linux-kernel - Re: [RFC PATCH V1 09/13] mm: Add heuristic to calculate target node

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Message-ID: <17b5d869-d1f7-4427-a293-aef42a37d639@amd.com>
Date: Mon, 24 Mar 2025 20:24:49 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Hillf Danton <hdanton@...a.com>
Cc: dave.hansen@...el.com, david@...hat.com, hannes@...xchg.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, ziy@...dia.com
Subject: Re: [RFC PATCH V1 09/13] mm: Add heuristic to calculate target node



On 3/24/2025 4:35 PM, Hillf Danton wrote:
> On Sun, 23 Mar 2025 23:44:02 +0530 Raghavendra K T wrote
>> On 3/21/2025 4:23 PM, Hillf Danton wrote:
>>> On Wed, 19 Mar 2025 19:30:24 +0000 Raghavendra K T wrote
>>>> One of the key challenges in PTE A bit based scanning is to find right
>>>> target node to promote to.
>>>>
>>>> Here is a simple heuristic based approach:
>>>>      While scanning pages of any mm we also scan toptier pages that belong
>>>> to that mm. We get an insight on the distribution of pages that potentially
>>>> belonging to particular toptier node and also its recent access.
>>>>
>>>> Current logic walks all the toptier node, and picks the one with highest
>>>> accesses.
>>>>
>>> My $.02 for selecting promotion target node given a simple multi tier system.
>>>
>>> 	Tk /* top Tierk (k > 0) has K (K > 0) nodes */
>>> 	...
>>> 	Tj /* Tierj (j > 0) has J (J > 0) nodes */
>>> 	...
>>> 	T0 /* bottom Tier0 has O (O > 0) nodes */
>>>
>>> Unless config comes from user space (sysfs window for example should be opened),
>>>
>>> 1, adopt the data flow pattern of L3 cache <--> DRAM <--> SSD, to only
>>> select Tj+1 when promoting pages in Tj.
>>>
>>
>> Hello Hillf ,
>> Thanks for giving a thought on this. This looks to be good idea in
>> general. Mostly be able to implement with reverse of preferred demotion
>> target?
>>
>> Thinking loud, Can there be exception cases similar to non-temporal copy
>> operations, where we don't want to pollute cache?
>> I mean cases we don't want to hop via middle tier node..?
>>
> Given page cache, direct IO and coherent DMA have their roles to play.
>

Agree.

>>> 2, select the node in Tj+1 that has the most free pages for promotion
>>> by default.
>>
>> Not sure if this is productive always.
>>
> Trying to cure all pains with ONE pill wastes minutes I think.
> 

Very much true.

> To achive reliable high order pages, page allocator can not work well in
> combination with kswapd and kcompactd without clear boundaries drawn in
> between the tree parties for example.
> 
>> for e.g.
>> node 0-1 toptier (100GB)
>> node2 slowtier
>>
>> suppose a workload (that occupies 80GB in total) running on CPU of node1
>> where 40GB is already in node1 rest of 40GB is in node2.
>>
>> Now it is preferred to consolidate workload on node1 when slowtier
>> data becomes hot?
>>
> Yes and no (say, a couple seconds later mm pressure rises in node0).
> 
> In case of yes, I would like to turn on autonuma in the toptier instead
> without bothering to select the target node. You see a line is drawn
> between autonma and slowtier promotion now.

Yes, the goal has been slow tier promotion without much overhead to the
system + co-cooperatively work with NUMAB1 for top-tier balancing.
(for e.g., providing hints of hot VMAs).