linux-kernel - Re: [RFC PATCH v2 8/8] mm: sched: Move hot page promotion from NUMAB=2 to kpromoted

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Message-ID: <b13fc805-728a-494e-93ea-f2dea351eb00@amd.com>
Date: Mon, 6 Oct 2025 11:27:21 +0530
From: Bharata B Rao <bharata@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
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Subject: Re: [RFC PATCH v2 8/8] mm: sched: Move hot page promotion from
 NUMAB=2 to kpromoted


On 03-Oct-25 6:08 PM, Jonathan Cameron wrote:
> On Wed, 10 Sep 2025 20:16:53 +0530
> Bharata B Rao <bharata@....com> wrote:
> 
>> Currently hot page promotion (NUMA_BALANCING_MEMORY_TIERING
>> mode of NUMA Balancing) does hot page detection (via hint faults),
>> hot page classification and eventual promotion, all by itself and
>> sits within the scheduler.
>>
>> With the new hot page tracking and promotion mechanism being
>> available, NUMA Balancing can limit itself to detection of
>> hot pages (via hint faults) and off-load rest of the
>> functionality to the common hot page tracking system.
>>
>> pghot_record_access(PGHOT_HINT_FAULT) API is used to feed the
>> hot page info. In addition, the migration rate limiting and
>> dynamic threshold logic are moved to kpromoted so that the same
>> can be used for hot pages reported by other sources too.
>>
>> Signed-off-by: Bharata B Rao <bharata@....com>
> 
> Making a direct replacement without any fallback to previous method
> is going to need a lot of data to show there are no important regressions.
> 
> So bold move if that's the intent! 

Firstly I am only moving the existing hot page heuristics that is part of
NUMAB=2 to kpromoted so that the same can be applied to hot pages being
identified by other sources. So the hint fault mechanism that is inherent
to NUMAB=2 still remains.

In fact, kscand effort started as a potential replacement for the existing
hot page promotion mechanism by getting rid of hint faults and moving the
page table scanning out of process context.

In any case, I will start including numbers from the next post.
>>  
>>  static unsigned int sysctl_pghot_freq_window = KPROMOTED_FREQ_WINDOW;
>>  
>> +/* Restrict the NUMA promotion throughput (MB/s) for each target node. */
>> +static unsigned int sysctl_pghot_promote_rate_limit = 65536;
> 
> If the comment correlates with the value, this is 64 GiB/s?  That seems
> unlikely if I guess possible.

IIUC, the existing logic tries to limit promotion rate to 64 GiB/s by
limiting the number of candidate pages that are promoted within the
1s observation interval.

Are you saying that achieving the rate of 64 GiB/s is not possible
or unlikely?

> 
>> +
>>  #ifdef CONFIG_SYSCTL
>>  static const struct ctl_table pghot_sysctls[] = {
>>  	{
>> @@ -44,8 +50,17 @@ static const struct ctl_table pghot_sysctls[] = {
>>  		.proc_handler	= proc_dointvec_minmax,
>>  		.extra1		= SYSCTL_ZERO,
>>  	},
>> +	{
>> +		.procname	= "pghot_promote_rate_limit_MBps",
>> +		.data		= &sysctl_pghot_promote_rate_limit,
>> +		.maxlen		= sizeof(unsigned int),
>> +		.mode		= 0644,
>> +		.proc_handler	= proc_dointvec_minmax,
>> +		.extra1		= SYSCTL_ZERO,
>> +	},
>>  };
>>  #endif
>> +
> Put that in earlier patch to reduce noise here.

This patch moves the hot page heuristics to kpromoted and hence this
related sysctl is also being moved in this patch.

> 
>>  static bool phi_heap_less(const void *lhs, const void *rhs, void *args)
>>  {
>>  	return (*(struct pghot_info **)lhs)->frequency >
>> @@ -94,11 +109,99 @@ static bool phi_heap_insert(struct max_heap *phi_heap, struct pghot_info *phi)
>>  	return true;
>>  }
>>  
>> +/*
>> + * For memory tiering mode, if there are enough free pages (more than
>> + * enough watermark defined here) in fast memory node, to take full
> 
> I'd use enough_wmark   Just because "more than enough" is a common
> English phrase and I at least tripped over that sentence as a result!

Ah I see that, but as you note later, I am currently only doing the
movement.

> 
>> + * advantage of fast memory capacity, all recently accessed slow
>> + * memory pages will be migrated to fast memory node without
>> + * considering hot threshold.
>> + */
>> +static bool pgdat_free_space_enough(struct pglist_data *pgdat)
>> +{
>> +	int z;
>> +	unsigned long enough_wmark;
>> +
>> +	enough_wmark = max(1UL * 1024 * 1024 * 1024 >> PAGE_SHIFT,
>> +			   pgdat->node_present_pages >> 4);
>> +	for (z = pgdat->nr_zones - 1; z >= 0; z--) {
>> +		struct zone *zone = pgdat->node_zones + z;
>> +
>> +		if (!populated_zone(zone))
>> +			continue;
>> +
>> +		if (zone_watermark_ok(zone, 0,
>> +				      promo_wmark_pages(zone) + enough_wmark,
>> +				      ZONE_MOVABLE, 0))
>> +			return true;
>> +	}
>> +	return false;
>> +}
> 
>> +
>> +static void kpromoted_promotion_adjust_threshold(struct pglist_data *pgdat,
> 
> Needs documentation of the algorithm and the reasons for various choices.
> 
> I see it is a code move though so maybe that's a job for another day.

Sure.

Regards,
Bharata.