMA_TIERING_FAULT_COLD, nr);
>> +			return false;
>> +		}
>> +
>> +		if (numa_promotion_rate_limit(pgdat, rate_limit, nr)) {
>> +			task_numa_tiering_fault(p, NUMA_TIERING_FAULT_THROTTLED, nr);
>>   			return false;
>> +		}
>>   
>> -		return !numa_promotion_rate_limit(pgdat, rate_limit, nr);
>> +		task_numa_tiering_fault(p, NUMA_TIERING_FAULT_HOT, nr);
>> +		return true;
>>   	}
> I like the separation of scan-period updation of tiering mode from the regular
> mode as the existing update scheme was dependent on private/shared faults which
> made little sense in tiering mode.
>
> However I am not sure if you should drive the scan period updates from
> should_numa_migrate_memory(). The scan period belongs to the hint fault
> scanner. I am pulling the promotion heuristic out of
> should_numa_migrate_memory() in the pghot series. The hint fault path
> only reports an access and shared engine does the classification and
> migration for every source. The other sources may not have numa_scan_period,
> so this feedback has to stay with the hint fault path.
>
> pghot supporting multiple sources may not see the light of the day in
> near future but it is still good to keep the source specific
> aspects and promotion engine separate.


Agreed, that layering makes sense.

I am wondering whether that separation is better handled together with
the larger promotion-engine rework, though.  Without the shared engine in
place, moving the outcome reporting out of should_numa_migrate_memory()
would add interface changes mainly for a future direction.

For this series, my goal was narrower: keep tiering-only scan-period
feedback separate from normal task-placement stats while staying close
to the current numabalance implementation.

Thanks,
Zhe

>
> Regards,
> Bharata.