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

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Date: Thu, 28 Mar 2024 13:21:58 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Bharata B Rao <bharata@....com>
Cc: <linux-mm@...ck.org>,  <linux-kernel@...r.kernel.org>,
  <akpm@...ux-foundation.org>,  <mingo@...hat.com>,
  <peterz@...radead.org>,  <mgorman@...hsingularity.net>,
  <raghavendra.kt@....com>,  <dave.hansen@...ux.intel.com>,
  <hannes@...xchg.org>
Subject: Re: [RFC PATCH 1/2] sched/numa: Fault count based NUMA hint fault
 latency

Bharata B Rao <bharata@....com> writes:

> On 28-Mar-24 7:26 AM, Huang, Ying wrote:
>> Bharata B Rao <bharata@....com> 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?

I have plotted the total number of hint faults per second before, and it
changes a lot along the time.  So I don't think it is a good
measurement.

--
Best Regards,
Huang, Ying