linux-kernel - Re: [LSF/MM/BPF TOPIC] Unifying sources of page temperature information

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Message-ID: <de31971e-98fc-4baf-8f4f-09d153902e2e@amd.com>
Date: Wed, 5 Feb 2025 11:54:05 +0530
From: Bharata B Rao <bharata@....com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Raghavendra K T <raghavendra.kt@....com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
 lsf-pc@...ts.linux-foundation.org, gourry@...rry.net, nehagholkar@...a.com,
 abhishekd@...a.com, ying.huang@...ux.alibaba.com, nphamcs@...il.com,
 hannes@...xchg.org, feng.tang@...el.com, kbusch@...a.com,
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 k.shutemov@...il.com, mgorman@...hsingularity.net, vbabka@...e.cz,
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Subject: Re: [LSF/MM/BPF TOPIC] Unifying sources of page temperature
 information - what info is actually wanted?

On 31-Jan-25 6:39 PM, Jonathan Cameron wrote:
> On Fri, 31 Jan 2025 12:28:03 +0000
> Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> 
>>> Here is the list of potential discussion points:
>> ...
>>
>>> 2. Possibility of maintaining single source of truth for page hotness that would
>>> maintain hot page information from multiple sources and let other sub-systems
>>> use that info.
>> Hi,
>>
>> I was thinking of proposing a separate topic on a single source of hotness,
>> but this question covers it so I'll add some thoughts here instead.
>> I think we are very early, but sharing some experience and thoughts in a
>> session may be useful.
> 
> Thinking more on this over lunch, I think it is worth calling this out as a
> potential session topic in it's own right rather than trying to find
> time within other sessions.  Hence the title change.
> 
> I think a session would start with a brief listing of the temperature sources
> we have and those on the horizon to motivate what we are unifying, then
> discussion to focus on need for such a unification + requirements
> (maybe with a straw man).

Here is a compilation of available temperature sources and how the 
hot/access data is consumed by different subsystems:

PA-Physical address available
VA-Virtual address available
AA-Access time available
NA-accessing Node info available

I have left the slot blank for those which I am not sure about.
==================================================
Temperature		PA	VA	AA	NA
source
==================================================
PROT_NONE faults	Y	Y	Y	Y
--------------------------------------------------
folio_mark_accessed()	Y		Y	Y
--------------------------------------------------
PTE A bit		Y	Y	N	N
--------------------------------------------------
Platform hints		Y	Y	Y	Y
(AMD IBS)
--------------------------------------------------
Device hints		Y
(CXL HMU)
==================================================

And here is an attempt to compile how different subsystems
use the above data:
==============================================================
Source			Subsystem		Consumption
==============================================================
PROT_NONE faults	NUMAB		NUMAB=1 locality based
via process pgtable			balancing
walk					NUMAB=2 hot page
					promotion
==============================================================
folio_mark_accessed()	FS/filemap/GUP	LRU list activation
==============================================================
PTE A bit via		Reclaim:LRU	LRU list activation,	
rmap walk				deactivation/demotion
==============================================================
PTE A bit via		Reclaim:MGLRU	LRU list activation,	
rmap walk and process			deactivation/demotion
pgtable walk
==============================================================
PTE A bit via		DAMON		LRU activation,
rmap walk				hot page promotion,
					demotion etc
==============================================================
Platform hints		NUMAB		NUMAB=1 Locality based
(AMD IBS)				balancing and
					NUMAB=2 hot page
					promotion
==============================================================
Device hints		NUMAB		NUMAB=2 hot page
					promotion
==============================================================
The last two are listed as possibilities.

Feel free to correct/clarify and add more.

Regards,
Bharata.