Re: [RFC PATCH 1/2] sched/numa: Fault count based NUMA hint fault latency

[Bharata B Rao]

On 28-Mar-24 7:26 AM, Huang, Ying wrote:
> Bharata B Rao <bharata@xxxxxxx> writes:
> 
> [snip]
> 
>> @@ -1750,25 +1753,20 @@ static bool pgdat_free_space_enough(struct pglist_data *pgdat)
>>  }
>>  
>>  /*
>> - * For memory tiering mode, when page tables are scanned, the scan
>> - * time will be recorded in struct page in addition to make page
>> - * PROT_NONE for slow memory page.  So when the page is accessed, in
>> - * hint page fault handler, the hint page fault latency is calculated
>> - * via,
>> + * For memory tiering mode, when page tables are scanned, the current
>> + * hint fault count will be recorded in struct page in addition to
>> + * make page PROT_NONE for slow memory page.  So when the page is
>> + * accessed, in hint page fault handler, the hint page fault latency is
>> + * calculated via,
>>   *
>> - *	hint page fault latency = hint page fault time - scan time
>> + * hint page fault latency = current hint fault count - fault count at scan time
>>   *
>>   * The smaller the hint page fault latency, the higher the possibility
>>   * for the page to be hot.
>>   */
>> -static int numa_hint_fault_latency(struct folio *folio)
>> +static inline int numa_hint_fault_latency(struct folio *folio, int count)
>>  {
>> -	int last_time, time;
>> -
>> -	time = jiffies_to_msecs(jiffies);
>> -	last_time = folio_xchg_access_time(folio, time);
>> -
>> -	return (time - last_time) & PAGE_ACCESS_TIME_MASK;
>> +	return count - folio_xchg_fault_count(folio, count);
>>  }
> 
> I found count is task->mm->hint_faults.  That is a process wide
> counting.  How do you connect the hotness of a folio with the count of
> hint page fault in the process?  How do you compare the hotness of
> folios among different processes?

The global hint fault count that we already maintain could
be used instead of per-task fault. That should take care
of the concern you mention right?

Regards,
Bharata.